Good evening, everybody. I'm getting ready to do a valve job on my DOHC 750 and cannot find the correct size shims. I don't mean thickness, I know those are supposed to change, but what is the diameter of the shims. I've seen 29mm -- is that right? Can't find the info in the service manual or a search of the forum.
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Valve shim size
- Thread starter brettp
- Start date
Looking on Ebay, the diameter for shims for a '79-'83 DOHC motor appears to be 25 mm. Lots of various thickness shims available on Ebay once you know the thicknesses you'll need.
vintagecb750.com lists shims individually or in a kit on their website. Also states that they are 25mm in diameter.
vintagecb750.com lists shims individually or in a kit on their website. Also states that they are 25mm in diameter.
Thanks for the reply. I had to do some more digging because I saw both 25mm and 29mm options. I did eventually see they are 25mm. I bought a kit from dimecitycycles. More than I wanted to spend but I'll have this bike forever, I think, and hopefully all the shims I ever need.
Kab wilcox
CB750 Member
I did a top end rebuild on my bike. I had to go down to 260 or 265 thickness to get my shims to fit but it was worth the math I had to do. Just remember that the clearance is.002 to .005. Use the smallest shim in thickness to determine what thickness you need for each valve. It takes a bit but it will run smoother.
you need to set clearances to .005 minimum, the stock spec is too tight...you will burn valves if left too tight. Get used to adjusting valves as the valve material and seat material are subpar and do wear in fairly quickly. The most common issue with this motor is people never adjusting valves and running them at stock spec which doesnt allow them to seat when running and burn the valves.
I do know the minimum I'm looking for is .005" (0.15mm), siding on the loose end for shims that aren't exactly what I need.
If anyone is interested, it is a 1981 cb750k with 48k miles, and the results are (in mm):
#1 in: .09 and .1
#1 ex: .04 and .06
#2 in: .08 and .08
#2 ex: .06 and .06
#3 in: 0.1 and 0.1
#3 ex: .08 and .04
#4 in: 0.1 and .08
#4 ex: .08 and .08
They are ALL tight, and in some cases way too tight. My #1 cylinder is running rich; probably be because the exhaust isn't letting enough exhaust out. #4 is lean; not sure why but I'll reset the mixture screws and idle, then resync the carbs after all this is done.
If anyone is interested, it is a 1981 cb750k with 48k miles, and the results are (in mm):
#1 in: .09 and .1
#1 ex: .04 and .06
#2 in: .08 and .08
#2 ex: .06 and .06
#3 in: 0.1 and 0.1
#3 ex: .08 and .04
#4 in: 0.1 and .08
#4 ex: .08 and .08
They are ALL tight, and in some cases way too tight. My #1 cylinder is running rich; probably be because the exhaust isn't letting enough exhaust out. #4 is lean; not sure why but I'll reset the mixture screws and idle, then resync the carbs after all this is done.
Quick question for a sanity check: the thicker the valve shim, the lower the valve drops into the combustion chamber, right?
If that's right, and my intake valve clearances are tight, the intake valves should be open longer than normal spec. Does that mean they are pulling in MORE fuel and air than normal and I woulld be running rich?
If that's right, and my intake valve clearances are tight, the intake valves should be open longer than normal spec. Does that mean they are pulling in MORE fuel and air than normal and I woulld be running rich?
In theory yes, but the amount is minuscule. you also alter valve timing. tighter clearances will increase duration and open valves longer and more up to a point, you are at the point where If anything the tight clearances are causing lower compression when running causing incomplete combustion.
Thanks, dirtdigger. The valve timing I had not thought about, directly, anyway.
When it comes to combustion, the valves should be fully seated, right? A tight or lose valve clearance should be immaterial, right?
And how do these valves tighten over time? I would think the valve springs would weaken, increasing the valve clearance.
When it comes to combustion, the valves should be fully seated, right? A tight or lose valve clearance should be immaterial, right?
And how do these valves tighten over time? I would think the valve springs would weaken, increasing the valve clearance.
The valve will be closed when the engine is off yes, but when the engine is running the cam bearing clearance is more than what your valve clearance is...so when the cam is moving around running it effectively makes you valve clearance 0 or actually less than zero and can hold them open, that is why you use .005" minimum. This clearance will still allow the valves to close even with wide bearing clearance. Your clearances as they stand make you valves barely close or stay open when running. Normally the clearance will tighten over time because the valve and seat in the head wear which makes the valve go deeper into the head pushing it closer to the camshaft which decreases clearance. Valve springs have nothing to do with valve clearance. All they do is close the valve.
Oh, man. I need to keep replaying this in my head. I think I'm starting to see how all of that works. The spring not being in the equation makes sense and the valve seat wear, too. I appreciate the quick replies.
I'll keep everyone updated on the progress (for those that care). Valve tool and shim kit come late this week. With any luck I'll have the shims out and replaced, and bike back together by next weekend. And hopefully the bike runs better.
I don't remember ever reading that in the manual, but thank you for bringing it up. I'll keep that in mind. I did plan on pulling the shims and replacing them while the tool was in place, then taking the measurement again.
Thanks, Bo82CB
Valve shim tool came in, shims came in. Most of my damn shims were 272 so I needed a lot of 265. Of which, I ran out so I need to order more. I put the bike back together with the old shims in and she has a hard time revving through the entire range. Is a carb resync in order now that the shims are changed? It definitely acts like a vacuum issue, but I don't know if a carb sync will fix that. All the boots are on the carbs correctly, the airbox and air filter are stock. Kind of surprised by this outcome
I wouldnt resync anything till you get all the correct valve clearances set. Then recheck compression see where you stand.
170 is spec
If its lower it could be bad rings, could be blown head gasket could be valves leaking
