Slant 6 Exhaust/Intake stud

[stroker416]

I thought all head sides on the 13 slant 6 exhaust/intake studs were 5/16 - 18. I've got a head off a 1970 Slant 6, the back top hole in the head is defintely larger than 5/16. Is it possible it came 3/8 or is this just someone elses mess up. I did not comfirm yet, but Im thinking its 3/8-24.

No big deal, just wanting to confirm.

 

[RustyRatRod]

Some are 3/8 and some are 7/16. I don't know what the rhyme or reason was. The threads in the manifold are 3/8 16 or 7/16 14 though. The top is usually fine thread.

 

[Charrlie_S]

There is only one reason for the stud to be larger than 5/16. The rearmost upper stud seems to be the most prone to breakage. So that stud was broken sometime in the past, and someone just did a drill and retap to a larger size. It is really not a good idea to use a larger stud. The exhaust manifold is designed to "float". As it heats up, it expands and gets longer. If an oversize stud is used there might not be enough clearance between the stud and the hole in the manifold, which could possibly snap the stud, or crack the manifold. The best repair is to remove the broken stud, and use some type of thread repair method (helicoil, nutsert, etc), to bring it back to 5/16-18 NC. Just remember the studs do go into water, so make sure to use a sealant.

 

[RustyRatRod]

There is only one reason for the stud to be larger than 5/16. The rearmost upper stud seems to be the most prone to breakage. So that stud was broken sometime in the past, and someone just did a drill and retap to a larger size. It is really not a good idea to use a larger stud. The exhaust manifold is designed to "float". As it heats up, it expands and gets longer. If an oversize stud is used there might not be enough clearance between the stud and the hole in the manifold, which could possibly snap the stud, or crack the manifold. The best repair is to remove the broken stud, and use some type of thread repair method (helicoil, nutsert, etc), to bring it back to 5/16-18 NC. Just remember the studs do go into water, so make sure to use a sealant.

Charlie, maybe I misunderstood. I thought he was talking about the stud that holds the intake and exhaust manifolds together that's closest to the cylinder head.

 

[stroker416]

Ill open that hole up and put a timecert in it then. Thanks for the clarification.