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GMC V12 Design Features

The stock 702 ci V12 was delivered from GMC as a robust powerplant.

GMC's own sales literature claimed that its use in a semi-truck would provide continuous trouble-free use for 200,000 miles. In 1960 that was quite a claim for any gas engine, much less one that runs 90% of the time at full power.

It would be anyone's guess how long these engines can last in a lightly loaded hot-rod.

The 702 ci engines that ran in stationary power applications far exceeded that level of expected durability. These engines were run on propane or natural gas and were routinely run (with rebuilds) over 100,000 hours and one user has documented closer to 200,000 hours. To convert that into miles would mean somewhere between 5 and 10 million miles. These engines are robust.

Some features that define its ruggedness are:

  • The cam lobes spin in an oil bath.

  • 56 head bolts.

  • 180# forged steel short-throw crankshaft.

  • Oversize bore spacing to allow water around the cylinder.

  • Three thermostats to aid cooling flow.

  • Seven main journals.

  • Four bolt mains at center and output journals.

  • Heads designed for ample cooling around the exhaust.

  • Top piston ring away from the top of piston for ring cooling.

  • Top ring in steel expansion control band.

  • 1.24" wrist pin.

  • Stiff and stable rocker shaft design.

  • Hydraulic valve lifters to reduce adjusting requirements.

  • Short pushrod design.

  • Head bolts placed well away from the edge of cylinders to prevent bore deflection when torquing bolts.

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