[dman14]

We had our compressor mounted upside down all of last year, (with very minimal usage as the only piston was a small 3/4" bore) but had no problems, if you do mount it upside down make sure to lock it in place or at least check the bolts often, as the vibrations and weight of the compressor tend to loosen them.

Note: Don't take my experience as a guarantee that it will work, because as stated we only used it for a single 2" stoke 3/4" bore that was fired maybe 10 times per match at most. As such, on time was minimal at best (other than on startup).