Rocker box bolts

[trevinoz]

I think that this subject has been discussed before.
A friend was confused as to what thread form should be on these bolts.
He brought his alloy head around because his Wassell bolts wouldn’t start in the inserts. We ran a 5/16 x26 tap into the inserts and cleaned the threads.
The bolt still no go. This bolt is from a new set from a parts supplier.
On having a good look at the bolt it turned out to be 5/16 BSF.
These bolts however, fit his iron head.
I spoke to my mate at Goldstar Repairs and he said that he has seen heads with the BSF thread.
I don’t believe that the BSF threaded heads came out of the factory as such, I can’t find any listing for the bolts in my parts books.
I checked two iron and an alloy head and they are all cycle thread.
My guess is that some owners in the past have retapped the heads.
Any thoughts?

[BagONails]

My alloy head has bronze threaded inserts which I believe are original, they were certainly strained enough from PO's over tightening. These are tapped BSCY 5/16" x26Tpi. I machined these flush with the rocker box mating surface again to eradicate oil leaks and re tapped the threads to clean up. So far so very good.

I re-used my old screws so didn't experience your friend's problem. 

[Swarfcut]

 I have a wide fin iron head which has had a weld repair to one of the tapped holes. Considering that this fixing is not under any gut busting torque, and this sort of mishap being not as rare, my reckoning is that both BSCy and BSF threads can be found on head castings, and forcing the wrong bolt down the hole splits away the side of the casting.

 My thoughts are not to take things for granted and as mentioned, identify the thread before trying a new bolt.

 Swarfy.