Topic: cylinder wear help? 78 CB750 SS F -- HOW FAR TO HONE?

[HondaMan]

Yeah man! I'm worried about everything, I've never done this before.

The .003 does go into the honed cylinder; however it does not go into #2 & #3. So my guess is I overdid it. Or maybe I did what I was supposed to do (i.e., get rid of all wear marks), but there was no helping it.

Any suggestions?
Thanks.

You didn't really 'overdo' it, as the normal process is to hone until the wear marks are gone, then measure to see how much clearance there is, afterward. If the clearance is too big, then an overbore is called for, unless the engine is not to be ridden hard or long (i.e., touring or racing apps). If the clearance is more than .0025", it will not develop full compression for very long, and if it is more than .0032" you'll have trouble will oil control because the rings gaps will likely end up too big on the new rings as they approach the top of the stroke. The piston slap when cold will also be annoying, because the worn pistons have already lost some of their camber.

All this said: you can always just plunk in a new set of rings and try it out, if you're too broke to go after a rebore (and you can get MC rings for about $25 on eBay - see "Mrssuperdeals", the seller). This is/was my "college kid's rebuild" back in the day, and it will last a little while, maybe 10k miles or so, if you stay with top quality oils in the 20w50 range and 1000-1500 mile change intervals. Don't expect dragrace performance from it, as running it to redline or above will sometimes damage the rings if the clearance is too much.

Just be sure, when setting ring gaps on a non-round bore (and yours are, now) that you set the rings more than halfway down the bore when checking the gaps. Your hone will not straighten out the wide top-vs.-bottom of the bore, so it will be narrower at the bottom. If the ring gap is less than minimum down there, the rings can crack as the piston forces them down the overly-tight bore. So, just set the gaps at the point where the piston is furthest down, for best results.

[filipo]

Thanks for the thorough response, HM. Do you think it's worth doing as Scottly says -- i.e., honing the rest of the cylinders and having them professionally measured? Or do you think I should look into overbores (provided I can afford them)?

I'm assuming I'm doing the feeler gauges correctly: I slide it in alongside the piston, piston is upside down, skirt is at the very top of the bore, and I see if I can pull out the gauge. If I can, the gauge is too small and I need the next size up. If I can't pull it out, I'm very close to the tolerance. And of course if I can't even get it in there I need to go down in gauge size.

Another question, because I am a simple feller: If I overbore, I can go one over stock (i.e. 0.25), correct? And all I'd need is new pistons and rings? Whereas if I go to, say, an 836 kit, that means all kinds of new gear -- studs, valves, cam, all of it?

Thanks everyone.

[scottly]

You should have the bores professionally measured. The taper and out of round are specified as a maximum of .002" in the Honda manual. These specs are more critical to lifespan. The Honda manual also states the wear limit before re-boring is 2.406. The manual does not specify a maximum clearance, only a minimum clearance, which is from .0004 (farking tight!) to .0016, on a fresh re-bore and hone.

[HondaMan]

Thanks for the thorough response, HM. Do you think it's worth doing as Scottly says -- i.e., honing the rest of the cylinders and having them professionally measured? Or do you think I should look into overbores (provided I can afford them)?

I'm assuming I'm doing the feeler gauges correctly: I slide it in alongside the piston, piston is upside down, skirt is at the very top of the bore, and I see if I can pull out the gauge. If I can, the gauge is too small and I need the next size up. If I can't pull it out, I'm very close to the tolerance. And of course if I can't even get it in there I need to go down in gauge size.

Another question, because I am a simple feller: If I overbore, I can go one over stock (i.e. 0.25), correct? And all I'd need is new pistons and rings? Whereas if I go to, say, an 836 kit, that means all kinds of new gear -- studs, valves, cam, all of it?

Thanks everyone.

When you measure with the feeler gage, make sure you are at right angle to the piston pin. The piston is narrower at the sides where the pin goes in/out, widest at the longest part of the skirt. Since you're measuring worn pistons in worn bores, maybe try turning the cylinders upside down, insert the pistons their correct way, and measure at the mid-point(ish) of the bore with the feeler gage routine. You will find it slightly tighter there, as opposed to the top where most of the wear already occurred.

