A high quality motor, properly operated AND maintained will last a long time. Fox's are not known for being of utmost quality nor ability to last. It has been 30 years since you ran one. What about your other Foxes? Were they ALL working perfectly for you over 700 flights? Were they bone stock?
Yes to 1 and 3. They did not run perfectly all the time, particularly when I started. I started out with them on a profile model, so of course there were many failures and near-disasters from the burp, since there was no cure for it at the time. When I switched to Nobler, I still had some problems that I eventually traced to the tank design. I switched to a Sullivan round tank (one of the few that actually fit in the narrow fuse). That pretty much fixed my run issues and after than I had excellent reliability and repeatability.
When I started running them there was no such thing as an aftermarket Fox part and modified engines of any type in a stunt plane were extremely rare. The only parts that were not the actual original pieces that came the box were then needle and spraybar, since I had destroyed so many of them from burps (and then later I discovered the business with the clocking of the flats with the threads) I had a *huge* number of flights on one of the engines, at one point I had something like 40 fuel jugs lined up against the basement wall. At 4 oz/flight that's 1200+ flights, and that was not all the fuel I had run.
I am no apologist for Fox (or anything else), but the Fox 35 is pretty durable when you can hit the needle reliably. And one thing it rarely does is just suddenly change the way it runs, which was the thing that drove us crazy with the ST46. If a Fox is running right, it will run right until something else changes. If it's running wrong, it will run wrong (and the same way wrong) until something else changes. This led to the corollary theory that said "if it's right, don't touch it!"
I may have misunderstood your comment - I thought you were saying (well, you actually said) that it was rare to have a ball-bearing engine go for 700 flights, which was generally a favorable comment about the Fox reliability.
Brett
p.s. I admit I missed the "AND maintained" part. As far as I can tell there is nothing to maintain on a stunt engine, replace the plug occasionally maybe, but there's no routine maintenance required as far as I know. It *might* help to devarnish some AAC engines from time to time but I haven't and usually I get stuff to work. You just shove fuel in it and flip. That's not to say that you sometimes want to change it for performance reasons, but not just to keep it going.
BTW. unless you run the Fox on the recommended 5% fuel, there is some chance that you will break the crankshaft (with the likelihood rising sharply if you go over 15%), but run per the directions in stunt trim even the crankshaft is durable enough.
I am aware that there have been some durability issues on Foxes used in racing events (that's why Marvin Denny invented the stuffer backplate), but not in stunt.