Author Topic: Turning piston undersize above top ring  (Read 564 times)

Offline Brian Rupnow

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Turning piston undersize above top ring
« on: May 06, 2021, 04:23:53 PM »
Last week someone mentioned that it was wise to turn a piston slightly undersize above the top ring to help getting the top compression ring on without breaking it. I've been playing Mr. Mechanic for almost 75 years, and I've never heard of that. Has anybody else heard of that, and if so, how much undersize do you turn it. I am dealing with a 1" diameter piston (actually 0.998") dia.) and it's up on my lathe right now.---Brian

Offline fumopuc

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Re: Turning piston undersize above top ring
« Reply #1 on: May 06, 2021, 05:03:03 PM »
Hi Brian, I am doing it with all my pistons too.
But the reason is to get compression pressure quicker to/behind  the top ring.
This could give a better sealing.
So 0.05 mm less is my choice.

Kind Regards
Achim

Offline kuhncw

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Re: Turning piston undersize above top ring
« Reply #2 on: May 06, 2021, 05:34:09 PM »
I cut my top lands back as well, for the same reason and by a similar amount.

Chuck.

Offline Brian Rupnow

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Re: Turning piston undersize above top ring
« Reply #3 on: May 06, 2021, 09:31:00 PM »
I didn't do that on the piston I just made, but it's interesting. Nobody else on the other forums I post on has heard of it either. I like it when I ask about things and somebody gives me a believable answer.  Gee, I love learning new stuff.---Thank You----Brian

Offline kuhncw

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Re: Turning piston undersize above top ring
« Reply #4 on: May 06, 2021, 10:04:33 PM »
Brian,

High performance racing engine pistons sometimes have what they call "gas porting" .  One version has holes drilled through the piston crown into the top ring groove.  Another version has a cut back top land. 

https://auto.jepistons.com/blog/vertical-and-horizontal-gas-porting

I doubt the cut back top land has much effect in our model engines.  I carried the practice into my model pistons just because it was sometimes done on full scale pistons. 

I  cut the top of the skirt back just a bit, right below the bottom ring groove.  This is for oil accumulation below the ring. 

Another feature I add to pistons is to take the final OD cut on the piston skirt as a fairly quick feed leaving very shallow grooves (tool marks) on the skirt for oil retention and hopefully better skirt lubrication.  This is similar to a process sometimes applied to full size piston skirts to prevent cold start scuffing.  My piston experience comes from the diesel engine field (20 years back)  and is different that automobile engine technology.

Again, I don't claim these tweaks are needed on a model engine piston.  I just add them because I can.

Regards,

Chuck

Online Dave Otto

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Re: Turning piston undersize above top ring
« Reply #5 on: May 07, 2021, 12:29:42 AM »
Attached are two different charts that give proper piston clearance. The extra clearance above the top ring is because that is where all the heat is and needed thermal expansion.
Like Chuck says probably not needed on a model engine, but it certainly can't hurt.

Dave

Offline Brian Rupnow

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Re: Turning piston undersize above top ring
« Reply #6 on: May 07, 2021, 01:34:20 AM »
Okay, Thanks guys.---I learned something new today. Even if I don't use it on my little engines, it's one more bit of knowledge for me.----Brian

Online lohring

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Re: Turning piston undersize above top ring
« Reply #7 on: May 07, 2021, 03:40:21 PM »
We always do that on our 26cc RC boat race engines.  See my other reply.

Lohring Miller