For a 21T spec motor and a stock, what size comms do they have new out of the package? How small can you cut them before they are too small to run? Thanks!

.300 is new

.270 is about the min. I've seen some come apart at .272 I've seen some as low as .265

Is there an Advantage to having a Larger or Smaller Comm? like say a .290 over a .275?

Yes and no...a smaller comm induces more brush wrap which in turn makes more power but will reduce overall torque and reduce efficency.

When I was a hard core stock/fixed timing 19t racer a few years back. I would start my brand new motors at .285, I felt on those generations of motors they ran the best at .285-.275

But on thing before you go hacking comms away is to remember a GOOD motor is good regardless of comm size.

I agree with Danny , unless your a PRO driver which very few really fall into, don't waist your arms life by cutting the comm down in diameter.

The power difference is barely noticeable on the track ,,,, just keep the comm true and with fresh brushes.

Just my opinion ???

R.S.

I have found that a motor runs significantly faster just before it spits the com. Whether or not it will finish a race is another question. Like Danny said... a good motor is a good motor no matter what the com size.