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Message: [QUOTE="motopuffs, post: 153330, member: 22281"] How well do we really understand the physics of breaking in a piston? If you look at some of the older manuals, they say to run the engine easy for a few heat cycles, then take it apart and remove the high spots on the o.d. with emery paper. This is a pain in the pooper, but really the best method, particularly if you have tight clearance. Let me ask this: what does break in do? I mean, what is actually going on in there? For sake of argument, let's say that during break in, we are wearing the high spots off of the piston with the cross hatching on the cylinder wall. I might note that there won't be any cross hatching on many cylinders (well used and not honed). According to the accepted plan, the motor isn't revved or loaded like normal, so expansion of the piston will be less than full race conditions. In other words, the high spots won't have a reason to wear down any significant amount, because they aren't forced against the cylinder wall (also lubrication is present). So now your break in is done, you go out and rip on it, the piston expands way more than during break in, and you seize it...follow my logic? Heat cycles: isn't or couldn't this be already done at the factory? Couldn't I do it in the oven before ever assembling? The crown of the piston gets pretty hot, but the o.d. shouldn't see more than a few hundred degrees. Aluminum transferrs heat and expands so quickly that I don't think it's too likely that you can really wear down the high spots during break in, in a controlled manner. There are many reasons for high spots/areas of tighter clearance: 1. relief of stresses after machining 2. cylinder profile doesn't match piston (note that they don't both have to be round) 3. nonuniform transfer of heat/nonuniform expansion of piston 4. overheating exhaust bridge area (this is a bummer) 5. improper maching or casting of piston (not likely, but could happen) A mechanic for which I have much respect will heat-cycle a few times, up and down the trail for a few minutes, and say "it's ready to go". As far as warm up time/cold seizure, this is always a point of debate. This also comes back to piston clearance (I'm ignoring the crank bearings for now). If you have a generous clearance, and you are starting the bike on a hot day, the engine is 90 F, and the most the cylinder might reach (within say 10 minutes) is 160 F, I have doubts that there is enough difference in the cylinder bore size due to thermal expansion to make any difference. My opinion: if you cold seize an engine in the summer, you clearances were too tight already (on the verge of destruction). The bozos revving the bike on the starting line for 15 minutes are just building more heat in the motor which they don't need. Some time for fun I will do the calculations on this. If you really want to get serious, take measurements before assembly to check piston to cylinder clearance. Or how about using some thin shim stock between the two? I admit I don't always do this. When I have, I found a wiseco piston to be a tighter fit (this was just on one bike, but...) approaching the point of being too tight, so between this factor and several others, I decided to stay with OEM. Anyway, as you can tell, this has always bugged me, because many folks have all sorts of opinions on how to break in a bike, but very few can explain why and how. [/QUOTE]

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