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Trying to identify cylinder thrust surfaces

Julian Sudano

Julian Sudano

Tuned and Synch'ed
GT Contributor
Joined
Jul 25, 2021
Messages
61
Location
Los Angeles, CA
Hi All,

I've just finished rebuilding the cylinder heads on my 1984 V65. I have upgraded to the later single-spring valve arrangement and hope to put the engine back together tomorrow. Three out of 4 valves were leaking so new valves have been lapped in, the cylinders have been deglazed and carbs, cylinders, heads and inlet elbows have all been vapor blasted so it's looking good!

In order to properly clock the rings, I'm looking for some help. Based on my diagram, can someone advise if my assumption that cylinder surfaces B & D would be the "thrust" surfaces?

Thanks!
 

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First off, I’ve never performed engine work on a small block like yours so this is not 100% certain on my part.

Is there an offset on the wrist pin of the piston? The biased side is the major. Another thing is the pistons should be marked for forward orientation.

Normally, I would expect A and C as major and B and D as minor.

Please wait and see if John Zibell or Amboman or V700Steve or GTM himself, respond as I am not completely 100% positive here. I would like other opinions.


Piston Side Thrust - Engine Repair

>During the power stroke, the force of combustion on the piston's dome and the connecting rod angle force the piston's thrust side against the cylinder wall. As a result, this cylinder area wears more than the other areas; the thrust side of the piston travels on the thrust side of the engine.
www.freeasestudyguides.com

 
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Scott is correct on the forward orientation. There should be an arrow on the piston for forward facing. Guzzi uses letters for piston size variation. That is an A piston, would go in an A cylinder. Since you are rebuilding, my assumption is you are re-using the piston. As for ring placement, I set the gap of the oil ring at 12 o'clock as placed in the engine, then the gaps for the compression rings at 120 degrees or so from the 12 o'clock position. The rings will move on their own, but this is a good starting position.
 
I forgot to mention just to be specific. The 120 degrees from 12 o'clock for the compression rings go to opposite sides.
 
I agree with John, the rings will move/precess, is normal that they walk around the piston ring groove. Learned that last century building my race motors. Sometimes is easier to install with all the gaps in one place during a rebuild, just easier to deal with in the field/at the track.
 
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