rc pilot
I just used a carbide cutter small enough to get inside like 1\4 inch diameter. its aluminum so it cuts easy be careful its real easy to go to far. I dont want to be responsible for you destroying your engine so let me explaine somethings to you to ease my concience.
you need to check the piston skirt at the exhaust port to make sure you have enough to work with BEFORE you do any grinding.
heres how I do it ,remove the back plate and muffler.bring piston up to t.d.c. draw a line on the piston skirt at the bottom of the port ,now bring the piston down so you can see the line you drew on the piston skirt ,this material below the line is what seals the exhaust port from the crank case,when you take material from the exhaust port floor you are moving this line down on the skirt ,please make sure you have enough to spare so you dont ruin your engine by grinding the port so low that the skirt uncovers the crankcase..the book i perchased says you need at min. 2 mm overlap for an efective seal .anyway,just take the least bit you can from the port so you dont have problems.
If you are going to grind on your port a word of caution ,GO SLOW,small steps ,be careful.I found its pretty easy to want to do "a little extra " when grinding because reasembling and checking it out is such a pain in the posterior.but resist the temptation dont forget to leave some material to smooth and polish.
while you have your degree wheel on the engine note how much you need to remove to obtain the durationyou want
how much metal do i have to remove before you pull the jug so you are not guessing when it is time to grind . the way I did it is to figure your duration as it is , lets say its 125 degrees we want 140 degrees that is a difference of 15 degrees I move the piston to the bottom of the port,even with the floor or use a feeler gage to find the piston top , rotate it up 15degrees then measure how much the piston is above the port floor with a feeler gage. I grind about 1\2-3\4 of this depends on how soft it is and how much is to be removed ,the rest I save fore smoothing and polishing operations and finaly dont forget to chamfer the port inside the cylinder lightly so you dont catch a ring on it.Ihave found that the emery cloth will get most of it for you as you smooth the port for polishing but dont forget to check just to be sure so you dont hook a ring on the port edge and seize the engine
So like I said, go slow, remember a little bit under is better than a little bit to much ,when it comes to grinding
GOOD LUCK. let us know how it works out for you.