Topic: 1st over instead of 836 ?

[srook]

If you increase the size of the bore, you will need larger pistons to fill that larger bore.  Rings won't take up the extra space and you would be lowering compresion in each cylinder by allowing more space for the mixture to fill.  The rings are to "seal" the combustion chamber from oil entering, from mixture sneaking by when compressed, and from the burned mixture from entering the lower end.  Cast pistons have gotten a bad rap on this forum.  Stock pistons are cast.  The OEM Honda replacement pistons and overbore pistons are cast.  Many race bikes of the 1970s used cast pistons.  Unless you are building a high compression motor (what for?), cast pistons are up to the task.
Scott

[MCRider]

Re: Srook's comments. cast in a street bike are absolutely fine. Build it and ride it.

I would say that all else being equal, I think that effective compression (that which makes the power) is increased when overboring. You are drawing a higher volume of charge into the larger cylinder, then squeezing it into the same size combustion chamber. Unless you have enlarged the combustion chambers to match the increase in the cylinders proportionately, your effective compression is increased.

[754]

A few thoughts,
 If you are riding with others, espescially 836s, you will probably wish you had gone 836... unless you like being last..

 I am thinking a hotter cam with stock pistons (lower compression) is leaving a bit of the cam effect behind.

 Cast pistons can hold up a long time, unless you are at the very extreme end of how hard you run it.

 I am riding a stocker roght now, and it is pretty sluggish compared to an 836.. if you ride double its nice to be able to keep up with modern sedans on the hiway, and be able to pass cars safely..

[MRieck]

Re: Srook's comments. cast in a street bike are absolutely fine. Build it and ride it.

I would say that all else being equal, I think that effective compression (that which makes the power) is increased when overboring. You are drawing a higher volume of charge into the larger cylinder, then squeezing it into the same size combustion chamber. Unless you have enlarged the combustion chambers to match the increase in the cylinders proportionately, your effective compression is increased.

 Right. Cylinder fill is also enhanced by moving the cylinder wall away from the intake valve

[MCRider]

Re: Srook's comments. cast in a street bike are absolutely fine. Build it and ride it.

I would say that all else being equal, I think that effective compression (that which makes the power) is increased when overboring. You are drawing a higher volume of charge into the larger cylinder, then squeezing it into the same size combustion chamber. Unless you have enlarged the combustion chambers to match the increase in the cylinders proportionately, your effective compression is increased.

 Right. Cylinder fill is also enhanced by moving the cylinder wall away from the intake valve

Thanks, a lay person appreciates the confirm.   

[Really?]

I learned a lot off of this thread.  Dang it! 

[Old Scrambler]

I have not learned enough......but trying!  I did the 1st-over with new pins and rings and like the results.  All else is basically stock except for the K&N air-filter.  My K3 now has the same or more power than before the rebuild when it was running a 4-1 header.  The bores cost $50 per hole; the piston kit was $150; the gaskets were $85 with seals. The original cam chain had less side play than a DID replacement so I stayed with it.  One mistake was the cam chain masterlink. It has larger side-plates that hit the adjuster. Lots of loud ticking but now solved with a little grinding wheel. 

I am now getting ready to do a 3rd-over (.75mm) rebuild to stay within AHRMA size limits. This motor will have F2 rods with head porting, an F2 cam, and pods running into unstamped stock pipes (lots of patch-welds). I am building it for endurance rather than all-out power since I don't have the budget for that. 

[wannabridin]

why the f2 cam?  does it make power in a different range than a stock k cam or a 41 grind?  and the f2 rods are stronger than the k rods correct?

[MattFreeman]

This is the best thread on this subject yet.

So lets jack it.

I'm interested to know as well if the f2 rods are stronger. And if they can be used with the earlier cranks or if you need the f2 crank as well. AND are the f2 cranks and rods compatible with the earlier cases.

Sorry for the detour.

[wannabridin]

sorry for the thread jack, i didn't even realize...  20 lashings for me!   

[MattFreeman]

sorry for the thread jack, i didn't even realize...  20 lashings for me!   

I'll take 10 of them.

[sangyo soichiro]

Okay, since we've jacked this thread for learning purposes...

What's a 41 grind?  I heard that mentioned.

