1970’s -1980’s Evin/John 4 hp piston /rod diameter specs

[PugetSoundBoater]

Looking for the specs for the above piston, Conn rod, wrist pin. Thinking (way) outside the box, as oversize pistons are NLA for a 2 hp and they are availiable for a 4 hp ,any chance they would fit /with boring? Also the 4 hp rods have needle bearings ,as the 2 hp dosn’t. I know this is a real long shot. Thanks, Jim

"Some people want to know how a watch works, others just want to know what time it is"
Robbie Robertson

[frankr]

I think you have to be more specific. For instance STANDARD piston # 384651 fits the following:

1R69,70
2R71-78
4RW, W, WF69
4R, W70-71

It would seem reasonable to think o.s. ones for those models would also work. But who’s a reasonable thinker?

[dan-in-tn]

It takes quite a bit of homework to figure all that stuff out, but basically the rod & crank set up changed around 1980. The piston did not, except in 1971, the piston rings went to two rings. The two ring design changed to pressure back style rings. That changed the part number. It also changed the sizes available. In all the years piston have been std., .020 OS, .030 OS. After pressure back rings came in the price of the rings are really expensive. I try to watch for any OS pistons I can find. I have over a dozen JW and Lightwins and love the little motors. Parts are hard to find at a deal, but you get lucky every now & then.

Dan in TN

[PugetSoundBoater]

Thanks for the info. I will find and oversize piston somewhere

"Some people want to know how a watch works, others just want to know what time it is"
Robbie Robertson

[Mumbles]

I have a set of +.020 pistons here which are for the later motors but will work in the earlier ones to. They are two ring, same as the early ones,but they use thinner rings. They are square and the thickness of the older rings is 3/32 while the newer ones are 1/16. I don’t think the pressure back style ever made it into the small bore motors, at least in the 70’s.

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