I have 2, 302 blocks and no pistons. I have found a new set of pistons that are .040 over size. One of my blocks measures 3.96 the other is 4.052 and the numbers seem strange. What do you think? Jay D.
A true 302 engine would have a 4-inch bore. A 3.875 inch bore SBC would be either a 283 or a 307.
Kevin 1951 Chevy 3100 work truck Follow this saga in Project Journal Photos 1929 Ford pickup restored from the ground up. | 1929 Ford Special Coupe (First car) Busting rust since the mid-60's If you're smart enough to take it apart, you darn well better be smart enough to put it back together.
302 Chevy V8, or a GMC six? Standard bore for either one is 4.00".
"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt! There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway Love your enemies and drive 'em nuts!
From the factory, GMC 302 engines had either a 4.000" or (if needed because of machining problems) a 4.005" bore. Check the Engine Serial Number Suffix chart to see which your engine left the factory with. Even though the factory Maintenance Manual says the letter is placed AFTER the serial number (a suffix), I've only seen the letter as a prefix, never a suffix.
At the factory the actual bore of each cylinder was measured and then had a properly fitted piston installed. See the "Cylinder Bore Selected Sizes" chart.
Last edited by Bill Hanlon; 05/16/20253:24 AM.
'57 GMC 102, Original 347 V8, HydraMatic, 3.08 rear gear, added A/C, disk front brakes, HEI, AFB carb, '98 Honda Black Currant paint. T-boned and totaled 10/12 '52 GMC 152 Stake Bed, Original 228, SM420, added A/C, HEI, disk front brakes, '67 Chev 3.55 rear gear. Gets used as a real truck.
Some mid-1950s Buick V8s also had the .005" oversize cylinders- - - -oil burners from day one! The only good warranty fix was a rebore to .010 oversize and new pistons and rings! Dealers even tried file-fitting .010 over rings to the .005 bores- - - -made the problem worse! Jerry
"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt! There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway Love your enemies and drive 'em nuts!
The 3.96 engine cannot be a 302. It's 40 thousandths UNDERSIZE. Maybe a bored-out 270?
"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt! There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway Love your enemies and drive 'em nuts!
The 3.96 engine cannot be a 302. It's 40 thousandths UNDERSIZE. Maybe a bored-out 270?
i just did another measurement with my new digital calipers not the most expensive but not cheapy's ether. come up with 3.987 the block is stamped 302. maybe these calipers are not accurate out that far ??? anyway this block should accept a .040 over bore.. Jay D.
'57 GMC 102, Original 347 V8, HydraMatic, 3.08 rear gear, added A/C, disk front brakes, HEI, AFB carb, '98 Honda Black Currant paint. T-boned and totaled 10/12 '52 GMC 152 Stake Bed, Original 228, SM420, added A/C, HEI, disk front brakes, '67 Chev 3.55 rear gear. Gets used as a real truck.
Standard bore for a 270 is 3.781". 302 is 4.00" Both the 270 and the 302 have a 4" stroke. A 270 MIGHT accept a standard 302 piston (a 0.219" overbore) (resulting in a VERY thin cylinder wall, but putting oversize pistons for a 302 into a 270 block would be VERY risky. A .080" overbore in a 270 would end up with 281 cubic inches. That would be a 3.861" bore size.
"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt! There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway Love your enemies and drive 'em nuts!
Bill, both blocks are stamped 302 and NO LETTERS. They have numbers 2324004 on the other side just above the pan rail. i was thinking military blocks but they have provisions for a mechanical fuel pump. I got this stuff from a guy that had dump truck and other GMC powered vehicles. i bought probably4 complete 270s and 2 bare blocks, 2 302 bare bocks, and probably 8 heads, 4 big port, 1 was cracked. also a pickup load of misl. parts. Jay D.
The 3.96 engine cannot be a 302. It's 40 thousandths UNDERSIZE. Maybe a bored-out 270?
i just did another measurement with my new digital calipers not the most expensive but not cheapy's ether. come up with 3.987 the block is stamped 302. maybe these calipers are not accurate out that far ??? anyway this block should accept a .040 over bore.. Jay D.
Maybe you’re getting an incorrect measurement due to carbon ridge at top of cylinder.
John
Last edited by Gdads51; 05/18/202512:39 PM. Reason: fix double quote
~ J Lucas 2006 GMC Envoy 4.2 I6 2023 Ram 2500/Cumins 6.7 I6 2010 Jeep Grand Cherokee (Wife’s) 2002 Silverado 2500 Gas 6.0 1941 Chevy 1/2-Ton 1942 Chevy 1.5-Ton SWB In the Gallery My Flicker Photos!
Digital calipers, even brand new accurate ones do not measure inside diameter accurately.
Agree - hard to find the real diameter, but probably close enough to tell whether a cylinder is standard or 10 thousandths over. I got a set of telescoping gauges to do that kind of measurements.
Last edited by klhansen; 05/19/20253:13 AM. Reason: spelling
Kevin 1951 Chevy 3100 work truck Follow this saga in Project Journal Photos 1929 Ford pickup restored from the ground up. | 1929 Ford Special Coupe (First car) Busting rust since the mid-60's If you're smart enough to take it apart, you darn well better be smart enough to put it back together.
