V12 Thunder

Referred to as the GMC Twin Six. Very low production numbers between 1960 and 1965. The Twin Six was a 702 cubic inch V12 that looked like two V6 engines conjoined. The use of four heads from V6 engines made it look like two engines but it was actually one block. It had one long crankshaft and one camshaft.

Fifty years later it exemplifies the term "rare".