Honing indicates how much wear happened, and where it is in the bore: the outside bores have more wear toward the front-outside fins, where it runs the coolest and presses in toward the piston(s). The inner bores wear toward the outer pistons and toward the cam chain tunnel. These will be the 'widest' areas, and you will be able to actually feel this with your gage. This is the out-of-round spec, which occurs naturally in these engines. If you can hone the "ridge" above the top wear marks away and still have less than .0024" clearance in that area, new rings will work OK: otherwise they will be less than great because they will struggle to seal well.

Hone until you no longer can catch a fingernail at all on the top wear mark, and at least a little more, at the minimum. Normally, you want to hone (like you have) until all the marks are gone, which shows new metal.

Going oversize (up to +2mm) uses all stock parts, plus oversize pistons. I am on the .25mm bore myself, about to go to .50mm with the nice pistons from Z1 Enterprises (maybe this Fall). This bore work requires only new pistons and rings, a gasket set, and the bore work. If the engine has more than 20k miles on it, I'd recommend going to 0.5mm overbore, as I have noticed some machine shops don't have rigid-enough small-bore machines, and they tend to miss center by a couple of thousandths. This isn't important on a .5mm overbore, but it sure is on a .25mm (0.010") overbore! At least a couple of the shops around here have been 'big' enough to admit it, when I have brought them 400F or 70cc engines with 0.25mm/.010" overbore pistons. One shop almost begged me to give them 1.00 instead, which I was able to accommodate. They did a good job in the end, but they weren't sure it would come out in the beginning - I appreciated that!

So - measure that clearance with the feelers in the well-honed bore you have, mid-stroke, and see how it comes out. If it is an outer cylinder (#1 or #4), those have the most wear and will indicate how much work might be required.

[cgswss]

I think your question should be "can you buy overbore pistons?", Sorry about the link, I didn't look to see it was a 78.  Before you go ahead, check every where you can to see if you can come up with pistons you can afford.  When your pistons wear, the bores become oval and tapered.  The only to correct this is to re bore.

Of course the other option is to go on e-bay or to one of the many motorcycle salvage yards and get a new jug.  The problem with this is that the replacement used jug may be worse then yours, and I have always tried to put the same piston back in the same hole.  Even a jug that doesn't look good on the outside is fine as you can always swap sleeves 

Of course if you can't afford to "do it right" you can hone the other cylinders and throw it back together and start saving for an overbore kit while you ride it.  I'm assuming you aren't racing the bike and don't plan to put 100,000 mile on it, so the "loose" rings will work for for the rest of the year.  You may get some piston slap, but I doubt it.  Just be aware that you should "do it right" in the future if you want to keep the bike for a number of years.

[filipo]

Thanks for the responses, everyone. Sounds like I have three options:

1) hone the remaining cylinders similarly to the first, measure, and if they're under .0032", install new rings and hope for the best. I'll lose some power and have maintenance issues, but it's a temporary fix, and at least I can get the bike going.

2) have the cylinders over-bored to .5mm, replace the pistons, rings and gaskets. Obviously the next-cheapest option, though still up there at probably $500, but I'd have a professionally machined setup and *slightly* bigger engine.

3) go crazy, spend $$$ and go for an 836 kit. 


My issue with cheapie Option #1 is the risk I take with the rings. If there is more ovalization or change in inner diameter than I am able to measure, I could mess up a ring and fry the cylinder entirely, no? The other issue is I've already got the bike stripped down entirely (great idea, newbie!!), so might as well make things "right" while I'm at it.

Issue with #2 is, I've only found pistons/rings in overbore (0.5mm) at Z1, and their site says "While these will fit a 77-78 supersport, the compression will drop slightly." Is this because those pistons are for K models, not specifically F models? Anyone have any thoughts on how much "slightly" is? Is it a big deal? I'm not racing, but surely the point of having a 750 is to blow it out on the road every now and then, no?

One note: I have no idea how many miles are on this engine. I bought it from a drug addict, the clocks were mismatched (thus probably had been replaced who knows when and how), I changed the clocks out myself without noting mileage, etc. And I'm not nearly qualified enough to gauge how many miles by the looks of the internals. But my guess is, if the bores are out of spec, surely the rest of the engine has a similar (i.e. significant) amount of wear as well.

[HondaMan]

My issue with cheapie Option #1 is the risk I take with the rings. If there is more ovalization or change in inner diameter than I am able to measure, I could mess up a ring and fry the cylinder entirely, no? The other issue is I've already got the bike stripped down entirely (great idea, newbie!!), so might as well make things "right" while I'm at it.

