oversize piston to bore

[Tacomaboy]

Does the piston to bore clearence increase with an oversized piston to account for a larger mass of thermal expansion? Just checking on my KA assem.

[nismo dr]

I'm not going to lie to you

[ggzilla]

I'll take a stab at it. No, it doesn't change. An oversize piston is a just a tad bigger. Use the same clearance specs. It also won't make a noticeable amount more horsepower unless it's a major overbore (normal overbore is .5 or 1 mm).

[Tacomaboy]

Was just a little curious, sence were working with a bit larger mass of piston. Went to .020/50 over. Plan on adding a turbo inabout a years time so I'm gapping ring ends a scunt on the snug sidefor a turbo build. .017 top .020 bottom

[ggzilla]

My Datsun factory service manual (for A15) lists the same piston-to-bore clearance for stock, 0.5, and 1.0 oversize pistons.

 

If you are rebuilding the engine, don't you have a FSM for the KA?

[datzenmike]

Does the piston to bore clearence increase with an oversized piston to account for a larger mass of thermal expansion? Just checking on my KA assem.

 

Stock hypereutectic pistons don't expand much anyway. This is why they are used for quiet cold start up.

[datsunaholic]

If you're worried about heat you want bigger ring gaps, not smaller. On the race engines I've worked on, which are supercharged, we double the factory ring gap- like between 040-055. We clearance stuff as if it had 50,000 miles on it. Found that generating so much heat tended to snap rings and/or score up bores. Pistons were not the problem.

[zed]

None of my workshop manuals, nor Honsowetz, gives a larger piston/bore clearance for any oversize. Haynes KA24E piston/bore clearance is .0008-.0016" or 0.020-0.040mm. Honsowetz says that a motor set up with piston/bore clearances on the tight side of the scale will only develop full horsepower after a few thousand miles running. I always ask the machinist to bore for the tightest spec clearance

[Tacomaboy]

Really not worried about heat ,I just want it right the first time. Plastic-gage on cranck and rod's was just shy of spec which is spot on piston to bore fit is spot on. I'm sizing ring end gaps now. DatsunMike linked me to the FSM fo this enging,so all is good there.I maintain some pretty large naturag gas compressors at work. 12'' piston,depending on size and metal composition it will determin an over bore. Thanks to all in this matter.