Topic: First timer top-end rebuild gone wrong, yippee! Ugh...

[markreimer]

EDIT: This project grew into much more than simply replacing the rings. The cylinders were worn beyond spec and required a re-bore and oversize pistons. After completing that and putting the engine back in the frame, I ended up cracking the engine case along the main oil galley by over-tightening the tapered oil pressure switch. I wasn't able to repair it, and therefor sourced a donor engine, took the bottom half of it and bolted my now fresh top end to the donor bottom end. A two year saga is finally complete and my beloved 750 is back on the road just in time for winter snow 


Hello fellow SOHCers,

It's been too long since I've posted here. I spent the summer letting my cb750 K4 mostly sit under a tarp other than a few less than great rides.

Last year I re-did my head and valve train - new valve guides, seals, had the valve seats re-cut and lapped, replaced a bunch of seals, etcetcetc. Sadly my nagging problem of oil contamination in cylinder #1 continued regardless. Guess it wasn't a bad valve seal after all 

Long story short, I've diagnosed that I should replace the rings in my engine. I've never pulled the cylinder block off before and have precisely no idea what I'm doing, as usual. Exciting!

I thought I'd quickly outline my plan of attack here. Perhaps one of the many engine experts here could point out if I've missed something obvious or important? Many thanks!

1 - I've ordered one of Frank's super sweet weld-less frame kits, which should arrive today! No more plying my friends with beer to help pull the engine. First step will be to install the kit.

2 - Remove the top-end of the engine. This shouldn't be too difficult as I did it when I removed the head, I'll just have to go one step deeper.. Any tips for pulling the jugs off? I've got my rubber mallet ready.

3 - I don't know how to measure if the cylinders are beyond wear. Same for the pistons. So I therefor don't know if I should just order replacement rings, or if I'll need to go one size up for pistons/rings and get it bored and honed as a result... Am I thinking about this correctly? If the pistons and cylinders are both within spec, all I'd need to do is replace the rings and hone the cylinders?

I've got a quote from a local motorcycle shop to measure, bore and hone for $175. That's for all four/total. If I did need to go that route, I'd consider doing an 836 conversion, but only if one of the cheaper eBay kits was available.

4 - replace the base gasket, head gasket, cam cover gasket, and the infamous 'pucks'. I replaced all of those the last time I was in the engine (other than base gasket) and had no leaks. Hopefully I can repeat that..

5 - I've heard many people suggest replacing the studs as well. If I don't do an 836, what is the popular opinion here? Stock studs ok at this point? Haven't failed me yet.

6 - While I'm in there, is there anything else I should check for? I already know my head is flat, and the valves should all be good..

7 - rebuild, fire it up, burn rubber.

And yes - I have four different manuals for the CB750 already, including the original shop manual in all it's glory. I've also requested Hondaman's book for Christmas from my wife.... fingers crossed...


I'll post records of my progress here as it happens. Frame kit shortly, engine break down over the holidays, then parts ordering accordingly, and rebuild in early january.

[flybox1]

#3 - my spare engine was low-mileage, and my pistons showed little to no skirt wear, so I picked up a ($20) 220grit blade hone off the internet, and gave each bore a nice cross hatch, new rings on the old pistons, and put everything back together with APE HD Studs.  No leaks or smoke here. 

All of your bore/piston measurements are in the HM book. 

[markreimer]

#3 - my spare engine was low-mileage, and my pistons showed little to no skirt wear, so I picked up a ($20) 220grit blade hone off the internet, and gave each bore a nice cross hatch, new rings on the old pistons, and put everything back together with APE HD Studs.  No leaks or smoke here. 

All of your bore/piston measurements are in the HM book. 

Thanks flybox1 - mind if I ask a few questions about that?

Unfortunately I'm not sure what typical skirt wear would look like, nor do I understand how to measure bore/pistons without specialized tools. I have some calipers, so I suppose I could measure the pistons easily enough, but perhaps the best thing would be to have a machine shop do all the measuring, and I could hone it myself if that's all that is needed.

I've currently got about 40,000 miles on the engine. When I bought it, there was around 20,000 miles. The PO had replaced the rings right before I bought it. I don't think it had even been fired up, which I didn't know until the first time I dropped the oil pan and found pieces of a ring a year after I started riding it! It ran perfectly until about 35,000, then started blowing a bit of blue smoke, then fowling the #1 plug, occasionally #2. Now I can't ride it more than 15km before the plug is covered in wet black oil.

[flybox1]

For about the price of a set of new rings for your current pistons, one could get the well known 836 piston/ring kit off feebay, and have a machine shop bore your cylinders to precisely match the new set. 
The time and money spent to have them measured might be best spent on fitting these new pistons 
Existing mileage, fouling plugs and blue smoke would sway me in this direction....

[markreimer]

Oh I agree! Sadly there don't seem to be any on feebay at the moment.. I have a couple weeks to keep looking. I only have access to a shop for about the next month or so, so I can't wait too long to get this finished.

