Topic: 1975 HONDA CB750 F PISTONS

[RodSOHC750]

I finally received my machined 1979 K pistons to go in my 1975 F engine. Can someone please assist me in sizing the proper rings for these pistons?  Following are some images of the original flat piston and modified 1979 piston in the cylinders.

Piston Diameter: 61.43 (top) and 61.89-61.66 (skirt across pin - with pin)
Cylinder Bore: 61.93 (top), 61.92 (midway), 61.93 (lower)
Piston Marking: 425   4

A reference chart indicates the 425 piston is a 62mm piston.  13101-425-000 62.00mm, 13102-425-000 62.25mm, 13103-425-000 62.50mm, 13104-425-000 62.75mm.  I'm thinking a standard ring set should work 13011-425-004.  What is the proper ring gap specifications?

[Jerry Rxman Griffin aka MuthaF'er]

Are the 79 K pistons stock size?

If so:

2   13011-425-004   RING SET, PISTON (STD) (RIKEN)   $35.46   


You would need 4 sets

[martin99]

Forgive me, I'm not an engineer, what am I missing here?

Out of interest, why haven't you just gone for a +40 rebore (ie to 62mm) and new sohc pistons?

What is the benefit in fitting those modified pistons? If it's to raise compression I would think the gain would be negligible unless accompanied by some thoughtful head work (but what do I know? ).

Seriously, this is a bit different, I'd be interested to know what the plan is.

[RodSOHC750]

I'm going back 8 months ago so I'm relying on my memory.  I'm working on a 1975F model and the cylinders needed to be bored because of wall condition. I was not able to locate oversize 1975 F model pistons. I bought some new pistons for a 1975 K model. I discovered soon afterward that the resultant compression ratio would have dropped from 9.0 to 8.5 or so.  A forum member suggested using modified 1979K pistons that would result in a compression ratio of about 9.4.    The 1979 pistons are stock size as far as I can tell.  The casting number is 425 and the diameter measures 61.98mm max at the skirt. I removed the top ring from one of the 1979 pistons and placed it a cylinder bore.  The ring end gap measured .381mm which is within tolerance. 

[martin99]

Thanks for taking the time to explain, didn't know oversize pistons were hard to come by.
Looking good, best of luck!