Posted by John Smith8N on January 28, 2013 at 18:12:28 from (75.23.125.17):
In Reply to: head bolt question posted by Wes(MI) on January 28, 2013 at 14:01:36:
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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.
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