On the block side of the cylinder heads, under two of the intake ports: there is on some heads a "F" or an "R" or other capital letters cast in. Some times there is no letter. Does any one have and idea what these letters stand for. Maybe a foundry? Just a curiosity type question. Thanks Jim

Most times those letters mean what level the tooling is at. As each tool/die gets repaired, modified, it gets a revision letter.

The 906 lore is that the “F” heads are the best.

My findings have been that not all F heads are anything special........ however the best untouched 906 I’ve tested was indeed an F head.

What’s odd is it’s not like there are A, B, C, D heads.........I seem to recall C, F, M, R, and no letter for 906’s.

On the closed chamber 915 heads an "L" means large 1.74 exhaust valve and stamped with a large HP on the alternator boss . "K" means 1.60 exhaust valve with no stamping. Over the counter heads had no letters under the runners but did have a small HP 440 stamp under the runner.. No differences in the actual castings.

Thanks everyone for the reply's. I've heard the old "F" casting lore. The last set ("M" casting 906's) I used on the car finally broke. They cracked under the center two exhaust ports. I have a good collection of head castings on the shelf. I'm going to use a set of "F" casting 906's. This is the nicest set I have, both heads cast with in 4 days of each other in Sept 1967. They both have no noticeable core shift and originally factory valve job with no valve recession. I have seen all the different markings on these heads, I was hoping someone who was around back then at the factory had some insight. Thanks again......Jim

I've gone this route on both SB and BB. In stock eliminator, there's always something that makes a head special. Chased that idea for many years. Bottom line, go thru many heads and pick the ones that flow,make the HP, best. Both of my motors have mismatched heads. Made a difference between 8 to 27HP. Stacking up the little gains to make bigger HP at the end.