Randy or any one who knows,what size is the allen wrench that fits the head socket bolts on the ST/51. I can;t believe I don't have one to fit. It might be metric. I measured the opening across the flats and it is around.100 ths.
Ed Ruane

Hi Eric  I have the bolts and some metric wrenches but not the one for this motor. I need to know what size wrench not bolt. I am going to fly the new plane tomarrow if it doesn't rain. I got the new muffler and the 13.4 props. I also got some gaskets and wanted to lower the compression. A 45 oz plane with a .51 in it makes for a powerfull combination.

Ed,

If all else fails, you can do what I've been doing for a few years...

Find an ACE, Lowes, Home D or other major hardware store that sells US and Metric keys. Bring along the engine, and try what they have until you find one that fits right.

The local Ace guys are used to me by now; you might have to break in some newbies...

Now, if THAT fails, there are specialty fastener shops around. If none local, S&H is guaranteed to be more than the items ordered... If they can even match from the bolt specs.

BTW, the key that came in the box with mine (China-made) measured about 2.9mm across the flats. A 3/32" key is too small; that's 2.4mm in metric.

Someplace to start, anyway.

I've run the Tigre 51 on a Tutor II and also on a combat style foamy wing profile designed by Doc Passen.  In both cases the planes were large but very light.  In both cases the 51 was not too much power.   Both planes used the exact same setup, Rev-Up 12-5 (the bulk gov't contract version without any printing on the blades), and a K&B long idle bar plug, with Sig 5% or 10% nitro Champion fuel.  The fine needle and broad torque curve of the Tigre allow for needle adjustment to dial in the power wherever it feels good.  I've never felt the need to de-tune it.