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Re: head bolt question

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01-28-2013 18:31:52

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John Smith8N said: (quoted from post at 22:12:28 01/28/13)
TheOldHokie said: (quoted from post at 02:22:19 01/29/13)
I'd like to address Zane's point. You can bend any cylinder head including a big thick warped cast iron OHV V8 head an awful lot by tightening down the head bolts. If you have any doubt just ask a machinist that has to clamp them to a machine table to resurface them. If the table surface isn't dead flat the head will bend when it is clamped down. Then after its machined and the clamps are released it will spring back and the freshly machined head won't be flat anymore.

So if we can easily bend any head when we tighten teh head bolts what's the big deal with being flat to start with? Simple - every pound of head bolts force that is used to bend the head back down towards flat is a pound of clamping force lost at the head/block junction. In other words using a warped head reduces the gasket clamping force and increases the potential for a head gasket failure. OK you say - why not just use a little higher tightening torque to up the force from the bolts and compensate for the loss diue to warpgae? Maybe but the standard head bolt torque value is already close to the limit the bolts can handle and you are now blindly guessing about how much more you need. That risks coming up short or over stretching and permanently deforming the head bolts making matters even worse. You can wind up with a warped head AND even less clamping force where you actually need it.

That's the engineering issues. In practice a flat head does as Zane says bend a lot easier than a big thick OHV head so it can tolerate a bit more warpage. How much more is the million dollar question. The "official" warpage tolerance on a cast iron OHV head is usually about ,005. I have never seen an official Ford specification for the flathead but .020 is more than I personally would risk. In other words you pays your money and you takes your chances using anything that isn't very nearly factory flat from the git-go. You are re-engineering Fords carefully calculated clamping numbers while flying in the dark - how lucky do you feel today?


I fully agree with Zane. If there are no defects in the surface it's a complete waste of time and money to mill a N head. Absolutely not true for OHV engines, but for a N flat head it's pointless and causes more trouble with clearance than it helps. The amount of force used to flatten that 1-1/2" thick hollow head is so small it's insignificant. It can be engineered to death by smart guys, but I've tested it in the real world.

This head was bowed upward in the center like most of them. About .008" bow and .003" twist. It's placed on a flat table with just one (1) bolt right in the center of the head and an indicator to read the movement.

Tightening the single bolt to 5 ft lbs bends the head down nearly .010" and eliminates all the twist. Tightening to 10 ft lbs bows the dead down nearly .020". That's just one bolt in the center with a slight amount of torque. The N head normally uses 18 bolts all torqued to 65-70 fl lbs each. At 65 ft lbs each bolt is exerting roughly 8,940 pounds of clamping pressure. At 70 ft lbs each bolt is exerting roughly 9,600 pounds of clamping force. These numbers are for dry threads. Since the head bolts are normally lubricated with some sort of sealer, the actual clamping pressure will be up to 25 to 30 percent higher. The 4-5 ft lbs of force needed to flatten this head will be spread over the half dozen or more center area head bolts. which means the "lost" torque from flattening the head isn't more than a couple of INCH pounds per bolt. A lot of people won't agree with this, but that's ok. If it gives you a warm fuzzy feeling, have the head milled flat.


Ok - that is good hard data from a smart guy and I fail to see how anyone could disagree with your analysis. Empirical measurements always trump engineering theory so I defer to your testing ;-) Did you just set that rig up for this discussion or is that something you conducted previously?

This post was edited by TheOldHokie at 18:42:14 01/28/13 2 times.

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