Replacing Pistons and Piston Rings

| October/November 1988

  • Sketch of the old piston

  • piston drawing

  • Sketch of the old piston
  • piston drawing

One of the biggest problems encountered in the restoration of old engines is replacement of pistons and piston rings. Figs. No. 1 through No. 3 illustrate my solution to this problem and I have had extremely good luck with this method. I have used it on engines that have to go out and do a day's work to justify their existence. The first operation is to draw a sketch of the old piston, recording the distance of the center of each ring groove, from the bottom of the skirt, the depth of the groove which will probably be different from original depending on the rings you use. However this is not critical except to be sure they are deep enough for the ring you use. This is a great plus for this method since you can use whatever width ring may be available in the required diameter. It will almost certainly be narrower than the original. As for diameter, if you can get even close to your over-bore diameter you will probably have no problems. I try to find rings large enough to have to file the ends to fit.

I chuck the piston in the lathe and take a rough cut the full length of the piston. How much I take off is determined by several factors, size of piston, amount of over-bore on cylinder and thickness of skirt. It is very important to round off both top and bottom of ring grooves because brass is reluctant to flow into and over sharp corners. While piston is in the lathe, if it does not already have a center in the head I put one in for centering after build up. I then remove piston from lathe and lay it in a vee-block or piece of channel iron and aim a homemade natural gas burner into the bottom and preheat to a blue temperature. Any source of heat will accomplish the purpose. It is not really essential to preheat but it does speed up the job and saves acetylene and oxygen. I then start flowing the brass on with a welding torch starting at the head of the piston and run on a bead about three fourths of an inch wide to the bottom of the skirt, turn the piston and repeat until I have a sufficiently thick layer of brass all over. Let the piston cool, turn to finished size, cut ring grooves centered according to my sketch and to fit available rings.

If you sawed the piston in two you would have a cross section similar to Figure No. 3. At any rate, you have a piston equal to or better than the original!

Michael G
3/23/2010 7:43:00 PM

It will only work with cast Iron pistons, and it also depends on when they switch to Aluminum pistons. I would look for a modern Aluminum piston of the same bore and piston pin offset and sleeve the pin to fit either the piston or rod depending on which one had larger diameter. Taking into account that the older engine is probably not running high compression and/or high rpm's, you could (depending on how much there was to machine)re-machine the top to fit compression ratio and pin offset. We did a set last week for a student with an older Datsun 4 cylinder to get him a certain compression ratio.


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