79 Yamaha YZ250 Squish

I suggest making a impression with clay. The squish on some of the older big bore bikes I own is not only wide,,but also quite different inside to out .020"+. Some difference is desired,,my new 18 Husky 125 has .003 difference inside to out. Then call & send it(both head and impression) to Eric Gorr. For the small cost he charges,,,,you'll save a ton and get the best set-up. He may narrow the squish, lesson it, change angle, add a radius...

Don’t know about your bike but I have my 03 rm cut by rb designs and it went great. Fast turn around. I was happy with it. Just and option

Most folk on here don't know how to make this bike haul and RIV187 is correct, send the top end to Eric Gorr and follow his advice. His prices are more than fair and you will be super happy if you follow his instructions exactly.

Having a '79 YZ250. What exactly is done and how would this improve performance? Low....mid.....high rpm?

I also have a 79 YZ 250 and am curious about the performance gains.

bump
performance gains?

The stock 1979 YZ250 head doesn’t actually have a “squish band” like more modern 2 stroke heads. The squish area in the combustion chamber has more of a parabolic curvature that varies from about .010 at the outer edge to .040, at approx. 6mm in from the edge. The combustion chamber is offset as is the spark plug hole. This is probably one of the more difficult heads to re-machine a custom shape because of how it would need to be fixtured in the machine. If any HP would be gained, would most likely come from raising the compression ratio, not necessarily the addition of a squish band as they sometimes create a situation of excess unburnt fuel. But if a proper squish band could be machined, with a reshaped chamber that would match the shape of the piston well enough and maintain the correct compression ratio, would probably help some because the stock combustion chamber shape is more proned to cause detonation and rounding on the forward edge of the piston.

I am planning to re-machine the head pictured below. It’s pretty beat up from a bearing failure. Although I’m still working on a proper fixture design for holding it in a CNC mill.

Hmmm........
My personal experience, the stock configuration generates more than enough horsepower than i can use......DOH!