I know nothing about this engine.
It is NEW, never run.
On the front of the head is inscribed "GMA Custom"
On the left lug is inscribed "GMA 55"
On the right side is inscribed"50" in a small circle.
On the bypass is cast JETT.
Anyone know about this engine?
I would like to know some detail before I donate it to the museum.
(I assume "GMA is George M. Aldrich)

Bob Z.

   GMA-Jett 50 ABC SE. If it is original, be *very careful* trying to turn of over, at Aldrich's insistance, and against Jett's advice, there is way, way, too much taper and it is far too tight at TDC and it is extremely prone to sticking. If it does, heat it up with a heat gun rather than try to force it. The taper is excessive, but apparently it is less than George wanted, Dub talked him out of some of it.  It is set up with incredibly low compression ratio and an huge venturi. In flight it might actually squeak through TDC if it is running rich enough. They were notorious for breaking cranks, not because the crank is weak - it's incredibly tough - but because of the shock loads trying to jam the piston through TDC. It doesn't run nearly hot enough in stunt use to take out the taper.

     In flight, it has the classic Fox-style break characteristics - instant switch to 2-stroking as soon as you touch the controls, and more-or-less no power increase, it's mostly just a sound. It may also quit on release, because it can't suck fuel well enough with the enormous venturi to make it through the takeoff acceleration.   

    If you take out almost all of the very many head gaskets, leave yourself with a .025 clearance, and get a small enough venturi (like a #12 cylindrical) it runs pretty normally for a modern muffler 50, not nearly as powerful as a PA, but way better than a ST46. Until it breaks the crank. Almost everyone got Jett to replace the cylinder/piston assy with a typical AAC with normal fits, then it runs very well and has none of of the other issues.

      We spent a remarkable amount of time trying to get The Commissioner's GMA-Jett 50 RE to work. We tried a compression tester, a PA61 pumped it up to 135 psi, GMA-Jett, same conditions, *35* psi. Larry eventually got the piston/liner replaced and a reasonable venturi, and it was pretty good and pulled some amazingly porky airplanes well enough.

    Brett

THANKS, Brett - that's exactly the information I need.
The owner of the engine bought it new quite a few years ago but he's now sadly deceased.
It will go to the museum shortly.

Bob Z.

   Glad to help.

   Mostly, I wanted to caution anyone who was going to try to run one that it has some issues, and if you want to use it, it would be very wise to call Dub Jett and see if he will redo it to proper fits. I talked online with GMA about the GMA-Jetts, and he made absolutely no bones about the fact that he wanted massive taper, apparently because he once destroyed his brand new Supertigre X-29 (one of the first production ABC engines) because he thought it was too tight at TDC, so he clamped it in his Sunnen honing machine and "fixed it". Of course this complete ruined it, because brass needs that much taper if it is running nearly incandescent hot in Rat. Ever since, apparently, he put massive taper in every ABC liner he ever made, and some of his reworks had very similar problems to this.

     It's GMA himself, he invented modern stunt, you really *do* want to listen to him and give him every benefit of the doubt. I think it is clear he let his early experiences color his later ideas. All I know is when we ran it, I could hear it squeaking over TDC *in flight*, and it had nothing like the power of a PA51, or even a PA40, and was borderline unusable as delivered.

    Apparently, for the GMA-Jett, Dub Jett talked him out of some of it, so it would have been even more tapered if GMA had his way. I know several people who got a 40VF with a GMA liner (since George had the facilities and knowledge to chrome liners, and hone them), and, same thing - the "cheap nickel" plating had been replaced, a bunch of taper added, and then chromed/honed to fit. The "cheap nickel" that several of us had run literally several thousand stunt flights over the period of 20 years, and while a bit down on power, still ran perfectly. These people ended up finding another, bone stock, piston/liner, and using that.

   George Aldrich did a lot of really good things for stunt, and he was and still is one of my stunt heroes and deserves a lot of leeway and respect.  This, unfortunately,  appears to have been a miss.

     Brett