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Reducing length of piston rings
Posted: Tue Feb 10, 2026 4:28 pm
by Pete.Silson
I have acquired a box of +060 piston rings, does anybody have any experience of shortening rings for smaller piston oversizes. I've happily trimmed down by 010" (+040 rings regapped for +030 pistons) in the past but how much shorter can they be made e.g. Is it OK to cut +060 down to +020?
Pete
Re: Reducing length of piston rings
Posted: Tue Feb 10, 2026 5:19 pm
by Roger Gwynn
Probably pushing it to go that far but would vary with diameter. I would suggest not for 50mm dia piston not but OK with 3' marine piston. I would be happy dropping .02"
Re: Reducing length of piston rings
Posted: Tue Feb 10, 2026 6:10 pm
by dave.owen
Ive only ever dropped them 20 thou piston size
Re: Reducing length of piston rings
Posted: Tue Feb 10, 2026 6:45 pm
by dave.owen
Pete, what rings are you looking for
Re: Reducing length of piston rings
Posted: Wed Feb 11, 2026 4:25 am
by nevhunter
.020 Max. Do it equally to both ends Nev
Re: Reducing length of piston rings
Posted: Wed Feb 11, 2026 7:20 am
by ian williams
hi Peter ,
im using +.060 filed down rings in my +.040" bored ajs 600 twin .
the story behind it is that i had a little knocking noise when i bought it 15 years ago and found the "new" gpm pistons in it had nipped up and then i found it was only bored with .001" running clearance , so i got some new GPM gandini pistons and rings and i honed it out for .006" running clearance , this then caused the ring gap of the +040 rings to be around .025" and the maximum [ worn out ] ring gap limit for the ajs is +030" .
so i bought some +.060" rings and filed them to fit the +.040" bore and give me .012" ring gap and i put the +.060" rings on the +.040" pistons , it has been running that combo since then and i even changed the rings again one year after i did the job so that i could see if all was ok still , and it was .
i dont normally give the engine a lot of stick but i have ran it almost flat out on occasions and it still runs lovely .
what i would say though is to use one of those rotary ring filers you can buy cheaply these days and dont grip them in a vice and run risk of breaking them , ive been there and got the tee shirt ! .
i know youve done lots of engines peter and you have a lot of skill in doing it , i would be coming to you for advice and information .
ian
Re: Reducing length of piston rings
Posted: Wed Feb 11, 2026 10:22 am
by Julian Murphy
What is a 'rotary ring filer' ?
Got a link to a supplier ?
Re: Reducing length of piston rings
Posted: Wed Feb 11, 2026 11:23 am
by Pete.Silson
Pete, what rings are you looking for
+030 SQ4 - I've already got a NOS half set.
Re: Reducing length of piston rings
Posted: Wed Feb 11, 2026 11:35 am
by Pete.Silson
What is a 'rotary ring filer' ? Got a link to a supplier ?
https://www.ebay.co.uk/itm/317286930624
Pete