Mill head on 91 CR250????

No direct experience with the '91, but I'd stick with the stock head if you want to continue to use pump gas.

BTW, if you mill the head, the squish band may also need to be altered for optimum performance, which is going to add to the cost.

FWIW, increased compression is usually most noticeable at low rpm.

Milling the head was vague, and not recommended by itself. By raising the compression, your 7 hour top end will be less, and you may need race gas to get it that far. Still game? Vintage Bob

No one can answer the question about how much to mill the head without measuring the current squish clearance, which can vary quite a bit on a stock engine because of machining tolerances. Your squish band clearance ought to be somewhere between 1.0 and 1.4 mm for optimal performance. If you mill the head and set the squish band where you want it, it could increase your octane requirements, but most likely it won't be a radical change. And sometimes the engine will run better on the same gas because the combustion chamber will be more efficient.. And Dave is right, you'll feel the difference in the bottom end.