Issue with #2 is, I've only found pistons/rings in overbore (0.5mm) at Z1, and their site says "While these will fit a 77-78 supersport, the compression will drop slightly." Is this because those pistons are for K models, not specifically F models? Anyone have any thoughts on how much "slightly" is? Is it a big deal? I'm not racing, but surely the point of having a 750 is to blow it out on the road every now and then, no?

The compression drops below 9:1 on the 77/78 F2/3 heads with those flat-topped pistons, but most of this can be restored by milling the head 1mm (0.040"), as those heads have an extra height between the top of the chamber and the cylinder deck, to clear the pistons and valves better. This is one reason why those late engines revved higher without crashing internal parts. In fact, I am now building such an engine for a fellow SOHC4-er, using the 1.0mm pistons and milling the head, then advancing the cam 4 degrees, to restore the compression into the low 9.0 range (9.03:1). The resulting HP loss will be on the order of 1.5% of the total, although the original engine was in such poor shape that overall, it will be a significant power increase.

[filipo]

Interesting. If you mill the head, does that hose it for a later, even bigger bore (e.g. 836)? Or does it change that equation at all?

One significant update: I flipped the piston like you suggested, pushed it down past where the ridge used to be, and found that, on cyl #1, in the rear/inner "corner" of the bore (i.e. closest to the intakes and to cyl #2) I can get as much as a .005" feeler in there. Tried it again on #2 (using piston #1, since it's the only one with the rings off) and found I could get to .004". So my guess is that an overbore is definitely necessary, just need to find an appropriate piston kit.

Thoughts?

[CB750F2]

Filipo, I am in a very similar situation with my 78 Cb750 F2. The bores are marginal at best even before honing so I am going to rebore using the cheap 836 kits now on ebay. $110.00 buys you rings, pistons, piston pins, piston pin clips and a head gasket. The stock CR is 9.0 : 1. My calculations show that the 836 kit will give the same CR which is down on the 10.0 : 1  that would be achieved if an 836 kit with domed shaped pistons was used. I am OK with this because I am after good reliable runner without spending a lot of money. The 9.0 : 1 CR will allow me to run 92 octane fuel that is available here in Australia.  No modification to the head is required unless you want to shave  the cylinder head to achieve a higher CR. Hondaman has confirmed the CR figure of 9.0 : 1 so I am confident in what I have decided to do.
I also suggest that while you have the motor apart check everything including :-
Valve guide wear
Valve wear
Cam chain and tensioner
Primary Chain and tensioner
Big end bearings
Main bearings
Transmission bearings
Transmission gears and bushes
Transmission forks
Clutch needle bearings
Transmission assembled correctly.
It is a fair job and expensive to do all the above but you will be giving yourself the best chance of having a reliable motor. The decision on whether or not to replace a part is up to you. If you are only going to ride once or twice/year you may not need to replace but if you intend to use the bike on a regular basis and ride hard and the part is say more than half worn out then replace.
Hondaman, as usual, has given you a lot of good advice and information. Pat

[scottly]

One significant update: I flipped the piston like you suggested, pushed it down past where the ridge used to be, and found that, on cyl #1, in the rear/inner "corner" of the bore (i.e. closest to the intakes and to cyl #2) I can get as much as a .005" feeler in there. Tried it again on #2 (using piston #1, since it's the only one with the rings off) and found I could get to .004".

Thoughts?

The measurement should not be taken at the corner, as the pistons were made to be smaller on the sides. If the largest feeler you can fit between the piston skirt and the cylinder at the front or back, on all 4 cylinders, with their matching pistons, is only .004", my thought would be to just do the ring replacement.

[filipo]

Pat: Thanks for this. Glad to know someone is in the same boat. If I understand you correctly, you;re basically saying that, instead of going to the 1.0mm overbore and milling the head, I can get the same compression by going straight to an 836? I confess I'm out of my league here, but let me know if that's the general gist. If that is the case, then why go to 1.0mm when you could do the big bore job (i.e. 836)? Maybe I'm missing something.

As to your other suggestions, yes, I plan on checking out as much as I can. What I'm running up against is whether I have the tools and/or expertise to accurately measure all this stuff. If I'm running into this much trouble with just feeler gauges, well -- makes me nervous.

Also: do you have a link to the 836 kit you plan to buy? Thanks.

