I've been running ST G-.51 engines since they appeared in 1994, I think it was. I used my first one in the first Jetco Shark.45 that I built. When I built it's replacement about 10 years later to take to VSC, I took the engine out of the first one and put it in the second one. It's gonna be real hard to figure out how many hours that engine has on it, but it's still in the second Shark and I hope to refurbish the airplane and it may be time for a new ring!! I may be one of the highest houred users of this engine on the list, between the Sharks and three other models. I have never had an issue with the rings in any of them. I always ran SIG 10% Champion fuel mixed 50/50 with SIG 10% All Castor fuel in these. Straight SIG !0% Champion or your favorite nitro content fuel with at least 20% total oil should work well for you. I just recently tried a ST.51 that I installed one of Brian Gardner's ABC sets in and it's a great running engine and have been pleased with the results so far. I think he changed the timing just s tad but I sense no loss of power so far. I hope to fly the model a lot more next year after some refurbishment this winter. After breaking n the engine to Brian's instructions, and after a couple of dozen flights before the weather started to get colder, the piston fit is still feels like the first day I ran it, so these may last a REALLY long time. There is a difference between the R/C and C/L versions of the piston and liner, so if the engines that you are looking at are R/C engines, I would opt for replacing the P&L with the ABC set up if you have it. I think you will just be a lot more happy with the way it runs. If the stock one is a C/L version, maybe just let price be your guide, but, what the heck, get 'em both! But if the C/L version is used and all carboned up and been sitting a while, the ring may be stuck in the ring groove so watch for that. If you have to take it apart because a ring is worn out, might as well do the ABC swap, again if you have one. I think the best set up to start with is the fuel I mentioned, hot plug like a Glow Devil R/C Long or Thunderbolt R/C Long, at least a 5 ounce tank, and APC 12.25 by 3.75 prop, with take off RPM in the 9500 range. Line lengths and such subject to model size and weight.
Type at you later,
Dan McEntee