Armen, I do a standard break-in with petroleum oil, usually Rotella T4 or Castrol V-Twin 20W-50.  Depending on the project, I break it in for about 200 to 1000 miles.  This 94mm flat-top got a 500 mile break-in.

Some habits are hard to kick.  I spent a lot of time fiddling with drag bikes.  You don't have the luxury of a lengthy break-in period.  I have been finishing the cylinders with a 180 grit hone with a brief touch up with a 280 grit.  The intent is to leave the surface a bit rougher than generally accepted practice so the rings seat ASAP.  I don't skimp on assembly lube.  So that's the finishing process I used on this particular engine.  It seemed to be working just fine at the 2250-mile mark when I first tore it down to start the Evolution of a Hotrod project.  It didn't start consuming oil until I reassembled it to continue testing.  When I reassembled it, it went back together as-is, no honing, no new rings.  That might have been a problem.

My breakin regimen is fairly simple, while it's on the dinosaur oil diet, I use a constant accelerate/decelerate technique.  Open throttle to accelerate which forces the rings into the cylinder wall, then chop the throttle to decelerate, pull vacuum in the cylinder, and draw oil up the cross-hatch.  That goes on for the entire breakin period.  Anyone that is following me doesn't stay there very long.  The constant change in speed drives them nuts.  It's easy-does-it for about the first hundred miles, and then gradually increasing power until it reaches my desired finish point.  On this engine, that was 500 miles.

I'm not too concerned about the oil consumption.  It was minimal and didn't seem to start until I reassembled the engine without changing the rings or honing the cylinder.  

I can tell you that on my current test mule (97mm bore pop-top), I changed my cylinder finishing process.  That change was not related to any sort of issues I noted on this 94mm flat-top engine.  I wanted to try and get my bore as round and straight as possible, set the running clearance right at .0025", and try a smoother finish.  So, after boring the cylinder (.006" under finished size), I got it within .003" with 80 grit and 180 grit, then took out the last .003" with the 280 grit stones.  That last .003" took a long time.  My back was killin me, but I was very happy with the finished product.  We'll have to see how it works out.  So far, oil consumption is zero.  My breakin was 1000 miles.

I really feel that the blowby I'm seeing on this particular piston is related to debris.  While the linear marks in my cylinder appear superficial, the fact that I can see them tells me they aren't superficial.  The marks are mostly on the forward side of the cylinder wall, and that's where I see the blowby.  A steady diet of gear-tooth fragments takes a toll, but as previously mentioned, it was still runnin great when I tore it down. There was no obvious loss of performance, but the squeeze was down a bit.  

Apparently I am, too.  Yes, you are right...  a 94mm piston, and the cylinder wall was honed for a clean up.  I am in the middle of three engine rebuilds right now, thus the confusion.  Currently starting the break in on a Kawasaki KZ750 as well.  I guess my brain is still trying to keep up...

LS650 hot rod motor
EJ25 Subaru sohc n/a
5.2l Magnum V8 Mopar

Hey Lancer, thanks very much for checking that measurement again.  I feel a little better now.