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Hi, This must be 'build your own piston' day. Another thread just a few above this one is asking about hopping up an M. Odd. Yes, my dad did this on the M I have. It wasn't done for any performance reason, but because the thing was a total wreck. Number 1 had a cracked top, and dad was not about to spend any money on the wreck until after he heard it run again. So, all wrecked engine parts had to be either repaired or built from scratch. It helped a lot that he was a machinist as well as a racer, and had his own machine shop business. Making hi-performance parts was his stock in trade. This was pretty straightforward: Stick piston in lathe. Cut a nice concentric circle out to the diameter of the wirst pin bosses. This one goes all the way through the top. Then, cut a second circle of a larger diameter than the first. This one goes to something like 1/4 inch in from the outer diameter of the piston and something like 1/8 (or was it 1/4?) inch deep. Now you have a stepped hole instead of a piston top. Next, take a piece of 1/2 inch thick (as least I recall it was 1/2 inch) cold-rolled steel and machine it to match the stepped hole in the piston. Now you have a stepped plug that matches your stepped hole. This wants to be a light to moderate press fit. Note that the cast iron of the piston and the cold rolled steel of the plug have very similar expansion rates. This is important. Press it in the hole. Next, drill and tap a bunch of holes, like for a 6-32 flat-head machine screw, all along the flange in the step plug, through the plug an into the cast iron of the piston. Lots of them. As I recall, there are sixteen in each of the 4" M pistons. He used stainless steel flat head machine screws. After tightening them down, he took a large center punch and distorted the shape of the heads (and hole, of course) so it was quite oval. They aren't about to back out of there on their own. This makes sure that the press fit plug stays in place. Balance was never a problem, at least on the small 4" M pistons. These engines turn too slowly for that to have been a problem. He did this to my M in 1987, and it still runs just great. The thought was to eventually purchase new pistons for it, but since it's never failed, why bother? Unfortunately, dad passed on in 1995, so I can't ask him about any of the finer details of the work. One thing is for sure, it'll never run on distillate again! It likes 93 octane and really likes 100 octane racing fuel. It doesn't 'pop' or 'putt' any longer, either. It has a 'crack' sound to it. It runs a 6' finishing mower better than any 420 or 430 I ever had, too. That's about all I use it for, but it shows that the piston repair job is solid, as I'm always running it wide open as a result. Oh, sometimes I have to pause for a few minutes and let it cool off when mowing in the summer. The M has no water pump, and sometimes it get a bit warm. Not that this is a problem, though. It's a good excuse to head inside and get a cool drink myself! :-) Stan
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