The 8011 also had a lighter crankcase and was availible with the "P" head for precission areobatics. The 4011 had the stronger crankcase and front case and was basically bullet proof even in modified form with pressure venturi on 10% nitro.
The ABC 4050 and 4055 ABC stunt had the 4011 crankcase bored out to fit the ABC liner which was fine for the intended aplication. In modified form for racing the clylinder of the 4055 would burst apart and send the top of the engine flying.
I was working with Mecoa to address this problem. Mecoa is sitting on a run of 4055 crankcases castings that have a thicker wall at the know break point but as of yet they have not done the finish machining on them. I have one in my shop also unfinished as the racing event for that engine, alas, is no longer flown.
As the story goes, the initial run of ABC engines from K&B were properly fit and ready to go when the decicion was made to loosen the piston fit a little more for the RC market. These were softer aluminum chrome plated pistons taken from the Sportster 45 line, chrome removed and machined down to fit the 4055 sleeves.
This was the death knell of the 4050 - 4055 as the piston fit was not great from the start and then wore out in a few flying sessions. The fix in the mod shop was to get a high silicon piston casting from a K&B 6.5 and machine it to fit the 4050 sleeve. I have one of those too but alas, no where to race it.
Motorman