The old one will run and operate like it's big brother, the ST.60, but does not make as much power. There are a fair number of them out there, so parts cows can be had to keep one operating. Both R/C and C/L versions were the same except for car and venturi. The newer G version came in an R/C version and a C/L version, with the C/L version having a different cylinder liner with a bit milder timing. I have operated the C/L version a lot since it came out about 1994, to the extent that I'll guess I have several thousand flights with them in both stock and "reworked" editions. And by "reworked" I mean that they were just optimized for fit and details, by both Tom Lay and Randy Smith. It is a direct replacement for the OLD ST.60, of which I have many and fly them also, but it does not fit the same motor mounts.  (After typing that, I do have to say that I have not tried to put one in place of another, but I believe I have measured the cases to know they are different.)  The engine is not in production any more but parts are still out there as are plenty of core engines for basic parts. They are fairly light for their displacement and pretty reliable. If you search out on the forum you will find threads where guys have put the old ones in big planes like the Top Flite Score and other .60 size ARFs with not many complaints and you just need to watch the weight of any airplane you put it in. I have flown my newer G-51's in models as heavy as 72 ounces. With both being ring engines, you need to feed the accordingly with fuel that has castor oil in it, but a 50-50 mix of castor/synthetic is probably best in my opinion with total oil in the 20 to 22% range and 10% nitro the most I have ever run. I would give the newer G-51 the edge on power, as it will pull like a .60 but I could never get mine to swing a 13" prop. The prop prop of choice for me on the newer one is the APC-12.25-3.75. Old school props like a 12-5 or 12-6 will suit the old model. I think you would like either engine when you decide to use them.
  Type at you later,
  Dan McEntee

  A guy I know that is knowledgeable told me the round head , old style is powerful. I have one just never tested it,   I do know the newer G51   from the 1990s is very powerful.
                                 John

Thank you for that great analysis! I was thinking about sticking one in my yet to be built Magnum. Sounds like the G model will be the ticket!

Matt

     The 72 ounce airplane I was referring to was a Magnum. I got too enthusiastic with some vinyl graphics and too much clear and added 10 ounces I didn't need to!. I won my first Expert contest with that airplane, and got my first real 500 point score with it also. The SIG Magnum and the newer ST.51 is a great combo especially if you can keep the weight down to as close to 60 ounces as you can.
    Type at you later,
  Dan McEntee

I suspect Matt's G51 is a Chinese made one because the Italian one's have a milled surface on that bulge under the G51. Also the Italian engines have Made in Italy by Super Tigre whereas the Chinese just have Super Tigre under the exhaust. I have an Italian G51 and it's been just superb with the only mod being fitting a Bolly FG Super Muffler. With zero nitro fuel and a Zinger 12x5 prop running a full 4 stroke it had more than enough power to haul around an 810 sq inch/59 ounce model with ease. 

Back in the late '90's ST's became unavailable for a couple of years but that was because the Italian labour market was making it uneconomical for ST to continue making them in Italy so they moved all their machinery to a factory in China but also sent their own engineers as well to supervise setting up the factory and training the Chinese workers. 

I never had much luck with Super Tigre of any size.  Then Tommie Lay sent me a new .51 Tigre in exchange for some transistor spark coil units.  I wasn'r particularly looking for another Tigre, but there it was.  It took another couple years before I got around to running it.  Impressive, to say the least.  So I built a Brodak Legacy for it.  This turned out to be the best of all my engines of that time.  I used a 6 oz. round plastic tank plumbed for uni-flow.  Perfect engine runs.