Scottly: Thanks for the clarification. I will measure again more properly tomorrow. Again, the two issues with just running new rings is whether it's not worth it to just get things sorted right while I have the whole bike apart.

Thanks again.

[scottly]

Let me put it another way; worst case from just putting new rings in, the motor may use a slight bit more oil than a fresh bore job, and be down a tiny amount on compression at cranking speed. It will not disintegrate from slightly loose bores.
Don't fall into the trap that we all have at one time or another: well, as long as I have it apart, I'll bore it out and replace the pistons. Well, the bore job costs the same, so I may as well go 836. Hmm, might as well add a hipo cam to take advantage of the bore. Do I need better rods, since I've upgraded the power output?
It can become a very expensive spiral... 

[cgswss]

OK, I think maybe we have got a little lost here.

As I understand it, your original intention was to simply restore the bike to something around the original stock performance.  If you could find a set of original first over bore pistons, you would hit this goal.  Now most people think of "big bore" as a way to get more performance.  My original suggestion of going to the cheap 836 kit was that it was the least expensive way to restore performance.  I wasn't suggesting it as a "hop up" item.

Now your bike has a higher dome and original pistons are domed to get a good compression ratio.  The compression ratio is the volume of the cylinder/head with the piston at the bottom of the stroke compared to the volume at the top of the stroke.  (the real compression ratio would take in the effects of the cam leaving the vales open during part of the compression stroke, but we will ignore that.

Normally we raise compression by reducing the volume at the top of the stroke (like with a higher dome on the piston.  But you can also increase the compression ratio by Increasing the volume at the bottom of the stroke.  By boring the cylinder out.

So if we made no other change to the shape and height of the dome, just increasing the volume of the cylinder will increase the compression ratio.  If we take away the dome, we reduce the compression ratio.  So what we have by going to the 836 kit is that the increase in volume (bigger bore) almost offsets the the lack of dome.

Now IF the reason for this was to INCREASE HP, the flat top 836 set is not a good investment. But as  understand it, your not trying to increase power, just restore it.

Now if I have the head off a 35 year old bike, I would normally have a small cut taken off the head to make sure it was flat before screwing the engine back together.  When you are paying for milling a head, what you are paying for really is set up.  The actual cut takes very little time.  In other words, it won't really cost more to have .025 taken off then to have .010 taken off.

So doing the low compression 836 kit and milling the head, should do an excellent job of RESTORING the power.

By the way, you would NOT want to go to a bigger cam with this set up.  A bigger cam would effectly lower the compression ratio and you would have to build the engine to rev a lot higher to get your power.

this is the low compression 836 kit

http://www.ebay.com/itm/69-77-HONDA-CB750K-836cc-BIGBORE-PISTON-KIT-W-GASKET-/130712839062?pt=Motorcycles_Parts_Accessories&hash=item1e6f17a396&vxp=mtr#ht_1194wt_1067

This is the high compression kit that would INCREASE your power. (but is costly)

http://www.dynoman.net/engine/pistons/custom.html#F2

[CB750F2]

Filipo, I agree with what cgswss has said in the previous post. This is the message that I was trying to convey in my previous post. In my case I am not looking for an increase in power but I want a motor that is reliable and will last a long time. You are limited in options with the F. The 836 kit happens to be very cheap and the feedback from customers has been good. The following is a couple of calculations of CR using flat top pistons:
1         CR for +1mm over bore
           CR = (Swept Volume + Combustion Chamber Vol) / Combustion Chamber Vol
                = (Pie*Bore/2*Stroke + 26) / 26
                =(3.1428 * 3.1 * 3.1 * 6.3 +26) / 26
                = 8.32 : 1
2         CR for +4mm over bore (836 kit)
           CR =(3.1428 * 3.25 * 3.25 * 6.3 + 26) / 26
                =9.04 : 1
The bore and stroke figures in the above calculations are in cm and the combustion volume is in cubic cm. The accuracy of the above calculations depends on the volume chamber figure. I got that figure from Hondaman. I was doing that calculation myself when Mark emailed me the figure and I think that it was similar to mine but I cannot find it to confirm. I am sure that Mark will confirm or otherwise.
So again it is what you want to do with the bike. Pat

[HondaMan]

