Chris:

Not all "Plain bearing" engines have bronze bushings in them, the older K&B Sportsters, almost all Cox engines, for two examples, run the cranks in the base metal of the crank case. By saying "Plain" bearing they are all covered. Remember please, whether it has a cast in bush, runs in the base metal, or has some form of "Anti-friction" bearing be it ball or roller type, all rotating shafts have bearings of one form or another.

Synthetic oils do not have high enough film strength to lubricate a plain bearing crank properly. The engines will run with 100% synthetic, but they will run a lot longer with castor in the fuel. I use the Omega fuels in most of my engines, it seems to have a sufficiency of castor for my applications. I've not noticed any excessive wear in the four stroke cams, that's another high load place where the castor's film strength is needed.

Running a plain bearing crank as a pusher you might want to go to a higher percentage because not too much oil comes out the front, and it will have to for lubrication of what will then be your thrust bearing.

If the engine you've selected has enough crank float to allow the crank pin to hit the back plate you will definitely need to shim the crank to prevent it.

In shimming the crank you can use one or more washers, but try to use as few as possible, and keep them at least 0.020" thick. The thinner washers tend to crumple under load. Also, use hardened steel washers, with ground faces. If absolutely necessary, you can't find anything suitable, you can use a plain flat washer and lap the faces, then temper it.

As you adjust the end play remember the crank case is going to expand more than the shaft with heat, if the play is too little the shaft will bind when the engine warms up.

Possibly you'll have to use a thicker gasket on the back plate, or cut the face of it to keep the pin from hitting.

On the other end you can lap the front of the case if needed to get a washer in that's thick enough to last.

Bill.