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Do I bore my cylinders due to wear?

BAMotobuild

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Hey Guys,

New to the 750 build but learning a lot. To keep things short I have a 1982 CB750SC. It had 36,000 miles on it and never saw it run before tearing it down. Compression in cylinder 1,2,3,4 = 75,75,80,60. Adding a small amount of oil raised each by about 5psi. Water testing the heads showed leaks on intake valves of cylinders 3-4, 4 being the worst. No leaks from cylinders 1 and 2 or exhaust valves. blowing air from the side of the valve face did show some bubbles thought out all valves. Removing the pistons shows signs of blow by in cylinders 1 and 2. Bore gauging the cylinders and the results are as follows.
Cylinder 1 - 2.4435, C2 - 2.4430, C3 - 2.4429, C4 - 2.4436. All are within .0014-.0021 of the .004 wear limit. These are the largest the bores are and this is at the top end of the bore. Max out of round is at C1 at .001". Max taper is C1 and C4 at .0007". Piston measurements are as follows. P1 - 2.4395, P2 - 2.4395 , P3 - 2.4392, P4 - 2.4396. Piston to bore clearance is as follows. C1 - 0.0037, C2 - 0.0035, C3 - 0.0037, C4 - 0.0040.

At first i was relived to see my bore measurements....still within spec. After measuring the cylinder + the pistons, either I'm doing something wrong or I'm riding the wear limit of the clearance. This is my first time rebuilding an engine and I want to do it right.

Would the best approach be to put on new standard size rings or to bore out the cylinder and new pistons? If so, what would be the best size to bore to and where can I find decent oversize pistons?

I really appreciate all the help and suggestions. If you would like to follow along, a link to our channel is below!
https://www.youtube.com/channel/UCAXNe7JK7IM-yilnFzyGAug?view_as=subscriber
 
Rering it after lightly deglazing the cylinder. .004" is not that loose, they will run out all the way to .007"-.008" easy. The problem today is finding somebody competent enough to bore correctly, it is quickly becoming a lost art.

The valves are where you have the most trouble on first gen DOHCs, set them at .005" instead of .003" and they last much longer. The seats can be cut but NOT the valves, they are heattreated crap steel below the heat treat and grinding them removes it, the valve then lucky to make 10K miles. Even lapping them too far can remove the heat treat if done too much.

Honda says valves set at .003" +- .001"; I can tell you for a fact that .002" CAN burn valves, you always assume two thousandths of whatever you get on valves is not real, it does not exist with engine running. Why most of them have burned valves. The rings by comparison are pretty hardy.
 
Thank you amc49. I really appreciate the help. I have already used a flex hone to deglaze. Pretty happy with the outcome so far. I also lapped all exhaust and intake valves by hand to the point where an even ring formed and the residual buildup was removed. Hopefully that's not too far like you speak of.
Also, the cylinders walls had some signs of wear (vertical / horizontal scratches and marring) that were not removed with the flex hone but can not be felt with a nail. I assume this will be ok?

Before tearing down the engine, I fell between .002 and .006 for my shim clearance. From everything I have read, .005" is what I will be shooting for upon reassembly.

To confirm we are on the same page, your saying my piston to bore clearance is ok, that I can get away with .007-.008"? Again .004" is in the top end of the cylinder walls. Based on the engine manufacture, they specify .004" as a wear limit. Do the rings take up for this difference or is it somewhat of an unwritten rule? I really want to make sure I build the bike so that it has some longevity to it. trying to keep things best practice. Using the least means necessary so if I don't have to bore and step up piston size I would prefer not to but I also would like a sound engine with good consistent compression.
 
If it is put together right it will last easily long enough to keep you happy. The Honda spec is one calculated to keep the engine putting out max rated power AND like all businesses to sell more parts. I typically call for .002"-.0025" when getting a bore job done, it prevents possible piston skirt excess wear if somebody doesn't follow break-in to abuse motor right off the bat. I go to .003" if the engine is race only.

You said 36K miles, the engines can run as much as 60K and higher (seen 100K more than once) as far as rings/cylinder walls only, it's the valves that bite on these. That should give some idea of OEM parts life untouched to wear more. Other parts like the tensioner mechanisms will give you trouble way before rings unless bike is abused. The tensioners are often cooked and beginning to crack rubber at 30K, at least here in Texas where it gets plenty hot. That's not good.

Using .004"-.006" works fine on valves. Cylinder walls OK.
 
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