Filipo, I agree with what cgswss has said in the previous post. This is the message that I was trying to convey in my previous post. In my case I am not looking for an increase in power but I want a motor that is reliable and will last a long time. You are limited in options with the F. The 836 kit happens to be very cheap and the feedback from customers has been good. The following is a couple of calculations of CR using flat top pistons:
1         CR for +1mm over bore
           CR = (Swept Volume + Combustion Chamber Vol) / Combustion Chamber Vol
                = (Pie*Bore/2*Stroke + 26) / 26
                =(3.1428 * 3.1 * 3.1 * 6.3 +26) / 26
                = 8.32 : 1
2         CR for +4mm over bore (836 kit)
           CR =(3.1428 * 3.25 * 3.25 * 6.3 + 26) / 26
                =9.04 : 1
The bore and stroke figures in the above calculations are in cm and the combustion volume is in cubic cm. The accuracy of the above calculations depends on the volume chamber figure. I got that figure from Hondaman. I was doing that calculation myself when Mark emailed me the figure and I think that it was similar to mine but I cannot find it to confirm. I am sure that Mark will confirm or otherwise.
So again it is what you want to do with the bike. Pat

Your numbers are right, Pat: if you mill the F2/3 head 1.08mm it puts the CR back to 9.0:1 with the +1.00 mm overbore.

[filipo]

Filipo, I am in a very similar situation with my 78 Cb750 F2. The bores are marginal at best even before honing so I am going to rebore using the cheap 836 kits now on ebay. $110.00 buys you rings, pistons, piston pins, piston pin clips and a head gasket. The stock CR is 9.0 : 1. My calculations show that the 836 kit will give the same CR which is down on the 10.0 : 1  that would be achieved if an 836 kit with domed shaped pistons was used. I am OK with this because I am after good reliable runner without spending a lot of money. The 9.0 : 1 CR will allow me to run 92 octane fuel that is available here in Australia.  No modification to the head is required unless you want to shave  the cylinder head to achieve a higher CR. Hondaman has confirmed the CR figure of 9.0 : 1 so I am confident in what I have decided to do.

Well, winter is here, so I'm back at it.

Pat, from this post in the "Cheap 836 kits" thread it sounds like you put a K head on your F2 engine? Is that correct? I'm sure this is a stupid question, but is that possible? Recommended? And if so can you describe what you did (or point me to the appropriate discussion)?

Thanks!

[HondaMan]

Filipo, I am in a very similar situation with my 78 Cb750 F2. The bores are marginal at best even before honing so I am going to rebore using the cheap 836 kits now on ebay. $110.00 buys you rings, pistons, piston pins, piston pin clips and a head gasket. The stock CR is 9.0 : 1. My calculations show that the 836 kit will give the same CR which is down on the 10.0 : 1  that would be achieved if an 836 kit with domed shaped pistons was used. I am OK with this because I am after good reliable runner without spending a lot of money. The 9.0 : 1 CR will allow me to run 92 octane fuel that is available here in Australia.  No modification to the head is required unless you want to shave  the cylinder head to achieve a higher CR. Hondaman has confirmed the CR figure of 9.0 : 1 so I am confident in what I have decided to do.

Well, winter is here, so I'm back at it.

Pat, from this post in the "Cheap 836 kits" thread it sounds like you put a K head on your F2 engine? Is that correct? I'm sure this is a stupid question, but is that possible? Recommended? And if so can you describe what you did (or point me to the appropriate discussion)?

Thanks!

IIRC, Pat is trying to install the 836 flat-top pistons in the F2 engine. This works out well IF the F2 head is milled 1.0mm (.040"), as it brings compression back toward stock numbers where the cam is happy again. While the K7/8 head might fit, it has smaller intake valves (and is the wrong color!), losing the power you would have gained in the big-bore. The K7/8 can also receive the 836 pistons, but the head only needs about .015" milling to bring it back above 9:1 CR.

I'd suggest 'backing up' a litte bit and maybe re-measure those bores and psitons? Install the piston with the wrist pin pointed across the engine (i.e., normal direction) and upside-down from the top end, then push it in (with the feeler gage) until the skirt is just below the deck about 1/4". This is where the widest wear spot (that can be measured with a feeler gage) will be found. On the 1 & 4 cylinders, You can "force" a further test for out-of-round by repeating it with the wristpin oriented at 45 degrees, with one side pointed toward 10 O'clock and the other toward 4 O'clock (with 12 O'clock being the front of the cylinders) to try to see how much extra space there is toward the outside front fins. If this difference is more than about .0010" from the front-back clearance, it will take a long time for the rings to seat because they have to wear off this whole .0010" before they can seal.

