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02 Sep 2019
Your admin has not been on the site much as of late as my main computer apparently seized up :( I have had limited access, but will be back soon. Thanks. Mako
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TOPIC: piston rings-AT1 and CT1's

piston rings-AT1 and CT1's 12 Sep 2019 15:10 #1

  • RRB
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I am running into ring thickness issues on my AT1- 125cc and my CT1- 175's .Seems Yamaha made different ring thicknesses . Some of my AT1 piston rings are too thick for one the piston and fit fine in others same for my CT1. the most comin ring measures .058 +or-inch thick . One piston is N/O/S Yamaha #248-11637-01 [ .75- 3rd over ]for my AT1and a .058 ring is to thick and ring set #248-11601-30 will not work .What ring set do I need for a N/O/S Yamaha CT1 piston #251-11637-00-00 [3rd over 75 ] Partzilla's parts information helps but does not connect part#'s to piston part super-ceding #'s 248-, 251-, 479-, part numbers saying AT1 and CT1 may be the same B/S. one is 56mm and other 66mm bore-- ANY help out there.

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piston rings-AT1 and CT1's 12 Sep 2019 15:32 #2

  • MarkT
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Since you only gave one dimension I'm thinking it might be a "ring type" problem

First thought is to check and make sure you aren't trying to fit a standard ring square profile ring into a keystone groove.

(Keystone ring will fit in standard groove, not vice versa)

Many pistons had a keystone top ring and a square bottom ring... some had two keystone rings.

Hard to see if groove in piston is keystone with the slight taper... so what I do is try to put the part of the ring that normally rubs on the cylinder wall into the groove first... like you're going to roll the ring around the piston. If the outside edge of the ring goes into the groove slightly and stops, it's a keystone groove. If you can push the ring all the way in, it's a standard groove.

Using a caliper you can measure the thickness of the ring on the outside and inside.. if the inside edge is a little thinner it's probably a keystone ring and should only be used in a keystone groove.

If inside and outside thickness is the same, it's a standard ring and won't fit a keystone groove.

Prefix of part number is the model code and will tell you if AT or CT. Model code lists are in tech library here.

If you post pictures of your pistons and rings... especially details of the piston markings inside and out... maybe an expert like Enduronut can tell you what you need.
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1963 YG1-T, 1965 MG1-T, Allstate 250, 1970 CT1b, 1971 R5, 1973 AT3MX, 1974 TS400L, 1975 RD350, 1976 DT175C, 1976 Husqvarna 250CR, 1981 DT175G, 1988 DT50, 1990 "Super" DT50, 1991 RT180, 2017 XT250
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