Need to know about how to handlap a very tight CS piston

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Need to know about how to handlap a very tight CS piston

Postby William Hanshaw » Wed Aug 31, 2011 8:56 am

I have a new CS Mills 1.5cc engine that's very tight. It runs well but gets very very hot, and is a little difficult to turn over, even after running about half and hour so far. How can I safely and lightly hand-lap the piston but avoid any damage to it? What's the correct procedure and materials to use? I don't want to hurt the engine, for sure! thanks, Bill Hanshaw
William Hanshaw
 
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Location: Fort Worth, TX

Postby Jim Thomerson » Wed Aug 31, 2011 6:14 pm

I use white DuPont rubbing compound. Some use jeweler's rouge or toothpaste. Take the head and contrapison off. Put on a prop. Put a little of your rubbing compound on the piston sitting just covering the exhaust ports. Wiggle the prop to move the piston up and back a little. As the process goes on,. you will be able to take the piston up higher and higher. When your compound gets grey, wash it out and put some new compound in. Leave about 1/16 in at the top of the stroke unpolished to be smoothed up by running. Wash the compound out well. Oil the engine and put it back together. Run it a few times. If it is not OK, take it apart and look for shiny spots in the liner and on the piston. That is where it is still rubbing. Go through the process again, concentrating on where the shiny spots are.
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Thanks

Postby William Hanshaw » Wed Aug 31, 2011 7:14 pm

Jim, thanks for the information. I'm a bit confused. I think you mean to put the paste on the top of the piston, leaving the bottom 1/16" unpolished to run it in. Is this correct, or do I have it backwards. I want to be sure before I do it, and not ruin the engine.

Is the DuPont you refer to, body polishing compound, or another product only for metal polishing? Or is it OK just to use plain toothpaste? Whichever is gentler is what I should do, I think.

Thanks for your trouble, Jim.
William Hanshaw
 
Posts: 7
Joined: Tue Aug 30, 2011 11:16 am
Location: Fort Worth, TX

Postby Jim Thomerson » Thu Sep 01, 2011 10:13 am

What I meant was not to do any lapping as such below the exhaust ports, and leave about 1/16 at the top of the liner. The compound I use is a paint rubbing compound. I think tooth paste would be a good substance to use. Before you start lapping take a look at the piston and liner for shiny spots where they have been rubbing. If you can find the binding spots, that will inform your lapping efforts.
Jim Thomerson
 
Posts: 103
Joined: Thu Jan 23, 2003 4:27 pm
Location: Austin TX


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