Machine work Question

[Va914]

Can someone walk me through the process of checking Pistons, ring gap and Cylinders for wear.
After honing the cylinders, how much clearance in too much to use the old pistons with new rings (total seal at the 2nd position)?


Thanks

[Joe Ricard]

well general rule that I lived by was .004 to .006 side wall clearance. any less and you will bind the piston as aluminum and steel heat at different rates. You can go more to make a "looser" engine but it won't last near as long. I think I rememer someone running like .013 racing sand rail.

[dman]

I am no expert in this area so any real info on this subject would be apreciated, but I just built an engine with some after market P&Cs with clearances of .004-.006. The engine was very noisy and had a skirt draging. In my persut of a solution I have found that the type of piston, forged verses cast makes a big difference. Mahles, which are the only thing I will run in the future run around .002.

[Jake Raby]

The skirt clearance is measured without rings on the pistons, and is the tolerance between the piston skirt and the side of the cylinder, with the piston installed 1 inch into the cylinder.

The ring gap is the measurement of the piston ring installed square into the cylinder. The gap at the ends should be .004 per inch of bore.

94mm cylinder= 3.7 inches
3.7 x .004=14.8 thousandths of gap
some rings run looser or tighter..

Loose settings on pistons do not decrease life, and are really helpful uin race applications. The engine will rattle when it is cold, and burn some oil, and have leakdown when hot, but it will last about the same time, and even longer in hard conditions..

[Va914]

Thanks guys.
So do I meacure the bore after the honing in done and buy rings to match. OR do I just buy the standered ring size and then check the gap?

Jake, you said the piston should be 1" into the cylinder. Just so I have this right, I insert the piston so the bottom of the piston is flush with the bottom of the cylinder...then push it up 1" and measure??

Is that another way to describe it?


Thanks for putting up with the newbie questions.

[JP Noonan]

I hate to talk for Jake but I think he means from the combustion side down one inch.

Usally you buy a set of rings and then file them down to fit. Machinest use a crank operated grinding disc that has a fixture to hold the ring square. You can do the same with a file and patience. You need to file the end of the ring square to the existing end. Also you can't add material so file a little, measure, file some more, measure, and so on.

Problem I've heard with re-ringing a motor is if the ring groves in the piston are too loose the new rings grab the new cylinder wall and bang around in the ring groove. This either can break a piece of the piston off (the part between rings) or break a ring in half.

Penny wise, pound foolish.

ten of one, ,half dozen the other.

[Va914]

Thanks JP.
Do you know fi there is a wear limit clearance for the gap you are talking about?
Can I buy over sizeed rings?

Thanks