40hp vs 50hp looper pistons and rods. Scratching my head!

[ct1762@gmail.com]

Below are 2 pistons. one on the left is from a 1986 Belgian-made 50hp. One on the right is from a 1984 OMC 40hp. Note huge cutout on the 84'. both blocks are the same 326100 series. what's the deal? Further, the rods on the newer 86' 50 have no oil holes on the small end, but the 40hp does and are both the same part # 322212. The piston on the 40hp with the larger cutout one has no part #, but is factory stamped "M". the factory parts book says they should be the same, but ain't! Anyone know if these are interchangeable ? looks to me like the cutout would be better, letting more exhaust volume out, plus its lighter.

 

[racerone]

Why do you say about the cut out ----" letting more exhaust volume out "----There is no exhaust in the crankcase !

 

[ct1762@gmail.com]

Below are 2 pistons. one on the left is from a 1986 Belgian-made 50hp. One on the right is from a 1984 OMC 40hp. Note huge cutout on the 84'. both blocks are the same 326100 series. what's the deal? Further, the rods on the newer 86' 50 have no oil holes on the small end, but the 40hp does and are both the same part # 322212. The piston on the 40hp with the larger cutout one has no part #, but is factory stamped "M". the factory parts book says they should be the same, but ain't! Anyone know if these are interchangeable ? looks to me like the cutout would be better, letting more exhaust volume out, plus its lighter.

i just realized that one on the left is a 3 cyl piston! how it worked and ended up in that 40hp i don't know as the bore sizes are different. now i'm really confused.

 

[ct1762@gmail.com]

Why do you say about the cut out ----" letting more exhaust volume out "----There is no exhaust in the crankcase !

i was thinking backwards lol. it would uncover more of the intake port when the piston is down i guess. but from my research, it looks to be a 3cyl piston if thats possible?

 

[racerone]

And why do you say the bore size is different ??----The 56 cube 3 cylinder might have the same bore as the 2 cylinder looper for many years !

 

[ct1762@gmail.com]

i'm sure it does. 3.18'' i think. My whole point of posting here was to find out what piston it was and why it was so different! i didn't realize they worked in a 2 cyl. was just trying to find out if anyone has experiece with this thats all.

 

[ct1762@gmail.com]

i commented bore size because in 84' there was only the 49ci motor with the 3'' bore. didn't think to include the later 56ci. as i always think those were a new design, but i see they simply used the pistons and rods from the 2 cyl. So back to my original reason for the post: if the 3 cyl pistons are lighter, do guys use those instead? seems to be a reason they are different and thus not interchangeable?

 

[racerone]

How did you determine that the 2 cylinder and 3 cylinder use the same rods ??----Me just curious is all.

 

[juno pierrat]

from data sheets 1984 and 1986 , 40 and 50 hp, bore 3.1875, stroke 2.820
1984 70 hp, bore 3.000, stroke 2.3437 don't have 86" 3 cylinder data
just info, trying to follow along, ct

 

[racerone]

The 56 cube 70 HP models also used the 3.1875 bore.----But good luck with your motors.

 

[Mechmagcn]

I ran into something similar on a '90 model 48 SPL. Mine cracked the crown on a piston and I bought another power head from the same year motor that had a bad crank to get a piston. One had the smaller cutouts and the other had the larger. Had to use both, but noticed no difference in performance.

 

[ct1762@gmail.com]

interesting thanks all. I think they must've used the 3 cyl 56ci pistons at different times from the factory as the 84' 40hp was never rebuild. weird! i just assumed the rods were the same as the 56ci motors by eye... didn't research further just figured if they used the same pistons, the rods were likely the same.