Steve, Phil and others...
The original OEM Fox 35 backplate was more like a pie tin than a good cylindrical plug for volume in the lower case. Working from different ideas, in different parts of the country, without coordination, about the same time as Marvin Denny to improve that situation, I had my approach. That was to "lower" the rear face of the casting (i.e., towards the prop end) to where the piston pin touched the backplate, then use a 0.010" gasket to provide clearance. I'd heard that the 'spec' fore/aft float for the shaft was about that 0.010"...
It may not have done much of itself, but I made several other 'touches' to Fox 35s of the mid/late 70's era, and since, that somewhat improved 'balance', 4-/2-cycling, economy and usable torque - or so it seemed to me, anyway. At least, working from as-assembled-and-sold Fox 35s - rarely called for replacement pieces - I did 'make-overs' rather than hop-ups or rebuilds. ...and got them through the critical initial break-in phase, for several other fliers. And I worked cheap, but good value for cost.
Marvin Denny developed the first GOOD cylindrical, teflon or other low friction material faced after-market "stuffer" backplate, and I believe he developed the first button hemi-head and clamp set-up for the Fox Stunt 35. Fox Mfg came out with a similar design, "idiot-proofed" with a locating pin to keep the button slot aligned with the piston baffle - which BigIron considers not as good as his design. I'll go with MD on that, but feel that for practical success, with a bunch of factory "blessed" button/clamp sets going to inexperienced hands, Fox Mfg was probably better off to keep the bits aligned...
However, for good success with a "stock" (iron piston/leaded steel sleeve) Fox Stunt 35, the most important thing is a GOOD break-in! The other touches add, of course, but at a level that a poor break-in will never let the engine benefit from. It takes time, and plenty of 28%-29% Castor oil fuel, and associated mess, to 'bring-in' a stock Fox 35.
The ABC piston/sleeve, and the counterweighted shaft, now available after-market (piston/cylinder occasionally from Fox Mfg), change what was the first "modern" stunt 35 - in 1949-50, anyway - into a much nicer engine, which is still late-1940's in design concept. And do so at considerable cost. The thing is, that the 4/2 run the Fox 35 "created" has become a very desirable thing, often demanded from engines designed for an entirely different power profile.
The 4/2 run IS a glorious thing to hear! It isn't necessarily a standard that MUST happen. Engines designed for the RC market, which calls for less fussy break-in, less oil, and more power in a broader RPM band, can serve beautifully for stunt - if we accept that we have to go with what the engines were designed to do. We also gain - usually - from less vibration. More recent engines have relatively massive crank-disks - flywheels - and schneurle porting... and light alloy pistons in expansion-matched brass or aluminum-alloy sleeves... that damp a lot of vibes. The Fox 35 is a lightly-built engine - someone called it the max engine in the minimum frame - so there isn't that much crank-disk mass, except with the Super-Zoot shaft and the lighter reciprocating-mass ABC upper end.
The Fox 35 can still do what it has always done - serve modest size/weight stunt models with CONSISTENT performance for a long time. Not that the RC-based high-production stunt engines are Kleenex items, but you do have to spend a lot more to get one that will last for a few seasons of serious practice and competition. They ARE the solution for models heavier and larger than a Fox 35 can handle well, but for such models a Fox 35 was never a proper choice anyway.
We have more choice available today, so we're not limited to what a Fox can give us. Still, for what it can do, that Fox Stunt 35 is still there. Stock, or goodied-up to do significantly more than Duke intended, it is still available.
May that remain true for a long time!