Ty,
Are there any markings on the case other than the "40"?
There were several versions of the Max H 40.  There was an "S" which was a pretty good stunt motor, a "P" which was an R/C pylon motor and not so good for stunt except with a very low pitch prop.  The unmarked engines were, I believe, the standard R/C engine.  Nice, well made engines but heavier than an FP with less power.

There was also a Rat Race version that had a huge venturi opening.
Steve

If it had an "s" on it I would think it was one of the early engines with a single bearing  - bushing in front. The "p" stands for pylon and is a different timed engine and is not useable for stunt. There was also an r/c which I have seen but do not own. All of the markings are the same except the "p" - the "s" came with a venturi and needle. I have sold or traded several and still have 11 of them - all the double bearing versions. None of mine have the extra letter on the side. I have not seen a newer version with the extra letter. They will give you a breaking run but not quite like the Fox. Try an 11 X 5 on them.