Stock Piston/Bore Clearance & Valve Guides

[dougB]

I installed a new factory shortblock 25,000 miles ago - it just ate a couple of lobes on my 206-206. It had always used a quart of oil every 500 or so miles - the rate increased (I'm using a catch can) before the engines demise. I've pulled it apart and found a couple of addl issues: 1) Some of my fancy Winona threaded exhaust brass(?) valve guide inserts (25K miles old) have opened to .006 (smoked on startups). 2) The piston bore diameters are slightly tapered and measure 3.800 to .3.802. The pistons, measured by my machinist and me, measure .004 to .006 under. The factory piston spec is .0008 to .0025 (or is it .002) clearance. The pistons were torque plate honed to ensure the oil consumption was not coming from egg shapes in the walls where the bolts pulled out the wall - there was no problem. Does anyone know if the factory piston/wall clearance actually measures .0008 to .0025? Or are most of our engines with factory pistons, running larger clearances? I pulled two other engines, with factory pistons, apart - one with about 75,000 miles and one with 300,000 miles. They measured anywhere from .004 to .007 under 3.800. I want to put the engine together and not screw with it for a couple of hundred thousand miles, (like my 300,000mile engine). I want it done right, but I tend to push boost and alcohol here and there.

 

[Nick Micale]

Only have experienced one factory replacement shortblock. It ran for about 1000 miles and we removed it. It was worn worse than 100K mile original engines, and the piston fit was very bad. We had to bore it 0.010" over for new pistons.