milled head 1985 250r
 

Does anyone know how much you can mill the head and still use 92 octane? i bought the quad with a bad piston, and am having it bored now, thinking about bumping the compression a bit, but i don't know how much i can mill and still run pump gas.

milled head 1985 lt250r
 

I had the head on my 86 shaved .020" and I'm still running 89 octane with no problem(riding every weekend for about 6 months). On 92 or 93octane (93 is available at the pumps in Fl) you could get away with .030" for sure and possibly more as far as the fuel used. The reason I have not yet gone past .020" is that I have not determined how much clearance is left between the piston and the spark plug at TDC.

milled head 1985 lt250r
 

Since you’ve got it apart now, it would be easy to determine the clearance on yours. When you put the jug/piston back on, turn it over to TDC (the point where the piston is the highest it goes) then put a piece of clay or a similar substance right in the middle where the plug goes and a piece at the outside diameter of the piston. You will need to hold the piston at TDC somehow, I have not determined the best way to do that yet. Torque the head down then pull it back off. Measure the thinnest part of both pieces of clay. Whichever is thinnest is the closest the piston gets to the head. I'm not sure what the minimum required clearance for safe operation is, but it may say it in the manual or someone else here may know.

milled head 1985 lt250r
 

Thanks for the info, I was hoping to get the head done while the other work was going on to speed reassembly. I will probably reassemble, check the squish and plug clearance and then have the work done. If anyone else has had this work done, and you want to share what you had done, and the results that would be great. How much compression (PSI) do you typically get out of these stock?

milled head 1985 lt250r
 

Anybody else done this? Got advice?......... got milk?