What is the purpose of mounting a engine upside down,on it's side or stright up. If your plans says to mount it upside down what happen if you mount it stright up.

Most of the time the idea is to have as small a frontal area as possible. Example, look at the profile on a p-51 Mustang. The spinner is at the top of the nose with an airscoop under it. If you mount it right side up
then most of the cylinder head sticks out the top. Upside down the head is enclosed but your muffler sticks
out the side. On it's side is the best compromise. The head may stick out a little but that helps cool it and you
have plenty of room under it for the muffler. In any case as long as the crankshaft centerline is lined
up with the fuse centerline the plane will fly the same. But why spoil the looks of your bird and give it
extra drag if you dont have to. later daveo

The only reason to mount any engine in any position is usually for scale or appearances. Glow engines tend to be a little harder to start if inverted. Four strokes are the exception to that rule. Side mounting will usually keep the profile you are after better than an upright mount. If a kit maker suggests a mount position it is usually because of appearance. The analogy of the P-51 is perfect. If you have a trainer type there is usually not an appearance problem and all model engines run better and start easier in an upright position.

Once the engine is running and properly tuned... it doesn't care if its upside down or not. The only place the difference is noteable is when starting and tuning the engine. (Properly tune it upright and then mount it inverted and the needle adjustments won't change.)

I've heard that four cycle engines shouldn't be mounted inverted - something about oil collecting around the valves. Yet I see it done all the time.

What's your take on that?

Roodester

Four strokes don't care about being inverted. I know what you are saying though. I've got 2 inverted four strokes, and when I check the valves, the pushrod housing and the valve covers are completely full of oil. I can just set the airplane upside down on a stand and let it sit for about 10 minutes before I check the valve clearances, or I can just be expecting all that oil and put some rags down before I begin. It won't hurt it. Actually, it probably helps it some since it gives the valves and the pushrods some extra lubrication. They get a little oil in there right side up anyway.