Originally posted by GlennCauley
Hiya, Palmaris !
Uhhmmm... still don't know what you mean by "lapping in the piston"... can you explain the process please? As well as benefits and drawbacks. 
About "changing the conrod after break-in"... I know you probably didn't mean it this way, but this makes it sound like you must change your conrod after break-in if you don't use a lap-in method. I have run in my engines in many different ways (best way is WOT method to heat-cycle) and have never had to change a conrod after break-in.
Lapping the piston is were you reduce the overall diameter of the outside of the piston. Its like sticking a bar in a lathe and turning it down to a smaller size. The big difference being lapping the piston is a more delicate process and requires extreme caution. Reducing the size of the piston also reduces the length of time the piston needs to bed itself to the sleeve hence faster run-in times.
The benefits are that the engine will not have the same stress inflicted on it so wear is reduced and longetivity of the engine increases. Due to not having the same friction between the P/S the engine will not have to work as hard during runs which in theory should also help increase RPM's.
The drawback of lapping is getting it wrong
. Take off too much material and you end up loosing all compression. Result is as you know an engine that doesnt run.
You are absolutely correct. I didnt mean it as it must appear. There is no need to change a conrod after break-in ever but this is recommended by the manufacturer because the stress from break-in weakens the rod or creates excessive wear on the bushing.