[flybox1]

Sirius has one over pistons/rings for $60 each
Cycle-x has a 749 kit for $249, and a 2nd over kit for $199
Wiseco will run you close to 5 bills....
Plenty of options

[cougar]

Mark ; If your Not re-boring any, check for ANY type of lip at the top of your cylinder walls. If you can catch/feel ANY lip there AT ALL (yes capitals, I know, POINT) ! You need to get/rent a "Ridge Reamer" from your local parts house and remove that lip. If you feel a lip and Don't remove it you stand a chance of busting either or your new rings or your piston ring land/grove. Just a paranoia of mine ! seen a couple of folks rebuild auto motors , Not removing that lip and soon make crap out of their motor (more $$$). I'll be quiet now, LOL.   ...cougar...

[ekpent]

Might as well toss in some new cam chain tensioner rubber while your in there if your making a parts list.

[markreimer]

Mark ; If your Not re-boring any, check for ANY type of lip at the top of your cylinder walls. If you can catch/feel ANY lip there AT ALL (yes capitals, I know, POINT) ! You need to get/rent a "Ridge Reamer" from your local parts house and remove that lip. If you feel a lip and Don't remove it you stand a chance of busting either or your new rings or your piston ring land/grove. Just a paranoia of mine ! seen a couple of folks rebuild auto motors , Not removing that lip and soon make crap out of their motor (more $$$). I'll be quiet now, LOL.   ...cougar...

Haha, noted!! I've never heard of that, so I'll make sure to check for it when I get to that point. Thanks!

[Jerry Rxman Griffin aka MuthaF'er]

If you have a local shop do the measuring may as well let them hone while they're at it if all is good. Otherwise take the new kit to them. Make life easy and accurate.

[markreimer]

Here's the latest.

I've started tearing the bike apart to prep for engine removal. I've decided to pull the motor clean out of the frame for this.

As part of that process, I dropped the oil pan. Ta-daa! I found a chunk of a piston ring down there. I guess my rings were pretty shoddy after all.

Now I'm not familiar with rings at all as I've never installed them. I know there is a wavy looking ring, some thinner ones, and a thick one, right? This looks to be a chunk of the big thick ring. What purpose does the thicker ring serve? It also looks like it got a bit chewed up on the edge. perhaps from when it snapped. Hopefully the cylinder walls aren't gouged to hell as a result. We shall see. I've got some buddies coming over in two days to help remove the engine from the frame and should have it all apart a few days after that.

[rtbmrgl]

Im not sure how a piston ring would fall out of a piston even if the ring was broke, unless the piston was destroyed. Unless someone broke a ring putting the cylinder on and didn't realize.   

[markreimer]

Im not sure how a piston ring would fall out of a piston even if the ring was broke, unless the piston was destroyed. Unless someone broke a ring putting the cylinder on and didn't realize.   

Hmm good point... guess I hadn't thought of that.

[flybox1]

Im not sure how a piston ring would fall out of a piston even if the ring was broke, unless the piston was destroyed. Unless someone broke a ring putting the cylinder on and didn't realize.   

Hmm good point... guess I hadn't thought of that.

or the PO broke a ring while taking the old ones off.
The RIK brand rings I used all had a microscopic R on one end/face, and this side faced UP when they went back on.

[rtbmrgl]

Im not sure how a piston ring would fall out of a piston even if the ring was broke, unless the piston was destroyed. Unless someone broke a ring putting the cylinder on and didn't realize.   

Hmm good point... guess I hadn't thought of that.

or the PO broke a ring while taking the old ones off.
The RIK brand rings I used all had a microscopic R on one end/face, and this side faced UP when they went back on.

Yeah good point about falling out removing cylinder,
I just installed rings, they also had the "R", for up side. They didn't have a 1 or 2 designation, the top ring had a gray trim

[markreimer]

I had a productive day in the shop today - got the engine out and torn down.

Took me about 30 minutes of cursing to figure out why the stupid head wouldn't separate from the stupid cylinder block. oh yeah, I forgot one bolt 

What I found inside I find surprising though. The impetus for this rebuild is that I was constantly fowling my #1 and #2 plugs. They'd be wet with oil. If I'd hit the gas hard, my 1/2 pipes would blow blue smoke. If I'd go on a long hard ride, after maybe 200km my plugs would be dead in those cylinders. In the city with start and stop traffic they'd be dead sometimes after one day.

On really cold days when the bike wouldn't want to start, or after sitting for a week or two, I'd see oil dripping out at the headers if it wouldn't turn over quickly. Yikes.

Last year I replaced the valve guides, valve seals, had the valve seats re-cut, and lapped the original valves into place, thinking the oil was dripping into the cylinders from above. It made zero difference after rebuilding.

So fastforward to now - I open up the engine and oddly enough cylinder #1 and #2 have the cleanest looking pistons, while #3 and #4 have carbon buildup. However, #1/2 valves are block and sooty, whereas #3/4 have more of a light brown exhaust valve colour, as seen in the photos.

None of the pistons had damaged rings. I checked to see if they were installed correctly, and so far as I could tell they were (logos facing up, 120 degrees from each other). The base basket was cracked around #1, but I don't see what that would do as it's below the piston.

I'm taking the pistons and cylinders to a shop for measuring. If they are within wear spec, I'll have them honed and replace the rings myself after.

