Piston Mandrel

Hobby Machinist Article ...

Learn how to make a simple mandrel...the perfect DIY tool for machining and releiving the ring area of pistons on a lathe.

If you own a Zenoah G260PUM engine, and chances are you do, you should know that if this engine is to be modified, the first thing that should be done is to "relieve" the piston in the ring area to prevent seizure. The fit of the piston in the cylinder is ok if the engine is run stock and broken in slowly, but for modified applications the fit is a bit too tight and seizing of the piston in the cylinder will most likely happen unless this modification is done to the piston.

Since I like to do a lot of my own engine modifications when I have the spare time, I decided to make a simple mandrel that would make it easier to machine the ring area of the piston on the lathe. To relieve the piston, you want to remove about .002" from the diameter of the piston, from the top to approximately .250" down. These are numbers that have worked for me. This is easy to do on just about any size lathe, but having a suitable way of holding the piston in the lathe is important.

Here is how I made my mandrel. It is very simple in design but serves its purpose well.

Here is a picture of the bar stock material I used to make the mandrel.
I cut it to approximate length as my lathe is small and this size rod will not pass through the center of the chuck.


I started by machining one end of the piece to the diameter I wanted. This end is the end that will be held in the chuck when I machine pistons. I kept it big enough for sufficient rigidity. The last thing you want is a weak mandrel that will bend or bounce. I also faced the end to get it nice and square. For  added precision, I  should have kept this end as short as possible. I might get to shortening it eventually!


I then flipped the piece around and held it in the chuck by the end I just machined. At this point I machined the outer Image diameter of the piece to get perfectly round. On the end I cut a groove that will accept the piston. The groove is sized to allow the piston to sit snug on the mandrel. If you look inside a piston you will see there is a step at the bottom of the skirts. This is machined and precise for manufacturing purposes. I decided I would use this step as the rest point of my mandrel. The I.D. of the piston at the bottom of the skirts up to this lip is 32.00mm. I made my mandrel 31.96mm and it fits just right inside the pistons. The groove in my mandrel is deep enough so that the piston skirts will not bottom against the mandrel. It centers on the I.D. of the piston and butts against the inside steps in the piston. This seems to work really well for repeatability from one piston to another so far.

The next thing I needed was a draw bar to hold the piston to the mandrel. I got a little bit lazy here and used a long 1/4" thumb bolt I had laying around. You can see it pictured here with the piston and the mandrel.
The draw bar must pass through the center of the mandrel, so I drilled a 1/4" hole all the way through.


For the draw bar to work we need some way of it holding the piston. This was easily accomplished by drilling is to accept a piston rod. That's it. All done! Here are a few more pictures to show how it all goes together and what it looks like in operation.


Happy Machining!

Read Carlo Catalanotto's (CC Racing Engines) explanation why this modification to the piston is required on Zenoah G260PUM engines. LINK TO ARTICLE.

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