Exhaust manifold Merc BBC stainless bolts from ARP
 

In case anybody is trying to go the route of stainless bolts for your exhaust manifolds, I did a bunch of measuring and research and here is what I found:

ARP part 623-1750, which is 3/8-16 x 1.75" under head length. This part is in a 5-piece packaging, and you need 16 total. It has a 3/8 wrench head. Part 625-1750 is the same bolt with a 7/16 wrench head. Typical torque for BBC exhaust manifolds is 20 ft lbs so either should work fine, but the 3/8 inch head seem to be more common.


In measuring the head and manifolds, I found that the bolt hole in the head is about 13/16 inch deep, plus you have the thickness of the gasket, plus a washer. The Manifold is exactly 1 inch thick at the bolt hole. So a 1.75 length will still leave about 1/8 to 3/16 spare room at the bottom of the bolt hole.

BBC exhaust studs are available from ARP as well, but they are not quite long enough for marine exhaust manifolds- the overall length is is 1.6 inches, but there is unthreaded part at the end, so the amount of stud actually in the head appears to only be about 1/3 inch. If you threaded the stud all the way in the head until it bottomed out (as it should) then the stud will not have enough threads for the nut. For this reason I found them unacceptable, although I would have prefered the studs. The BBC exhaust stud kit runs about $115.

I am getting ready to put these on my engine, so I will post my findings when done. I would hold off ordering anything - let me be the guinea pig :)

Are you planning on using any sort of locking device? Unless you have lots of room on either side of the motor, re-tighen bolts could be a major pita.

Better use some anti-seize on SS bolts in an iron block. Also they are not as strong.

MIO, you are better off using Grade 8 bolts.

learned my lesson years ago with stainless allen heads on an intake manifold. Not as strong. They didn't hold the torque after time. I think they continue to stretch. They do look pretty.

If you buy off-the-shelf 18-8 or 316 stainless bolts, they are not as strong as grade 8 bolts. ARP bolts, however, are made of a special material that's actually rated stronger than grade 8 bolts (170,000 psi vs 150,000 psi).
For what it's worth, I've had no problems using the off-the shelf 316 bolts from McMaster-Carr to hold my Eickert manifolds on.

ARP SS bolts using their Stainless 300 alloy are far stronger than Grade 8.

For those interested, I am ready to order those same bolts for my Eickert exhaust. ARP has agreed to drill them for safety wire use. For an extra cost of course. PM me and I can supply the part number.

Does the Eickert exhaust also have 1" flange thicknes ?

Like I said though, use anti seize. The problem is not strength when you put them in. They only get torqued to about 40#. The problem is when the seize and you can't get them out. Dissimilar metals seize much easier.

The Eickert cast manifold has a stepped exhaust flange. With the washer and gasket, 1" and 1.625"

Griff, You are sooooo right. When using SS on most iron, steel or aluminum materials, anti-seize compound is a must. That along with the mild torque value, heat/cool, vibration conditions is the reason for the safety wire restraint.