59/270 Pistons

[Stephen B.]

Gentlemen
Does anyone know the typical or possibly the correct distance from the top of piston to deck when piston is at top dead center on a 59 270 hp. motor ?Also what is the compression of this motor ?One more please, were the pistons forged or cast flat top with 4 notches ?
Thank You
Stephen Barrett (21558)

[William C.]

Re: 59/270 Pistons

OK, 1959 270 hp, the same piston was used in all 283 engines with the exception on the high-horse FI engine. That would give you a flatop piston with four valve reliefs, CAST aluminum. There is a minimum deck height published primarily for race engine builders, but it has little relevance for what a typical engine was delivered with. Typical deck I have found with original engines is .030-035 or more. Advertised compression 9.5 to 1, actual, ?? but much less as delivered.

[Stephen B.]

Re: 59/270 Pistons

William
Thank you for the prompt reply. You seem to be very knowledgeable about this . Could you also tell me what the comp. ratio of the fuelie was,and if it was a domed piston.
Stephen Barrett (21558)

[Duke W.]

Re: 59/270 Pistons

Nominal blueprint deck height for all SBs is 9.025", and OE piston compression heights are such that all have nominal .025" deck clearance, but as Bill said, decks are often machined high, and this will show up as greater than .025" deck clearance.

Aftermarket pistons may have different compression heights, which affects deck clearance accordingly.

The 290 HP FI engine's advertised CR is 10.5:1 due to the domed pistons, but typical Flint-built engines are usually about a half point lower than the advertised value due to high decks and other tolerance factors.

So most OE 283/270s don't even need premium gasoline.

The flat top pistons have four valve notches because the pins are offset .060", so a four notch piston can be used on both sides. Two notch offset pistons would require mirror image parts for the respective sides. Offsetting pins slightly toward the major thrust face was common back in the day because it was thought the offset reduced piston slap noise, but I don't think this is common practice, anymore. Forged, domed pistons have no pin offset, so the same two-notch piston can be used on both sides.

Chevrolet recommends a mininmum QUENCH CLEARANCE of .035-.040" to prevent piston-head interference under extreme conditions. Quench clearance is the SUM of deck clearance and head gasket thickness.

So if deck clearance is .025", a .015" shim head gasket will yield .040" quench clearance.

The 9.5:1 pistons are cast. I believe the higher compression FI pistons were also cast.

I'm not positive, but I think the first use of forged pistons were the '62 SHP/FI engines.

The aftermarket offers a lot of piston options for restoring OE engines, and the judges can't see them.

Deck clearance is always measured from the deck to the piston crown - the machined area exclusive of any dome, dish, or valve clearance notches.

There is an article in the Fall 2009 Corvette Restorer that explains all this in great detail.

Duke