Cylinder head bolt threads - BSF or BSCY?

[Steverat]

I'm cleaning up the threads on the head bolts and in the block, preparatory to doing trial assembly. The thread is 20tpi 3/8" and I started using a BSF tap in that size, it goes into the block without making swarf but is quite tight. So is the thread BSF or BSCY? I know the difference is between 55 and 60 degrees thread angle; is the thread actually BSCY, which would account for the tightness of the tap? Don't want to damage my engine.
Steve

[duTch]

 Should be BSF

[RDfella]

You say 'quite tight'. That would, unless the tap is very blunt, indicate to me that it's the wrong thread. Have you tried bscy? Remembering of course that BSA sometimes used random threads, although I doubt they did in this application.

[Slymo]

Can’t think of any other 3/8” with 20tpi other than BSF. Cycle is 26 UNF is 24. Probably just requires a clean out with some brake clean and an air gun first. There is also the possibility that there is a nominal difference in the size of the tool used to create it.

[morris]

BSF. No doubt.
It’s normal that it feels “tight”. Cast iron feels rather “stiff” when drilled or tapped and don’t produce the same type of swarf like regular steel.

[RDfella]

Obviously the way to solve the connundrum is to count the tpi on a head bolt. Don't know why I didn't think of that before (old age brain fade). I know bscy is usually 26tpi, but some diameters are also 20tpi. Plus a few odd ones thrown in. And then there's BSA's oddball threads - never did find one matching their single-cylinder crankcase threads for the cyl studs. When I was looking for a close match for a new project the closest I could find was 3/8 bsp.

[muskrat]

G'day Steve.
10000% should be 3/8 BSF. PO may have screwed in a wrong bolt and buggered the thread.
Cheers

[RDfella]

Tapping cast iron is normally a breeze - it cuts easily (in a similar fashion to brass). Never had a cylinder thread I couldn't clean with a tap (up to 3/4) that I couldn't wind in with one finger on the tap wrench (after drilling out the crud at the bottom first) so either the tap is blunt, or maybe someone has put a stainless helicoil in there? If the latter, be careful as you can end up in a situation where the whole thing jams up.

[Steverat]

Thank you everyone  for your help, yes it is 20tpi BSF, all 9 holes are now nice and clean and no threads buggered. Interestingly the bolts are still quite a tight fit, but all go right in with light asistance from a short spanner. But there is also a 20tpi 3/8" BSCY CEI thread too, isn't there?
Cheers
Steve

[duTch]

 The thread chart I have here shows BSC 26/20 TPI starts from 7/16" in increments of 1/16" to 3/4", then 7/8" is 20/24 TPI & 1" 24 TPI only

[BSA_54A10]

Head bolt holes get filled up with all sorts of crud so bolts oft bottom out which gets mistaken for tension and thus the thread gets distorted.
Just had to helicoil all of the M20 ones.

CEI is a shakeproof thread so it only gets used on things that are likley to vibate loose & fall off. Carbies, inspection caps & tinwear.

[bsa-bill]

Just had to helicoil all of the M20 ones.

One of my bikes has had this in the past, I had to helicoil two when I restored it and three had already been done, two of the original helicoiled threads are slightly off angle, they go in just but need more torque (bit of a guessing game) so I'm wondering about a fix, can't redo them now there off but wondered if a shop could fill the hole and then drill and tap.
Anyone been through this

[Slymo]

In the past I have tapped out the hole to the next size 5/16 to 3/8 for example and then made a threaded bush in a suitable material to aviod the use of a helicoil. I've never liked them as an option as they often come loose or fail on a high load position. The advantage of this method is that if for some reason the thread fails again you can simply easiout the bush. Thats the theory anyway as Ive never needed to do it and I generally loctite them in.

[bsa-bill]

Thanks Slymo
beyond my skills/tools but we have a good machine shop at Berwick, I'll take the barrels and your suggestion to them when I start the job (be next year now tho )

[RDfella]

Bill - helicoils are wonderful things - when used correctly. I usually loctite them in to mitigate the risk they'll pick up on the bolt. As for getting your helicoil out, best bet is to unwind it (from the top, otherwise it simply tightens) Try a LH drill just under the tapping size for the helicoil thread and hope it bites at the top. If done slowly, it should unwind. Best done in a mill or at least a pillar drill to ensure it's perpindicular. Then, as Slymo says, drill and tap oversize and fit a bush. That depends, of course, if there's enough 'meat' to do that.
Or, whilst it is non-standard, the fact 8mm is bigger than 5/16 and 10mm bigger than 3/8 might get you out of a fix (provided you don't mind going to metric bolts). And there are better quality bolts in metric than you usually get in Bscy - when you can find them (OK, so I actually hate metric with all its different pitches - cant beat BSW / BSF)