1935 4F cylinders

[camstevens]

Hi All
We are in the disassembly/assessment stage of Cammy project. Much of the bike is in surprisingly good condition, but a major issue exists with the cyl 3 crank flywheel having come unsquare on the crankpin, resulting in a damaged cyl 3 sleeve (a couple of divots removed), possibly bent con rod. Alpha has the cranks, and we await the verdict

Initial thoughts/questions on bores;
- the bores are at 0.040" over, with considerable taper and ovality (cyl 3 sleeve will need to be replaced in any case)
- if we go out to .060" there appears to be very little meat left in the existing sleeves......my sense is to re-sleeve and return to STD pistons to eliminate any risk posed by possibly compromised sleeves. Is this reasonable?
- is JP the only cab-on-the-rank with regard to replacement 4F pistons? I recall a 4F restorer on a YouTube posting saying he has used a modern replacement with 3 rings, including a spaced oil ring. He claimed much better oil control/usage.

Regards
Campbell

[Roger Gwynn]

Is the block currently fitted with sleeves (liners)? In which case I would be dubious about going out to +60 but depends on the wall thickness of the liners. If it isn't sleeved I am not aware of any problem with going to +60. You may find that you have to any way if wear is as considerable as you say. I am not aware of any other pistons. When the U tube restorer claimed better oil consumption etc does this mean that he ran the bike with JP pistons then swopped to something else, in other words was it a real comparison or just an opinion. JP pistons are perfectly OK.

[camstevens]

Thanks Roger
I believe the JP pistons have two rings. Does the second ring have a dual oil control/compression function?

Yes, one can see the sleeve at the block mating face. Replacing them seems prudent

Cam

[Roger Gwynn]

No, the 2nd ring is a plain one https://draganfly.co.uk/shop/66233/piston-020/#4162/20

[paul.jameson]

My current OHC 4F project arrived having been newly bored to suit the new +0.060 JP pistons which came with it. There isn't a great thickness of cylinder left below crankcase level but an adequate amount. The cylinder block has not been fitted with liners. I ran JP pistons in my previous OHC 4F and they were fine as the heavier gudgeon pin is automatically balanced out on a Four whereas it is very noticeable on a single. So I shall use the cylinder block and pistons which came with the project.

Oil consumption was much reduced with the new JP pistons in my previous OHC 4F but then the cylinder and pistons originally fitted were very badly worn. You may find that, if you fit oil control rings, you suffer from excessive wear in the valve guides as the oil passing the plain rings contributes to valve stem and guide lubrication.