302 built with 9-1 compression, forged pistons and HEI distributor. Engine is in a 1951 GMC 1 ton firetruck and the rear gears have been changed to 4:11 instead of 5:14 with stock Muncie 420. Motor kept having oil leakage issues from the rear main but was redone and placed back into the truck. It still leaked but not as much and ran really strong for 26 miles then…….
Guestimate is excessive timing and too much compression for the weight of the vehicle.
Fixed he distributor to limit total advance to 32 degrees instead of 50 +
Cut down the forged pistons to reduce compression.
If that piston picture is after the "Cut down the forged pistons to reduce compression" I'd bet the original was more than 9:1. Depending on the 302 year, stock CR was 7.5:1 or less. That's a big "lump" on the piston.
Interesting that the gasket was blown out between cyls 1-2, 3-4 and 5-6 and NOT where the extra water passages are between 2-3 and 4-5.
Stock distributor advance is 24-28 degrees at 3200 rpm. 50 degrees is way too much.
'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.
That much modification is going to require a couple of extra precautions- - - - -Grade 8 studs, not bolts, to hold the head on, and a solid copper head gasket. Find someone with a plasma cutter or a water jet, and use the original head gasket as a template to cut out about six layers of .040" or maybe .060" sheet copper. The machine setup time will be the same as making only one. and you'll have a supply of replacements. That's what we did on the blower equipped "426" hemi we ran on the truck pull circuit (which was actually 547 cubic inches). 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!
High compression makes for detonation, which in turn breaks the ring lands or bounces the main caps against the block. Blown head gasket is due to not enough bolt torque holding it down. Timing is critical, but I doubt you ran the truck hard enough to hurt it in 26 miles. If it was fully loaded with water and gear, and you were heading to fire at full throttle, then maybe. Next time, drive it with the dog house off if possible to hear what the engine sounds like. To me it looks like you need better head bolts or a stud kit with a better gasket. I have never had problems using Fel-Pro blue or black gaskets. 9.1 with premium fuel shouldn't be a problem.
