Lightening a Piston

Hobby Machinist Article...Submitted by Julius de Roo



Making piston weight reduction easy with the help of a mill and a rotary table (and some other suggestions).







I'm relatively new in the world of metal working and I am learning new things everyday. In this small "how-to" article I want to share a technique for reducing the weight of a Zenoah 260 piston by about 25% or that of a Zenoah 231 piston by about 20 %, in about 10-15 minutes. You need a mill, a rotary table and a 3-jaw chuck (or make your own adaptor plate).


Before starting any work I made an analysis of how much material could be safely removed from a piston. So how do you do this? Yes, first you measure. To make measuring easy I cut a piston in three cross-sections.


Based on this I made a drawing to guide me through the milling process.


Next I needed to work on the mill. As I don't have a 3-jaw chuck that would center the piston on the rotary table I had to make two adaptors which are show below: Adaptor #1 on the left, Adaptor #2 on the right.



The first one is to hold the piston. This is easily done by using a piece of aluminum about 10-15 mm thick and 6 x 6 cm square. Fix this in about the middle of the rotary table and mill a round hole 34,1 mm in diameter for a 260 piston and 32,1 mm in diameter for a 231 piston. With a metal or band saw make a cut (kerf) in the piece of aluminum and drill a hole lengthwise. Tap one side to M5 thread. Place an M5 allen head screw in it so you can thighten the adaptor around the piston or Adaptor # 2. With the cut and the screw in place the piston can be held very firmly.




The second adaptor (Adaptor #2) is to position the first adaptor in the center of the rotary table. This adaptor is made of a piece of aluminum about 30 to 40 mm long and with a diameter of at least 35 mm (depending how well you can place it in the middle of the rotary table). The rotary table has a tapered round hole in the middle and I'm going to use this hole as a guide. Measure the diameter of this tapered hole and start cutting the piece of aluminum round using the rotary table, taking care to only do this about 10mm deep; the other end will be needed to fit inside adaptor #1. When this is done you are already half way. You can put this in the middle hole of the rotary table and mill the other end to a diameter of 34 mm for a 260 piston and 32 mm for a 231 piston. (This can also be done on a lathe if you have one. The bar stock is all you need). Be sure this thicker part is at least 5 mm higher or thicker then adaptor #1, otherwise you will not be able to get it out of the rotary table once it is set up.


With these 2 pieces ready you're ready to start milling your piston.

Start by putting adaptor #2 in the rotating table, press it in firmly (don't hammer, it just needs to be there temporarily).



Next, slide Adaptor #1 over Adaptor #2 and fix it solidly to the rotary table. To make sure it is centered, tighten the M5 set screw in Adaptor #1.

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Next remove adaptor #2 (see why the extra 5 mm comes in handy?) and place your piston (dome facing down) in adaptor #1.

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Now with the use of the cross-section drawing of the piston above you can start milling. Be carefull not to damage the piston pin holder. I use a 6 mm milling bit, but with a smaller bit you can make tighter corners and remove even more material.


The end result will look like the 231 piston on right of the 2 pictures below (on the left is a standard 260 piston).

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Weighing before and after shows a weight decrease of 6,61 gram or 23% for a 260 piston. More material can be removed, but this was done using only the mill, no Dremel with special shaped bits or doming of the piston or boost ports.

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Using these types of adaptors for a mill, one can do other things as well, such as raising the compression by lowering the cylinder (machining material off the base of the cylinder - I mill off 0,5 mm). Image


Good luck with this milling project!


Julius de Roo

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