Author Topic: Cylinder head bolt threads - BSF or BSCY?  (Read 597 times)

Offline Steverat

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Cylinder head bolt threads - BSF or BSCY?
« on: 06.10. 2018 14:36 »
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

1951 BSA A10 - now returned to Germany
1972 Triumph T100R Daytona
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Offline duTch

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Re: Cylinder head bolt threads - BSF or BSCY?
« Reply #1 on: 06.10. 2018 15:01 »

 Should be BSF
Started building in about 1977/8 a on average '52 A10 -built from bits 'n pieces never resto intended -maybe 'personalised'
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Re: Cylinder head bolt threads - BSF or BSCY?
« Reply #2 on: 06.10. 2018 16:25 »
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.
'49 B31, '49 M21, '53 DOT, '58 Flash, '00 Firestorm, Weslake sprint bike.

Offline Slymo

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Re: Cylinder head bolt threads - BSF or BSCY?
« Reply #3 on: 06.10. 2018 17:53 »
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.
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Online morris

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Re: Cylinder head bolt threads - BSF or BSCY?
« Reply #4 on: 06.10. 2018 18:43 »
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.
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Re: Cylinder head bolt threads - BSF or BSCY?
« Reply #5 on: 06.10. 2018 20:02 »
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.
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Re: Cylinder head bolt threads - BSF or BSCY?
« Reply #6 on: 06.10. 2018 20:18 »
G'day Steve.
10000% should be 3/8 BSF. PO may have screwed in a wrong bolt and buggered the thread.
Cheers
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Re: Cylinder head bolt threads - BSF or BSCY?
« Reply #7 on: 06.10. 2018 21:06 »
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.
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Offline Steverat

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Re: Cylinder head bolt threads - BSF or BSCY?
« Reply #8 on: 06.10. 2018 21:23 »
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

1951 BSA A10 - now returned to Germany
1972 Triumph T100R Daytona
1924 B-S SS80
1965 Triumph SH Cub
1960 AJS M18CS

Offline duTch

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Re: Cylinder head bolt threads - BSF or BSCY?
« Reply #9 on: 06.10. 2018 22:47 »

 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
Started building in about 1977/8 a on average '52 A10 -built from bits 'n pieces never resto intended -maybe 'personalised'
Have a '74 850T Moto Guzzi since '92-best thing I ever bought doesn't need a kickstart 'cos it bump starts sooooooooo(mostly) easy
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Offline BSA_54A10

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Re: Cylinder head bolt threads - BSF or BSCY?
« Reply #10 on: 08.10. 2018 06:42 »
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.
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Offline bsa-bill

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Re: Cylinder head bolt threads - BSF or BSCY?
« Reply #11 on: 08.10. 2018 10:11 »
Quote
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
All the best - Bill
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Offline Slymo

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Re: Cylinder head bolt threads - BSF or BSCY?
« Reply #12 on: 08.10. 2018 11:30 »
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.
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Offline bsa-bill

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Re: Cylinder head bolt threads - BSF or BSCY?
« Reply #13 on: 08.10. 2018 11:43 »
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 )
All the best - Bill
1961 Flash - stock, reliable, steady, fantastic for shopping
1959 Rocket Gold Flash - blinged and tarted up  would have seizure if taken to  Tesco

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Re: Cylinder head bolt threads - BSF or BSCY?
« Reply #14 on: 08.10. 2018 11:52 »
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)   
'49 B31, '49 M21, '53 DOT, '58 Flash, '00 Firestorm, Weslake sprint bike.