In many postings on internet message boards, Larry Brumfield has shared a lot of his knowledge about engine operation, combustion, intake and exhaust theory, and cylinder head design, construction, and manufacture.
Larry recently posted this further explanation about the topic of flatness versus warpage of the gasket surface, as well as the history of an early flatness problem he experienced which led to a few claims that the cylinder heads he produced had a warpage problem. Many thanks to Larry for agreeing to let it be presented on Ford Garage as well!
His cylinder heads have been produced and sold for 22 years with some customers reporting as much as 60,000 miles of trouble free service.
Here is the flatness requirement explanation in Larry's own words:
I had one machine operator causing all the trouble. What made it difficult to discover, however, was that he didn't cause the problem every time. It would vary.
At the time we only quality checked the flatness at 25% of a batch. If they checked out o.k. then we assumed the rest were o.k. or within specs. High compression heads on Model A's should be flat overall to 1 thousandths of an inch (0.001 inch) or less in my opinion. I like to see the blocks equally as flat. On the other hand, stock heads (along with the blocks) can be off a bit more with no problems.
If more of our head castings had been checked, however, the insufficiently flat ones would have never gone out. Since they were not, the customers installed them as they came and when the gaskets blew out the customers and every other esteemed know-it-all, i.e., engine rebuilders and some customers, came up with all kinds of ideas like "green iron", weird metal that's not stable, the gasket won't work, etc., etc., etc.
It was quite a problem for awhile especially with competitor minions and other assorted backstabbers blowing their mouths!
I hired a metallurgical laboratory (at 350 dollars per hour back then) to determine how much movement would be possible with a B-F Head casting and what steps they would recommend to control or prevent it. To my surprise they came back and said that the amount of movement in THAT particular casting from it being "green" (recently cast) was so low and insignificant that it would be difficult to measure!!
Also, they told me that if I wanted to I could stop heat treating them (which is something I refused to do. All B-F Heads then and to this day are heat treated before being machined and are also run through our heavy shot peen machine to relieve stress. Moreover, an inspection tag is included on every head showing the specs of flatness and the date it was inspected). And by the way, anybody who runs a machined B-F Head through an oven will have no warranty.
Now back to the guy who was the source of the problem:
He was squeezing the head castings TOO TIGHT on the machine. Or in other words, he clamped it in position with so much force that it caused the casting to flex. The tool milled the surface off flat within specs like it is supposed to BUT when the part was released and the pressure was off, the part flexed back and was no longer flat (enough).
Some flexed as much as 12 thousandths of an inch (0.012 inch) or so. Bolt on a Model A high comp head that is off 12 thousandths of an inch and see how long the gasket lasts!
The remedy?
A torque wrench!
After I determined a lesser amount of clamp that would hold the casting without moving or flexing, I measured it with a torque wrench and from that point forward I made sure that every man running the machine used it. OR, they could pay for the re-surface jobs out there from customer complaints out of their own pockets!
I will admit, however, that much of the blame goes to me because I always ride the men to keep parts tight and I tell them, "Don't let 'em move." This just has to be.
Larry Brumfield
August 2008