Topic: 1974 CB750K4 Piston Ring Correct Size?

[Avalachegantch]

Hey everyone, I tore down the top end of my 1974 CB740k4 that I purchased last winter for a project bike. To my surprise it has a 836cc Wiseco Big Bore Piston kit installed. I need to get new piston rings for it but I am not sure what size to get. The sizes come in 64mm and 65mm. The bore size is 64.94mm. This is where I plead ignorance, I am not sure if I am supposed to measure the piston diameter, or the recessed part of the piston where the ring sits. I have read that I will have to file down the piston ring to get the correct fitting so everything logically tells me that I should go with the 65mm set of rings. But I thought I would double-check here first.

I bought the ?Hondaman book, as well as the service manual; most of the information in there just talks about tolerances and how to fit the rings. And everything I read online touches on STD size and oversized rings by.10, .20, etc.

Thanks all!

[scottly]

You need the 65mm rings.

[PeWe]

Are rings really worn?
Ring gaps too wide?
If ring gaps are OK, just assemble it. Maybe a very light hone.

Measure ring gaps, rings must be removed from piston and pushed into the bore with help of the piston to ensure ring sit correct where bore is worn.
Measure with feeler gauge.
If max is around 0.4mm, use the  rings.

[Avalachegantch]

Thanks, appreciate the info.

I snapped two of the old rings taking them off as carefully as I could.

[HondaMan]

Thanks, appreciate the info.

I snapped two of the old rings taking them off as carefully as I could.

Which rings did you snap? I have a set of 2 rings of a Wiseco kit, I think they are the 2nd and 3rd rings. Their top rings are as brittle as a potato chip and break easily while being manipulated: this kit was needed because when I got them originally, one of the top rings had a bad-cast 'divot' in it on the piston's side of the ring, and as soon as I flexed it, it snapped. They wouldn't replace it under any warranty, either.