[wannabridin]

the 41 grind is a mild camshaft that's made to be "drop in," as in an adjustable cam sprocket is not required.  it has 360 lift and 270 duration, so it's an overall improvement over the stock cams.  i don't know about the F2 cams, so i can't compare them this way. 

http://www.dynoman.net/bikepages/sohc/cams_web2.html

it's the first one there

[Old Scrambler]

The experts on this forum and my first hand inspection of the rods confirm they have more strength, not only in design but in the cap-bolts. They can be shot-peened and/or lightened for even better performance. No crank change needed. The F2 cam has the same lift as stock so you are not adding strain to the springs and retainers. It has longer duration which adds fuel-mixture at a greater rate over time than can be accomplished with lift alone. Its also in my hand versus buying the $200 performance cam. I will also use the F2 stator because it is lighter than my K2 core motor uses. That allows the motor to 'spin-up' faster with less stress on the crank. A heavy-duty cam chain is being considered.

Sure, it would be nice to add some high-compression pistons and titanium parts, but that's money I don't have for this project.  A good set of shocks, decent tires, and some traveling money will produce more fun!

[754]

Did your motor quiet down.. that noise, was really stumping me, not sure why it did that..

[754]

I am surprised no one mentioned a good valvejob.. that may give you more than a good straight overbore on a seasoned block.... them two together can make a big difference..

[Old Scrambler]

Frank.......as I rode more miles the noise (tick or tap in rythem with the cam) disapated. After 2000 miles it was almost gone. I changed oil and filter and found nothing. I got home and put a stethoscope to the running motor and could get the loudest tapping noise when directly on the camchain tensioner. Pulled the tensioner and used a light and a mirror to view the chain. The masterlink had a shinny spot so reached in with a Dremmel and touched it once.  Buttoned it up and changed the oil.........all is quiet.   

[wannabridin]

i know i'm going to pony up the cash for mreick to fix my head, valves and all!  i'll let him do it all so all i need to do is drop in the new cam and degree it!  hopefully it won't be in too bad of shape...

[Terry in Australia]

FYI, the F2/F3 cam has around .5mm more lift than any of the others. Don't use F2 pistons with an F2 cam with anything but an F2 head, or your valves will lightly "kiss" the pistons. I can show you pics if you're not sure.

I've just rebuilt my F2 engine with a K6 top end (K6 pistons, cylinder block and head) but I kept the F2 cam for a little more "poke".
It's not a great cam, but it was there, and as the K6 pistons have deeper valve pockets than the F2 pistons, I'm 99.99999% sure that they won't meet.

I was trying to decide whether to pull the valves apart to fit new guide seals, so for inspiration I tipped the head upside down and filled the combustion chambers with solvent, and 3 out of 4 leaked past the valves. I've now replaced 2 exhaust valves and lapped in all of the valves, and my solvent test and subsequent "compressed air into the ports" test has revealed that they're all sealing now, and are good to go. Hopefully, I won't have to pull that engine again. Cheers, Terry.  

[gtyler5]

"Hopefully, I won't have to pull that engine again." the SOHC4 prayer that we all say at one time or another 

[Terry in Australia]

"Hopefully, I won't have to pull that engine again." the SOHC4 prayer that we all say at one time or another 

Ha ha, having owned and rebuilt way too many CB750's over the last 32 years, that's not an occasional prayer for me mate, it's a bloody mantra!

[wannabridin]

terry,

what dance do you do?  don't you have one patented yet?  the dance to the CB gods for a fire on 1st or 2nd kick??   

[Old Scrambler]

Terry........Thanks for the info.........what was the 'solvent' on you leak-down bench test? And I don't want to imagine you doing a dance unless maybe for beer.

I will measure my cams when the weather warms around here. Seems that conflicting info has been posted by others regarding lift.....or maybe I just read it wrong.  That also goes for the F2-3 barrels fitting on the K cases. Did yours just slip in? And what gasket kit did you use?

I was planning on shaving a K head to increase compression with standard K pistons. Cam lift may interfere so maybe the shave should be no more than .003mm.  I also have a Mega-A grind cam for comparison. Sounds like your saying its slightly better for performance.

I have the option of using F2 pistons and would not shave the head. I saw your pics on the H-P thread and note the need for deeper valve pockets on the exhaust. My goal remains to build the motor(s) for endurance. A local rider was the AHRMA champ a few years ago in a smaller class and wants to move up. Thanks again........Dennis in Wisconsin

[754]

On the 41 cam by drop in, they mean no piston cutting, and no springwork, needed.  Degreeing the cam & springs would be an option.

 Correct me if that is wrong..

[wannabridin]

well i'm still going to use HD springs when a new cam.  i mean 36 year old springs, regardless of if they're in spec or not, are still 36 year old springs.  the HD ones didn't hurt anyone, at least i don't think?