BAMotobuild
CB750 Member
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
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