Do you think it's a good idea to remove the valves and have the head and valves cleaned up at the same time, or can I just leave this all as-is, as it was redone only about 1000km ago...?

Here are some photos. Curious to hear your thoughts. I don't have the skills or experience to diagnose probable causes visually.

[Vinhead1957]

You still are getting oil in the chamber.  Valve guides are the usual suspect with that  amount carbon.  You said they were done last year.  I would re measure the valve stem play also.  All four cylinders are burning oil across the board. That may indicate you are indeed on the right path with piston and cylinder clearance.  What do the intake and exhaust look like from inside where you can see the guides? You might consider the next bore size since you have it apart.  That ensures everything is new and won't have you wondering.  Off the cuff, the cylinders will come back with excessive wear and the ring gap will be out of tolerance.  Rings are for compression and oil delivery as well as removal for cooling.   A compression test would have indicated wear also. Too late for that.

[markreimer]

Thanks Vinhead1957. I haven't taken the springs off the valves yet, I was hoping to not have to 

I'll take them off and measure the stem play. I haven't found good instructions on how to do that though. I have a dial gauge setup that measures the back and forth motion in two axis, but I don't know how far to pull the valve out of the guide, which would have a big impact on the readings I would get. Any advice?

This is the only photo I have from the top of the head, for the moment. Probably not very useful though.

[Vinhead1957]

Check the manual but I think you measure side to side play with the valve about 10 mm above the seat

[markreimer]

In my shop manual, it doesn't spec the distance at all. It just says "measure the clearance"... I have three different shop manuals I've compared, none of them spec it. Very weird. Here is an example:http://www.dotheton.com/downloads/CB750%20K0%20K7%20SHOP%20MANUAL.pdf

Look on page 39 for measuring valve guide wear.

Found this video as well, looks like it's sitting at about 10mm.

I used these APE guides last time and asked to have them honed to size. Maybe the shop did a poor job 

http://4into1.com/ape-high-performance-bronze-valve-guides-honda-cb750/

That'll be a big bill just to re-do it. So it goes. I'll try measuring mine at 10mm and see what's what.

[Vinhead1957]

Hondaman book!   .0032" or .08mm. Max. 

[markreimer]

Oh man...

how is this for a coicedence.. My Hondaman book arrived in the mail JUST TODAY! 

[Vinhead1957]

Nice. Good buy

[markreimer]

I spent saturday disassembling the head and measuring the valve/guide play, cylinder bore, pistons, ring gap, etc.

I just received HondaMan's book in the mail the evening before, which is incredible (everyone NEEDS to have this book if they don't already!). So all the measurements were done according to the recommended procedures found there.

Bad news it it looks like basically everything in this whole damn engine is worn past spec! 

Thought I'd post my findings here before pulling the trigger on a pile of new parts, see if you all agree with me.

Piston skirts measured 60.9 60.89, 60.9, 60.9 (From cylinder 1 to 4)

The bore measured 60.96, 60.98, 60.97, 60.98 (measured 12mm down from the top of the cylinders). According to the book, anything more than 0.05mm difference is 'pretty loose'. These range from .06 to .09, so I interpret that as meaning the cylinders are worn past spec and need to be bored, therefore the pistons will also need to be replaced with one size up. Proper difference should be around 0.02mm smaller than bore.

I also slide the pistons into the cylinders from the top, with the pistons upside down, then used feeler gauges to see what size I could fit between the skirt and cylinder. I was able to fit a 0.051mm feeler. The book says anything past 0.046mm should indicate a re-bore.

Conclusion: Substantial wear on the block, probable cause for burning oil. Re-bore and replace pistons with one size up.


Moving on up to the valves...

I measured the valve guide play. Here is a question - I've read two conflicting reports on what the max acceptable play is - Most people say it's 0.003". But is that the TOTAL play, or is it allowed to wobble 0.003" in both directions, for a total play of 0.006"?

For example, on Page II-11 in the HondaMan book, in the section for measuring valve play, there is a graphic of a dial gauge ranging from -0.003 to +0.003, for a total of 0.006" of valve play.

All of my valves have a total play of 0.005", or in other words, -0.0025 to +0.0025. Depending on the answer, my guides may be just fine (Hooray!) or quite a bit past worn (boooo). Thoughts?

Here is a video of me testing the valves. Sorry for the vertical orientation..

I've posted some photos of the valves as well. You can see the intake valves are nice and clean. There is no crap building up behind them.

The exhaust valves are black and sooty behind them. Would I be correct in interpreting this as meaning the engine was running either quite rich, or was burning a lot of oil, which contributes to the buildup in the exhaust valves?



NEXT STEPS:
- Order pistons one size up, with rings
- Take pistons and block to machine shop, have it bored out to a MAX of 0.020-0.025mm piston-to-bore gap.
- Clean up the carbon buildup on the head and valves. Should I lap them into place again, or is that overkill? Based on my valve guide test, should I replace these guides already/again?

[rtbmrgl]

Measure the piston skirt and do the feeler gauge measurement where the arrow is. You may get a different reading on the piston to bore clearance. Also check the bore in different spots to be sure its not egg shaped.