modding puch head with squishband

So my Puch has been running great for the last few hundred miles, so I decided to modify the stock 50cc head to work better with the 70cc airsal. I decided to put a large 4 degree chamfer around the outside of the head making it the same bore as the piston. You can see in the first pic why that lip is a big deal:

With the head chamfered and "bored out" to the correct diameter, the fuel air mix will flow much better and would act as a squishband. Before I attempted this, I made a CAD drawing of both the piston and head, so I knew before hand that I would be able to mill the head down over .025" before getting too close of a tolerance. In the end, I milled about .02" to raise the compression from the initial 125psi to 150psi.

With this "squishband" it took out any and all of the ticking/pre-detonation that was heard at mid to high rpm. All I can describe the sound as would be what the kstar kits have as "engine noise" or ticking. It also gave me a small but noticable mid power. It now sounds smooth, like putting in fresh bearings and removing piston slap. When the head was off, all of the ports were smooth, crosshatching was still noticable, and no scratches or any imperfections were seen. Over 1,000 miles on the kit and still going strong. Joe

Re: modding puch head with squishband

Nicely done, makes me wish I had a mill and knew how to use it!

Re: modding puch head with squishband

Very nice, did the same kind of thing when building my muscle car motor. 11.5:1 with iron heads on pump gas, no detonation. Its all about the squish

Re: modding puch head with squishband

I usually like to run about 1mm of squish on these. You can run it tighter but I don't trust the rods and bearings at high rpms on these old engines. Working with that stock high comp head like that is a bit harder I usually just the raised part off and reshape it from there. Be sure to check the compression before you start it back up. Especially if you are just cutting some off to see what you get. It's very easy to cut too much off and have diesel like compression.

Want to post in this forum? We'd love to have you join the discussion, but first:

Login or Create Account