Maybe you guys can help me out. I own a 1970 with engine code GC006878 BUT has 4 intake studs PLUS a stamp of GB HEADS

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When i got my 76 it had a GC engine code correct for 75-76 but had 4 bolt bus heads
P/N 022-101-372G, also had been line bored to + .02" for main bearings, so had been reworked probably more than once. I have no history on the car

Rebuilt motor. All bets are off. Don't assume anything. Could be any size, any cam, anything could be inside.

I have heard many people conclude that in 76 when the 914 was being phased out for the 924, the factory used any part they could find to finish the last of the 914 production runs. Things like carpet was used under the gas tank in place of the felt pads. It's not impossible that the last of the 76 run (which were all 2 liters) was equipped with 2 liter bus heads because that's all they could get from the suppliers. There is no plausible reason for an aftermarket engine rebuilder to stamp anything on the case to indicate they had changed heads due to a rebuild.

If you look at the stamping on the case, the stamp "font" is all the same and, the GC serial number is lower than normal. So it would appear that the 2 lines of numbers were stamped by the same person, at the same time.

One can conclude that the GC case motor in a 70 was a factory warranty replacement or dealer replacement motor sent from the factory.

The last reported 76 GC motor was 6946 (www.p914.com). The factory did not pull motors in numerical order so one can conclude that the GC case motor in a 70 was a factory warranty replacement or dealer replacement motor sent from the factory or pulled from a very late 76 sometime in it's life.