I want to switch my saito into an inverted mounted position on my plane and whats the easiest way to attack this? Knowing that the thrust line has to be in line with the Crank shaft; how would one go ahead and make sure everything is lined up to accurately drill the new holes in the firewall.

Ive never did before in my modeling career but, to get this cowl and exhaust to work out the way i want it the motor needs to be inverted. Plus once i get my new YS for it I would have to mount it inverted anyway.

Mark your current thrust line onto your firewall. As you rotate the engine from
upright to side-mount, to inverted....the thrust line of the engine stays the same.

The propshaft is always centered on the thrust line.

FBD.

Don't forget to plug the original holes in the firewall and fuel proof the plugs.

I use dowel stock and epoxy for plugs, then cut the plugs flush with the firewall, followed by a coat of epoxy over the plugs.

You wouldn't want raw fuel getting into the fuse through the old mounting holes . . .

Flyboy, Thanks for the helping pics does clear things up greatly. So if seeing the pictures correctly the flat portion of the motor mount should line up with the lines drawn.
So in terms of thrust lines. Does the center line should be the exact center of the firewall?

If the back of that mount is round, just trace its shape onto the firewall with a pencil and keep it withing the drawn circle

You can always shift the engine slightly if necessary to get the propshaft
lined up, and the spinner to fit correctly aligned to the cowl. Changing the
thrust line up to 1/4" won't show up in the flight. In a perfect World this
wouldn't be necessary, everything would fit properly.

Here's an example. I crammed this .91FS into this Cessna 182. In order to get
the exhaust coming out the provided hole underneath the plane....I had to tilt the
engine at this angle. No problem, except the cowling wouldn't fit on....so, I had to
drop the engine down quite a bit. As you can see with the spinner in place, it looks
satisfactory.

But then I checked some real planes. Look at the 182's. See how the prop thrust
line comes back on the plane to about 8" or so under the side window on the fuse ?
Look again at the model....see the thrust line on the model is way too high ?

The moral of the story is....sometimes we have to adjust things to make them work
out.

FBD.

Thanks a billion. I havn't done anything like that before just wanted to make sure i understand everything.