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by Mal Marsden - 06/16/22 7:00 pm
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Allan G, Gordon Gray, NickL
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Original Post (Thread Starter)
by DAMadd
I am aware of the pitch angle difference between 1/4 BSF and BSC

From several vendors I was trying to source a thread repair for the primary case screws for a 68 a 65. The vendors have the various BSC and bsf helicoil kits. But for 1/4 they have the same kit. Where you're looking for 1/4 bsc, but when you click on the product it's a 1/4 BSF repair. Are helicoils of that size an in between angle or different profile that will accept any 26 tpi? Any engineers or machinist out there shed some light? I repaired some threads (I think rocker box) on my 65 tr6 with a 1/4 BSF kit which I still have but years ago before I understood the different threads. I certainly don't want to buy a duplicate to repair the a65 primary if there's virtually no difference. Any help appreciated
Liked Replies
by kommando
Originally Posted by dave j
There is an old thread where Stuart insists that the threads in the case of a 60s unit Triumph are in fact 55 degree 1/4 BSF rather than 60 degree cycle. This is because cycle was only used into steel. BSF was used into aluminium. As the threads get larger the tpi drops with BSF whilst the big steel threads on a Triumph remain 26 tpi. apart from the big rear spindle which is 20TPI. I wonder if it is the same for a unit BSA?

Yes, in parts books for the Unit Single rocker box studs pre UNF/UNC you see the 1/4 stud is threaded BSF one end which screws into the alloy head and Cycle on the end a steel nut is used. You only know which end is which by looking at the length of the thread. The same stud ends up UNC/UNF on later bikes, showing they finally realised 26 TPI in alloy is not a good idea.
2 members like this
by NickL
Most suppliers just sell 1/4 bsf stuff as bsf/bsc, nuts, bolts etc.
1/4 screws or bolts bsf or bsc will fit 1/4 bsf helicoils no problem.

Same with ww/unc in smaller sizes.

Personally if i was helicoiling a primary case i'd use unc or M6 you can
buy them anywhere and they are much cheaper, as are the bolts etc.
1 member likes this
by Villiers
Of course they sell them as the same thing. The difference is pedantic. Lets be brutal. You are repairing an unusable threaded hole with a piece of coiled wire. The result will be quite close to original but will not be a perfect replica of a theoretical thread form. You are then fitting a screw made within commercial limitations or possibly even the worn screw that came out of the repaired hole. Neither will have a perfect thread form either.

In this case all 26tpi threads are interchangeable, be they BSF, BSC or BRASS.

Save your money and use what you have.

1 member likes this
by DAMadd
Someone had replaced a few of the rocker cover studs with unf making for a variety of nuts which I was trying to avoid on the primary. For my own ocd and future owners. It's just one bad position so I'm going to use the bsf ones I have as per advice. Thanks for the replies! On a past thread or maybe a searched topic I had been put on to Tracy tools. Got my bsc tap and die set there.. I thought that might have been one I checked, but no matter. I wondered mostly about the pitch angle of helicoil. Certainly can't discern by eye on 1/4 26 piece of wire
Thanks again
1 member likes this
by Tridentman
Another +1 for Tracy Tools.
I ordered a 1/4" BSP die nut from them last Thursday---it arrived yesterday.
Great selection and great service.
Also pretty fantastic for unusual stuff like LH thread dies and taps.
The founder was an old style machine tool guy from Birmingham UK--not far from where I grew up.
Around there babies are not born with blood in their veins --they are born with suds oil in their veins.
Just my two cents worth of course.
1 member likes this
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