Ring tolerance

[Todd Humphreys]

Last year I purchased a 1982 75hp Johnny (the owner told me it was recently rebuilt .020 oversize) which compression was 100, 90, 90 (cold). About a month ago my head gasket blew and now reringing the motor. I just received new 20 over rings for the rebuild. After honing the cylinders I was checking the new ring tolerance which was 0.013 (mfg specs .007 - .017). Then I checked the old rings which averaged .045 - .050. How recent of a rebuild could it have been with .030 wore in the ring tolerance, or maybe standard rings was used on 20 over pistons. Is it normal to have this much wear and still have cylinder walls within .003 of .020 over std bore???

 

[rodbolt]

Re: Ring tolerance

forget about measuring the old stuff. measure the bore size as per the manual. then measure the piston as per the manual. then measure the ring side clearence and end gap as per the manual.also make sure all the carbon is out of the ring grooves. I usually soak the pistons in a vat of carb cleaner if I am going to reuse one.<br />remember a two stroke is not a 4 stroke. blowby in a 4 stroke may use a bit of oil. blowby in a two stroke contaminates the incoming air/fuel charge and can lead to crankcase related parts failures. that is why linear scratch marks are a no no for a two stroke. the hot combustion gases can travel down the scratch and start eating at the piston. as the piston eats the problem worsens and the incoming air/fuel gets leaner and carbon builds on the skirt and the vicious circle gets smaller and faster leading to the early demise of your baby.<br /> that is why scuffing you would not notice on your car motor will not work with a high performance 2 stroke.<br /> good luck and keep posting

 

[Dhadley]

Re: Ring tolerance

Might be hard to say but something isn't right. If the motor blew a head gasket and water got in the cylinders and created some surface rust, that can wear rings rather quickly. <br /><br />At any rate that motor at 3.023 is really close to being ready for a bore job. Especially if youre going to keep it.<br /><br />Good luck!