I used to think that one could not use a bushed crankshaft engine in pusher configurations. But they have done so many times over the years. I remember seeing airplane plans for pusher planes over the years, some from the early control line days and even free flight too. Some simple engines like the Cox .049's, Gilbert engines, and others used to have control line pusher planes that the engines were mounted on. So if they were doing it all this time in the past, it shouldn't be a problem, even today.

I would think the main risk is if the engine has enough fore and aft play that the crankpin on the crankshaft can touch the backplate. But one can use some thin metal shims to stop that from happening. Also the oil being forced out through the front end keeps the back of the drive washer and front of the crankcase well lubed too, so that shouldn't be a issue. More modern bushed crank engines have a thin steel shim or washer in between the drive washer and front of the engine, so users can use the electric starters on the engines better too.

There are a number of plans and many plans were built using them of the Curtis Wrioght Junior pusher plane, free flight, control line and RC. The British versions all show small diesel engines being used on them.

Here is a delta plane under construction, the plane and plans date back to circa 1952 http://www.rcgroups.com/forums/showt...557&page=2
There were not many ball bearing crankshaft engines back then either.