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Re: head bolt question

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John Smith8N

01-28-2013 18:12:28

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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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01-30-2013 08:54:51

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 Re: head bolt question in reply to John Smith8N, 01-28-2013 18:12:28  
Also agree with ZANE that surfacing the flat head cylinder cover is not required, even if there is evidence of warp using a machined straight edge.

However, do believe that surfacing is required if the cover has been subjected to overheating of the engine causing a leak between the cylinder and the coolant jacket.

Believe overheating of this engine is quite unlikely since the maximum rated rpm is 2000 and the compression ratio is only 6.7:1, unless the engine is run when low on coolant or no coolant.

Consider when the cover is manufactured as a sand casting, it cools non uniformly, creating temperature gradients in the cooling process. These thermal gradients result in a residual stress state in the cover. Now, the cover is machined flat and installed on the engine. The engine is run at normal temperatures. The cover is not at a uniform temperature and temperature gradients are developed. There is stress developed in the cover from the thermal gradients. When the engine cools to ambient temperature a portion of the stress developed at normal operating temperature still remains as residual stress in the cover.
When the cover is removed the residual stress developed at normal operating temperatures causes the cover to warp. If the warped cover is reinstalled from warp to flat condition additional stress are added to the residual stress so the cover is back to the original stress state before it was removed The the cover is now bolted down flat on the flat deck and seals at the fire ring and coolant jacket openings with no problems. The cover was sealing all right before it was removed.

Another condition
The engine is overheated and causes a leak between the cylinder and coolant jacket opening from warping between the bolt holes.
The leak can easily be detected by removing the radiator cap when the engine is first started before it is hot. Small gas bubles can be seen rising to the top of the coolant surface. The gas source is from the combustion chamber.
This type of warp between the holes is permanent.
Removing the cover and re installing it will not seal the passage between the cylinder and the coolant opening. A re surfaced cover or new cover is then required.

In addition, when the gas is present in the coolant jacket ,the gas bubbles decrease the film coefficient between the coolant and the radiator surface reducing the heat transfer and resulting in an even hotter running engine creating more permanent warp.

If the cover is removed for general engine overhaul like rings and valves and the engine was not overheated, believe the cover can be reinstalled, even if it shows warp with a straight edge off the engine. The cover will again bolt down flat against the block deck and seal. Believe this to be 99% of the cases for removing the cover.

Again, if the engine was overheated and gas bubles showing up with the radiator cap removed
the cover must be surfaced or replaced with a new one. Believe the probability of overheating on this engine to be remote, as evidenced by a 3 psig radiator cap.

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01-30-2013 10:57:35

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 Re: head bolt question in reply to Bulldozer, 01-30-2013 08:54:51  
'Believe overheating is remote " Clearly you never mowed/bush hogged tall weeds & had chaff/etc. clog radiator.........they do overheat!

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01-31-2013 08:33:47

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 Re: head bolt question in reply to JMOR, 01-30-2013 10:57:35  
Thank you for the clarification.

Understand overheating is likely with heavy PTO load and clogged radiator.

Understand the fan is a push type rather pull type ,so as to minimize pulling depris into radiator.

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01-28-2013 18:52:16

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 Re: head bolt question in reply to John Smith8N, 01-28-2013 18:12:28  
Thanks John ,

I don't recall saying I was going to have the head milled, I thought I said I was going to use the head with less gap. The only other flathead I had was back in the mid 60's with 59 Dodge pick-up truck flathead 6 cylinder , I put in many head gaskets in that never did get it planned. Last gasket I blew was I used too much either too start the truck in cold weather, wouldn't start.

9N 222933
2N with 8N motor 8N345567
8N 146710
8N 179555
8N 197904
8N 199000
8N 254079
8N 362039

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