The full 'SP' on 1972 cylinder head studs

Phil

Thank you for responding. BSC was my understanding of the thread type not a typo. 26tpi is more usually BSC at 3/8". 20tpi can be either BSC or BSF. The trouble with these standards is there are so many to choose from!

I'm not sure how easy it is with basic measuring tools and a pair of Specsavers-enhanced Mk1 eyeballs to tell the difference between BSC and BSF threads - the thread cut angle differing by just 5 degrees.

I needed to know as I'd like to ensure that I'm using the correct threaded stud and would like to check maybe using a die and then use a plug tap to ensure that the threads in the cylinder head reach in far enough for the stud used.

Hope this makes sense. Thanks again.

Andy

I've done a bit more digging on this on Access Norton.

The thread looks to be definitely 20tpi BSF on the part of the stud which fits into the cylinder head, i.e. as per Phil Hannam's response. The other threaded part is 26tpi BSC (aka CEI).

This list helps as it lists the thread type and tpi for the 067870 nut for the rear stud (No.2 in tightening sequence) confirming 26tpi CEI (aka BSC). I know this is not definitive but it appears to be a reliable source.

So if that's sorted the thread types what about the threaded and unthreaded dimensions?

Andy

I thought I'd try adding a photo to show the variability in cylinder head stud size. 067885 in order of decreasing overall length.

Studs A, B and D have all been purchased recently from reputable suppliers.C was removed by me from my 750 Commando's cylinder head (not using mole grips!). The differences are significant and can result in a range of 9-12 20tpi threads used in the cylinder head and a difference of 3/16" in thread available to the 26tpi cylinder head nuts.

Andy

A bit disappointed with the lack of responses about the cylinder head stud dimensions.

I guess it's a bit 'rivet counter' but the consequences of a badly specified and manufactured cylinder head stud are at least a leaky cylinder head joint (for compressed petrol/air and oil from the push rod tunnels).

On my Commando the stud at tightening position No. 7 had completely pulled the thread from the cylinder head. This is stud type 'C' in the photo above. Also there is the possibility of the cylinder head stud nuts (063192 at the front) 'bottoming out', i.e. running out of threads to tighten the joint, if the lower section of the stud is not threaded enough.

The existing studs are a bit 'chewed' so I ordered a new set. This started my 'line of enquiry' because I was concerned that the first set of studs I ordered might be made of stainless steel. So I ordered a 2nd set from a different supplier. These had different dimensions to the first set. A 3rd set were different again and all were different compared with the set of studs removed from the cylinder head!

I'm happy to continue with measuring and checking for the best fit. I've just got the repaired cylinder head back from BLR Engineering so will keep you posted.

Andy