2 and 3 ringed pistons

[ramboot]

just got my inline six block back with two cylinders rebored The pistons they gave me for those holes are two ringed, the other original four are three ringed, can I still use the two and three ringed pistons in the block?

 

[jimmbo]

Yes.
The two ring design is newer, and the engine doesn't know or care if some pistons are old style and other are the newer. The rings though, are design specific, and are not interchangeable between the two

 

[Chris1956]

The two rings for the 2 ring pistons are likely keystone shaped

 

[Faztbullet]

Need to check dome height as if the newer pistons are low domes, compression will be lower in those cylinders.

 

[Texasmark]

I may be out of line here but maybe not.

From memory: I had a 1988 115 "Direct Charged" 2 stroker. It was Mercury's modified cross flow scavenging answer to the OMC Looper for something adaptable to the engines coming off the assembly line at the time. The Charging system was similar but different, like the hole in the piston was in a different place and the 3 conventional piston rings were used rather than the 2 wedge shaped rings used with OMCs Looper. I think when Merc. did move on to Loop Charging, they adopted the wedge shape also for greater efficiency and better performance.

If your engine is of the Direct Charge type and your other pistons have holes, are they in the same place, same size? Does the piston have a (cross flow) crown or are they flat topped.....Crown used on Direct Charged and flat topped for Loopers!

Shooting from the hip, if it were my engine I wouldn't mix the two types, even if the clearances allow for it. I'd think that vibrations could/would occur due to non-uniform power pulses to the crankshaft from the different compressions????? Considering the tear down/re assembly effort and pistons/rings unable to be returned as they had been used, gaskets and such required if re-entry is required, I personally wouldn't try it. But that's your call!

 

[jimmbo]

Merc used the 2 ring design on the Direct Charge engine too, not just the Loopers. Omc used 3 rings on their early loopers, their 2 ring piston was introduced in 1970 when they upped the HP to 60hp

 

[racerone]

Only the top ring on early OMC CROSSFLOW and LOOPER motors was the wedge shape.

 

[jimmbo]

Only the top ring on early OMC CROSSFLOW and LOOPER motors was the wedge shape.

Early OMC crossflow and loopers used standard 3 ring pistons. The first use of the Wedge/Keystone in production engines was in 1970 on the 3 cylinder 60HP. In 71 they started using it in the V4s

Hey Racer, are you headed West this Year? The Snow and Ice are melting quite quick right now

 

[racerone]

edited

 

[Texasmark]

Merc used the 2 ring design on the Direct Charge engine too, not just the Loopers. Omc used 3 rings on their early loopers, their 2 ring piston was introduced in 1970 when they upped the HP to 60hp

Just keeping the chat going: I thought I recalled the 1968 or 69 Evinrude "Triumph" 55, triple, looper sales brochure advertised 2 wedge shaped rings and the reason for including them in their Looping design....no biggie, just idle curiosity. Interesting how it took awhile for the exhaust reflection (suited to triple cylinders) to be compatible with 4 cylinder engines for part of the loop exhaust scavenging.

 

[jimmbo]

There is no mention of the 'New' Pressure Backed Piston Ring until the 1970 Brochure,
http://www.boatinfo.no/lib/evinrude/brochures/1970evinrude.html#/8

The 1969 brochure makes reference to Thinner Rings

3 cylinder engines have an advantage regarding exhaust tuning having the pulses 120 degrees apart worked well timing wise as one cylinders exhaust port was closing as anothers was opening. Made it simpler, and more effective over a larger RPM range.

 

[GA_Boater]

Why don't we just answer the OP's question?

Can he use a mix of 2 and 3 ring pistons?

 

[jimmbo]

Why don't we just answer the OP's question?

Can he use a mix of 2 and 3 ring pistons?

I thought I did in post #2

 

[GA_Boater]

I thought I did in post #2

Then why are we doing the history of OMC motors in a Merc thread? :focus:

 

[Texasmark]

Then why are we doing the history of OMC motors in a Merc thread? :focus:

Because we are discussing the reason for 2 vs 3 piston rings on a piston because a naturally curious person would ask why would some pistons be supplied (to me) with only 2 and some with 3, and what difference would it make on 2 of the 6 cylinders on my engine anyway and why would the piston with 2 rings have a flat top and the one with 3 have a dome top, and how did 2 piston rings come to exist anyway as 3 had been the norm since the beginning of time, like cross flow engines with domed pistons and now the tops are flat and why..............................................................

 

[GA_Boater]

Come on, Tex. That is no reason to muddle the thread. Start another thread if you need your curiosity slaked.

 

[Texasmark]

Yes Sir! On a little diversion, you have knowledgeable folks right there, mind on the subject, new thread might go unnoticed if the title isn't in order, just a little coffee shop talk, come on man, ease up. Grin.