I've inherited what appears to be a never-run ST G21 .35.  It has a flat-top iron P&L, .155 ID sprinkler venturi, Black head, BB crank, and weighs 9.0 oz sans muffler.  There is a little oxidation on the crankcase in various spots, but it turns freely, removed backplate and there's no rust inside.Does anyone know anything about this motor?  Runner or Boat anchor?
Thanks,
Les

As Randy says.
Never heard of a blackhead, but never say never with ST!

Never posted a pic before, here goes...

hI LES
That is a G21 35  combat engine, but the head is not as it came, You may want to take it off and look at the inside, maybe someone just anodized it, or it may not be a stock head

Regards
Randy

It looks like the back plate is on correctly.  You can see the cutout to clear the piston skirt is up as it should be.

No - the backplate is not installed correctly.  It needs to be rotated counter clockwise so the round "button" is on the top.  The so called "cutout" goes on the left side of the engine looking forward and was intended to open up the bypass in the case to ease the flow of air and fuel.

Exactly correct   David ! :-)

Randy

Thanks guys, backplate now rotated as indicated.  Pics below on the black head.  The screws on this motor are "Merican", 5-40's with a 3/32 socket head!  I thought Tigre's of this era were all slotted metric screws?  I'm beginning to believe it's a rebuilt or modified engine.  Earlier I said "new" but upon closer inspection, there is a bright ring around the top of the piston wall indicating some running, although there isn't similar wear at the top of the liner, but instead the cylinder wall shows evidence of "horizontal" light honing.
There may be a slow combat sport plane in my future!
Thanks to all,
Les

That is one of the later ones.  Normally it would bring a tear to my eye to see a G.21 .35 on a sport plane-- kinda like Seattle Slew pulling a plow-- but by the time that engine was made, it had been "improved" to mediocrity.

The metric size of the original screws is close enough to 5-40 that we all replaced them with 5-40s.