High Altitiude Head

Howard Eisenhauer · Sep 1, 2022

Seeing a 7.8 compression ratio F4 head in the classifieds got me thinking- What's the exact difference between this & the standard head? Is there a difference in the combustion chamber or did the factory just grind a bit extra off the shiny side?

Enquiring Minds Want to Know...

mickeykelley · Sep 1, 2022

My guess is just sliced a little off. That’s what they did the the wagons L226.

PeteL · Sep 1, 2022

But... wasn't the compression ratio number cast into the heads? If so, they weren't merely taken from the regular production run and milled.

danielbuck · Sep 2, 2022

maybe a certain percentage was run through with modified casting text, and then machined farther than the others.

oldtime · Sep 5, 2022

The head Pete shows you can clearly see the casting number 648830. That is an early casting number.
So apparently that head removed from an early SW or PU. You can also clearly see a CR number cast, not stamped into the head of 7.4 /1 ratio. Note it has an early vented valve cover.

Scotties head shows the same early casting number of 648830 but it also shows a stamped finished part
number of 800395. 800395 is a post 1953 part number.

This head has a stamped CR of 7.8/1 ratio.

Note it has a late non vented valve cover.

On my own Hurricane I have a late casting numbered head with a 7.4/1 CR stamping.
7.4 indicates the head was made for 5’000 to 10’000 ft operations.
That said I shaved the head flat so now it’s much closer to being a 7.8/1 ratio head.