You probably got one of the in between US41s sold by Tower a few years ago...The taper in the flywheels was cast, not machined, and the hubs were drilled and tapped off center..Some of the hubs unscrewed the first time someone put a starter on the prop...Some of the cylinder castings were bad enough that the pulse hole under the carb block leaked....
The US engines have roller bearings on the crank and are not very tight when new...
I had good results straightening out the flywheels and making hubs that had no runout...
Anyone who remembers the old RC Online forum might remember the open letter I put on there about these engines
The instructions that came with the engines said all were run prior to shipping..Impossible, the leaky pulse and loose hubs would have shown up there...I think Tower was in denial, I don't know how many they got back to repair, if any.....
The problems disappeared after a while, maybe someone saw the light...[8D]