1964 Johnson 40hp Pistons?

[65JetStar]

Hello,
I'm in the process of rebuilding a 64 RDSL-26S Johnson 40 and need some background info on pistons. I purchased this motor off eBay with a blown power head and from what I see it looks like it was assembled with the incorrect pistons. According to the parts catalog it should have come factory with 3 ring pistons but for some reason it actually has 2 ring pistons. Did these 2 ring pistons ever supersede as replacements or am I correct in thinking they were incorrect for the PH? Part # on the pistons are 384692 and came in a set #'d 384698. Any help is much appreciated!

 

[F_R]

Original pistons were 378695 (three ring), replaced by 384698 (two ring) (set of two). So your powerhead has had the pistons replaced at some time in it's past.

 

[racerone]

Agee with the above information.------Have some NOS piston kits on hand here.

 

[65JetStar]

Thanks, thats good to know! Any disadvantage to the 2 ring over the three? Asking because I also have a good running 63 PH that I am going to use in the rebuild and may end up doing a re-ring to bring it back up to new.

 

[jimmbo]

The 2 ring design uses a ring design that provided better sealing and less sticking than a conventional ring. One less ring also makes for less friction

 

[F_R]

Top ring has a tapered side, right? It is called a pressure back ring because compression pressure is applied to the back side of the ring, which pushes it tightly against the cylinder wall (instead of spring pressure like a conventional ring.) As jimmbo said, better sealing and less sticking.

 

[65JetStar]

Thanks for all the help...I will start looking for a set of 2 ring pistons!

 

[racerone]

Standard or O/S ----New or used ?

 

[65JetStar]

Racerone, I haven't measured the cylinders yet but I'm hoping standard pistons will work with a re-hone. I've actually got a set of standard two ring pistons that were in the motor when it dropped a cylinder, pistons still look new. Might try to find a set of rings for them. Also, I'm under the impression that oversized pistons are nearly impossible to find.