I’m not sure if this is practical but here goes. I’ve been away from flying for about eight years. I just started building a Sig trainer for my kid. I found an old OS 40 engine in my old flight box and I need to see if it’s ok.
I would like to do this before I mount it. What is the best way to do this? I don’t even have a radio yet. Can I mound this on something outside and test it in a way that is safe? And how would I control the throttle? Thanks.

You can just set it on a piece of wood and trace the engine and then cut out on the lines so you can fit the engine into hole. After you get the engine situated into your mount, get the right nuts and bolts and make sure it is secure to the wood. Now, get a saw horse and secure the wood to the horse with some. Make sure you get the clamps as tight as possible so the engine will not vibrate off. To control the throttle just put a piece of wire into the throttle arm and you can control it that way, just make sure you know which way will make the engine throttle up and down. This way to test an engine can be made safe, but just be sure to watch the mount so it does not vibrate off of the saw horse.

Make sure you have the right prop for that motor...It is probably a 10X8 or
10 X 6.

Also, the fuel tank must be fairly close, and the fuel level should be just below the air inlet to the carb (1", as I recall...).

It is much safer to stop the motor using the throttle lever, than it is to throw a rag into the prop (as I did many times when I was a kid...LOL).

Good luck!

Like this....

Fly-Boy: Did you doctor that photo to try and take the prop out of it?? That is exactly how my engine test stands are made.

No, that's just how it came out with the digital
camera....don't ask me how ??? The engine was
running at about 2500 RPM.

I screw the mount board down, cause a clamp can
come loose from vibration.

Dave.