In the "old days" this was SOP for low-budget rebuilds. In those days, we would use real light oils during such a break-in procedure, watching to see when it stopped using oil. Then we'd go to the thicker, proper oils and hope for the best. In a car engine, this meant about 40k-50k miles of life: in these bikes, this is about 10k miles. I used to call it my "college kid rebuild" at Western Wheels, because the college kids there never had enough $$ to do it right. It got them through graduation, though!

The worst-case scenario is: if the oval is too great, the top ring can fracture. this shows up pretty soon, like in 2000 miles, as low compression on that cylinder. Then there is no choice but to bore bigger, as it will scratch the bore a little bit.

[CB750F2]

Filipo and Mark. Yes, I ended up putting a K head on instead of the F2 head. I had to slightly modify the F2 barrels so that the unused drain back holes would drain oil from the K head. I did this because I used the Cheap 836 from Ebay and I would lose too much compression if I reinstalled the F2 head. I could have machined 1.08mm from the F2 head but I did not want to do this. The K head was a K3 head - I think - and the standard size valves for a K head was used. I also smoothed out the intake and exhaust ducts - just to remove sharp edges etc. The cam I used was from a K8 and I also advanced the cam timing by approximately 5deg. The bike runs smooth pulls strongly and no oil leaks - so far!. Pat

[filipo]

I'd suggest 'backing up' a litte bit and maybe re-measure those bores and psitons? Install the piston with the wrist pin pointed across the engine (i.e., normal direction) and upside-down from the top end, then push it in (with the feeler gage) until the skirt is just below the deck about 1/4". This is where the widest wear spot (that can be measured with a feeler gage) will be found. On the 1 & 4 cylinders, You can "force" a further test for out-of-round by repeating it with the wristpin oriented at 45 degrees, with one side pointed toward 10 O'clock and the other toward 4 O'clock (with 12 O'clock being the front of the cylinders) to try to see how much extra space there is toward the outside front fins. If this difference is more than about .0010" from the front-back clearance, it will take a long time for the rings to seat because they have to wear off this whole .0010" before they can seal.

In the "old days" this was SOP for low-budget rebuilds. In those days, we would use real light oils during such a break-in procedure, watching to see when it stopped using oil. Then we'd go to the thicker, proper oils and hope for the best. In a car engine, this meant about 40k-50k miles of life: in these bikes, this is about 10k miles. I used to call it my "college kid rebuild" at Western Wheels, because the college kids there never had enough $$ to do it right. It got them through graduation, though!

The worst-case scenario is: if the oval is too great, the top ring can fracture. this shows up pretty soon, like in 2000 miles, as low compression on that cylinder. Then there is no choice but to bore bigger, as it will scratch the bore a little bit.

If the largest feeler you can fit between the piston skirt and the cylinder at the front or back, on all 4 cylinders, with their matching pistons, is only .004", my thought would be to just do the ring replacement.

Of course if you can't afford to "do it right" you can hone the other cylinders and throw it back together and start saving for an overbore kit while you ride it. I'm assuming you aren't racing the bike and don't plan to put 100,000 mile on it, so the "loose" rings will work for for the rest of the year.  You may get some piston slap, but I doubt it.  Just be aware that you should "do it right" in the future if you want to keep the bike for a number of years.

I'm sensing a pattern here...

Guys, thanks for all your patient replies. I re-measured the honed cylinder last night, and I think just honing the remaining three cylinders and installing the standard rings is the best approach for now. The .004" feeler only barely fits, and that's with the skirt 1/4" from the top of the jug (upside down). Clearly this setup won't last forever, but in the name of getting the bike going (and moving on to other things on my project list) it's the best option.

Another reason: I can't seem to find any .25mm or .5mm pistons for this model F, so that "small" overbore doesn't seem possible for this bike. Checked Dynoman, APE, Z1, CycleX.
Moot, at any rate, since I'm just going with the rings I have.
Thanks again for the help.

