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Exhaust manifold hole

Here's a strange one. Motor is in the car. 383-4V. I need to remove the exhaust manifolds and send them out for coating. The 3 bolts that should have extended nuts but have hex nuts. How someone did that and why I don't know. I need to turn a 14mm 12 point socket down to fit in the hole, except I can't get a good reading even with calipers. Can anyone measure that hole and let me know. I think it's .640. Many thanks.

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So same OP some 13 months later, after 5 pages of suggestions back then...

I don’t get it. Didn’t you have this same problem in Dec 2023?

 
Here's a strange one. Motor is in the car. 383-4V. I need to remove the exhaust manifolds and send them out for coating. The 3 bolts that should have extended nuts but have hex nuts. How someone did that and why I don't know. I need to turn a 14mm 12 point socket down to fit in the hole, except I can't get a good reading even with calipers. Can anyone measure that hole and let me know. I think it's .640. Many thanks.

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Can't remember how I got mine out. Sorry
 
I kind of doubt it’s a 9/16” bolt head if the hole is .64” +/-. Looks more like a 1/2” head to me - possibly a header bolt as suggested. A 1/2” hex head is .577” +/- across opposing hex points.


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I would soak it in penetrating oil and grind down a 1/2” socket to fit and see if it will engage the bolt. A Craftsman 1/4” drive 1/2” head socket is .067” dia. A 3/8” drive 1/2” head socket is .073” dia although I have an old Thorsen 3/8” deep socket that measures .070”. I suspect it’s a 1/2 hex-head - they had to get it down there somehow and tighten it.
 
Not saying you are incorrect Wayne but it would be a heck of a coincidence to have all 3 of them broken off down at the stud. I think it’s just rust and crud and a fuzzy picture concealing the flats of the head. Pretty sure it’s not a 9/16” hex head though. I could be wrong on the rest though.
 
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Not saying you are incorrect Wayne but it would be a heck of a coincidence to have all 3 of them broken off down at the stud. I think it’s just rust and crud and a fuzzy picture concealing the flats of the head. Pretty sure it’s not a 9/16” hex head though. I could be wrong a out the rest too.
They could be some reduced head size MS nuts, but if that's the case a socket from a 1/4" socket set should get in there.
 
That is kind of a mystery because most of the reduced head size, 3/8” bolts have a 3/8” or 7/16” hex head. A 1/2” hex head 3/8” bolt is pretty rare. Looks bigger than a 7/16 head in the hole but could be??
 
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