The truth of the matter is that modern model engines are incredibly tollerant of bad break-in procedures and virtually all of them will run just fine, regardless of the technique used.

Generally I just follow the manufacturer's instructions (which do vary significantly from one brand/model to another).

Saito says to run their engines *very* rich for the first 10-20 minutes-- in fact most of the time they run so rich that the glow-connector has to be kept connected or they die and you can easily hold your hand on the (only slightly warm) cylinder head during this process. I've used the recommended break-in on both my Saitos and they run *brilliantly*, now having *excellent* compression, stacks of power and a wonderfully low idle.

On my ABC/N engines I simply run a tank of fuel through them on the ground with the needle set just a few clicks lean of 4-stroking and vary the throttle from full to idle at intervals of about 30 seconds -- then I go fly with the same needle settings. I have a TT46Pro here that was broken in that way and after more than 210 hours (and a set of bearings 10 hours ago) it's still running perfectly, hauling a 4.5lb profile 3D plane around with authority.

Even the engines that have been somewhat "abused" during breakin (ie: an accidental lean run on the first flight) seem to suffer no permanent ill effects.

Maybe it's because I choose reasonable quality engines (Saito & Thunder Tiger) for most of my planes that I have no problems. The cheaper engines (GMS, TigerShark) that I've tried have been dysmal regardless of the careful breakin I've given them. For example, I had *one* slightly lean run with a GMS32 and it lost its compression. My TT42GP has (over the period of many years) had some even leaner runs -- so lean that the engine has actually sagged and stopped -- yet it's still 100% as good as new and hauling a 5.5lb trainer around for the club.

In short, unless you're *really* stupid, I don't think it matters too much at all exactly how you break in a modern engine of reasonable quality -- just follow the manufacturer's instructions and then go flying without stressing out about it.