Where to start?

OK so it's my first time out here but I need some help with this ST .40 I know it's not exactly your basic PA power plant, but it's my only hope for now in a SIG Mustang. The model is nearly complete with a Brodak 4oz. tank, adj. tip weight, but heavy. I'm an old combat guy, (the 60's) and stunt is coming slow for me. The G21 runs a little fast, yea well, I had it on a Flite Streak with a bladder 'nuff said, 90+ mph. Now I need a place to start with venturi size, prop, fuel you know.

Hey, the G21-40 is a decent engine to start with. I'd go with a fairly small venturi to start, say a .225 or so. 4 oz tank is plenty. I'd try an 11x5 or 10x6 wood prop. Maybe a BY&O or Zinger. Fuel is your own decision. But make sure it's got enough oil maybe 25% half castor, half synthetic. Depending on the altitude and heat, 5% to 10% nitro should be plenty. Set it at a nice, easy, fast 4 stroke and off you go. With the ST40, you may need to decompress it a bit with some additional head gaskets, but try it the way it si first.

The ST G.21/40 isn't quite as tame as the 46 is, but Randy's right, it is capable of a fair stunt run. In addition to Randy's suggestions, I'll just add that it needs a tube muffler, not a chip. You will get a MUCH better run with the tube (or chamber) muffler due to the sub-piston induction, where you can see below the piston into the case at top dead center. Some folks hemi out the head, to lower compression and improve the run. By the way, rings are available from Frank Bowman out in New Mexico.

Thanks for the info. I bought this motor on Ebay about 6 months ago returning to the hobby after a 30 year absence. It came with 2 venturis, .310 and .330 . I'm not sure what it was used for before, I know back in the day, they were Rat Racers. I see now that technology has long passed them by. I've already made a tounge muffler for it and a .280 venturi. Looks like I'll be making some more smaller venturis. It has just been re-ringed but some spares wouldn't hurt. The rest of the engine is near mint. How about head shims, are they available somewhere or at least interchangable with something. No gaskets in ST's.

Mike

My opinion is that the 40's are a LOT less tame than their bigger sibling, the 46. But it is much less fuel-thirsty. I've had three G21-40's, of which two couldn't be made to perform in a stunter no matter what I did, within the bounds of bolt-on or bolt-in adjustments.

The first two were the usual flat-top piston type, with the usual PDP porting. They wouldn't break back to a four cycle until the power had been just about totally subtracted with multiple head gaskets, leaving way too "soft" of a two-cycle. I had one with ordinary loop-scavenged style porting (no piston fence that I can recall inside, but there was an external tipoff that it's not the same as the others).

The bypass is straight-sided instead of flared. There is also supposed to be a straight-sided 46 that is "economical" on fuel as well, but rather hard to locate. I would love to find one of those for a stunter of mine that I built with a permanently mounted 4 ounce tank in it!

I also tried lowering the pitch on the props and letting the first pair of 40's rev up, but AFAIK there weren't any carbon fiber props being sold when I experimented with the first two (1975 or perhaps the year after). Even on 12-4's, they still wanted to run as lean after a maneuver as they did before it, so they seemed to gradually be faster and faster, on any fuel (and yes, on a uniflow vented tank).

The 46 I tried instead is/was such a sweet-running one I often think it might be worth the rebuilding effort to cut the nose up for a 5 1/2 ounce tank to be installed.

Kiwi

I have a couple of single ring ST-40s that are the best stunt engines that I have ever had!

I got them from Big Jim Greenaway in the early 80s, as used engines, for $50.00 each.

The venturis are 273s , OS MAX NVA, and they have both been decompressed with three gaskets. They are completely user friendly and have a super stunt break. I think that they are just as friendly as the 46s but not nearly as powerfull.

I have used mine for a long time and they just keep going and going!

PS

Both of my engines have baffled pistons!

I would also concur in that these engines really like tube mufflers. Dont even bother with a chip... the tube is that much better!