Help! > Hints, Tips & Tricks
Turning piston undersize above top ring
Brian Rupnow:
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
fumopuc:
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.
kuhncw:
I cut my top lands back as well, for the same reason and by a similar amount.
Chuck.
Brian Rupnow:
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
kuhncw:
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
Navigation
[0] Message Index
[#] Next page
Go to full version