[bryanj]

Just to throw you another curve the 77/78 F2/3 were notorious for wearing out exhaust valve guides quickly

[HondaMan]

Filipo and Mark. Yes, I ended up putting a K head on instead of the F2 head. I had to slightly modify the F2 barrels so that the unused drain back holes would drain oil from the K head. I did this because I used the Cheap 836 from Ebay and I would lose too much compression if I reinstalled the F2 head. I could have machined 1.08mm from the F2 head but I did not want to do this. The K head was a K3 head - I think - and the standard size valves for a K head was used. I also smoothed out the intake and exhaust ducts - just to remove sharp edges etc. The cam I used was from a K8 and I also advanced the cam timing by approximately 5deg. The bike runs smooth pulls strongly and no oil leaks - so far!. Pat

That's a nice collection of parts! Those rubber cylinder seals will make for a nice seal against the K3 head, should keep it dry a long time. Let me guess: is your compression number about 140 PSI?

[HondaMan]

I'd suggest 'backing up' a litte bit and maybe re-measure those bores and psitons? Install the piston with the wrist pin pointed across the engine (i.e., normal direction) and upside-down from the top end, then push it in (with the feeler gage) until the skirt is just below the deck about 1/4". This is where the widest wear spot (that can be measured with a feeler gage) will be found. On the 1 & 4 cylinders, You can "force" a further test for out-of-round by repeating it with the wristpin oriented at 45 degrees, with one side pointed toward 10 O'clock and the other toward 4 O'clock (with 12 O'clock being the front of the cylinders) to try to see how much extra space there is toward the outside front fins. If this difference is more than about .0010" from the front-back clearance, it will take a long time for the rings to seat because they have to wear off this whole .0010" before they can seal.

In the "old days" this was SOP for low-budget rebuilds. In those days, we would use real light oils during such a break-in procedure, watching to see when it stopped using oil. Then we'd go to the thicker, proper oils and hope for the best. In a car engine, this meant about 40k-50k miles of life: in these bikes, this is about 10k miles. I used to call it my "college kid rebuild" at Western Wheels, because the college kids there never had enough $$ to do it right. It got them through graduation, though!

The worst-case scenario is: if the oval is too great, the top ring can fracture. this shows up pretty soon, like in 2000 miles, as low compression on that cylinder. Then there is no choice but to bore bigger, as it will scratch the bore a little bit.

If the largest feeler you can fit between the piston skirt and the cylinder at the front or back, on all 4 cylinders, with their matching pistons, is only .004", my thought would be to just do the ring replacement.

Of course if you can't afford to "do it right" you can hone the other cylinders and throw it back together and start saving for an overbore kit while you ride it. I'm assuming you aren't racing the bike and don't plan to put 100,000 mile on it, so the "loose" rings will work for for the rest of the year.  You may get some piston slap, but I doubt it.  Just be aware that you should "do it right" in the future if you want to keep the bike for a number of years.

I'm sensing a pattern here...

Guys, thanks for all your patient replies. I re-measured the honed cylinder last night, and I think just honing the remaining three cylinders and installing the standard rings is the best approach for now. The .004" feeler only barely fits, and that's with the skirt 1/4" from the top of the jug (upside down). Clearly this setup won't last forever, but in the name of getting the bike going (and moving on to other things on my project list) it's the best option.

Another reason: I can't seem to find any .25mm or .5mm pistons for this model F, so that "small" overbore doesn't seem possible for this bike. Checked Dynoman, APE, Z1, CycleX.
Moot, at any rate, since I'm just going with the rings I have.
Thanks again for the help.

The last time I looked, only CycleX (Ken) had the right pistons for 'stock' bore "F" engines. Haven't looked for a while, though.

[filipo]

Just to throw you another curve the 77/78 F2/3 were notorious for wearing out exhaust valve guides quickly

Just had to tinkle in my Cheerios, didn't you...

[CB750F2]

Mark, I have not checked my compression pressure as yet. I have less than 250 miles on this engine and will wait until I have travelled 500miles before checking. However the way it feels when I manually kick it over I think compression pressure would be more than 140 psi. I have another F2 with a similar setup and it measures 185 psi. Pat

[flybox1]

Just to throw you another curve the 77/78 F2/3 were notorious for wearing out exhaust valve guides quickly

Just had to tinkle in my Cheerios, didn't you...

For the F2/3 heads..
New APE bronze valve guides are easy to install.  An oven, a freezer, and a drift are all you need 
Use OEM guide seals, not the cheapo aftermarket ones.
brandEn clued me in on a thread MRieck posted where the aftermarket seals are taller and don't seal as well.