3917291 heads

[pjbizjak]

OK, someone tell me what's up with the 'dual' format 3917291 heads. As far as I know the 67 built heads came in two versions based on the valve sizes and they didn't have the temp sensor fitting. The 68 heads also came in the same two versions but had the temp sensor fitting. So why don't  head castings dated late 67(for 68 model year) have the temp sensor fitting?  Was it only after Jan 1 08 castings did the heads have the temp sensor fitting?

[Boston14]

According to Colvin's Chevrolet by the Numbers (1965-69), "This cylinder head was the change-over head in moving the temperature sending unit from the intake manifold to the cylinder head. The production of this head started in March/April 1967. All early #3917291 (1) heads dated from March to April will have the temperature sending unit boss, but it probably will not be drilled and tapped. Sometime in May or June of 1967 the boss was drilled and tapped in preparation for the 1968 model year." Hope that helps.

[JohnZ]

291 heads had the boss for the temp sender, but it wasn't drilled and tapped for that usage until August, 1967 production of 1968 model year engines. The 291 heads on my '67 Corvette were cast on May 23rd, on a June 1st-built engine, and they aren't tapped.