Ok I am new in this area, I'm used to a couple of small wood screws to hold an .049 onto an 1/8 inch plywood firewall..
this echo is I think 21.6cc and has 3 screw machined holes that I would have to get to from inside the plane because ..well you cant get there from here if you know what I mean....and spacers to get the rear nut off the firewall,,,will this hold up ok with just 3 bolts?....Rog

Cut off the back of that crankshaft, and then the engine can mount flat against the firewall, using studs and washers/nuts behind the firewall as you mention. The three will hold it just fine, but use nyloc nuts, or Loctite on them. If you don't want to disassemble the engine, you can cut the crank off with a dremel and a cutoff wheel. Just go a little at a time so you don't heat the crankshaft up enough to damage that rear seal.

The three bolt crankcase was used on 16cc and early 21cc engines.

AV8TOR

Do I not need the counter weight on the back of the crankshaft, or is that for the pull starter?....Rog

Nope, not needed. You can take off any mechanism on the back of the engine. Threads might be left hand...

When I am feeling ambitious, I cut the crank off just inside the rear bearing. Then I make a plug to go where the seal was. Loses a little bit of weight done like that, and there is one less place for the engine to possibly leak, but it is a bit "overboard" and not necessary.

AV8TOR

Rog, on a plane large enough for an engine this size getting to the back of the firewall is rarely a problem.

...or...

You could make a flat plate mount that would bolt to the back of the engine and extend outward from the sides allowing you to bolt the engine to the firewall from the front. Look at a Zenoah G26 and you'll see what I mean.

Brian

My converted to electronic CDI G23 has this arrangement also: a plate bolted to the back of the engine that extends out to allow bolts from the front to go through the firewall into blind nuts. Afternarket conversion parts suppliers have these for most engines, or they are easily made.

Yeah, just a 1/8 or 3/16" piece of aluminum plate with some countersunk holes in it to mount to the engine. Then the plate mounts to the airplane. I rarely do this however, as it is just more weight. It does make it easier to remove/replace the engine however....

AV8TOR

Hey Brian, didnt know you hung out here...going to the pylon races over the weekend?...I got my bud down the street taking the back part of the crankshaft and counter weight off....then we gotta tear it down and mill down the front end for enough room for the prop and nut and washer....funny thing I just remembered I have a Zenoah engine downstairs on a GoPed....Rog