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Hi Folks - busted off one of these in the fork leg today. It's a 5/16-26 and it's on a 1968 BSA fork leg, so I'd prefer to salvage the leg.

Would there be a problem, other for a subsequent owner, if I have to repair the threads, with using a 5/16-24 insert and an SAE fine bolt?

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its one of those things Jake .......if you are going to have a mixture of threads thats fine (pun intended) while you own the bike and remember but if the get mixed up well not so good

you know 3 x 26 tpi 1x 24 tpi ........visually there is sod all difference unless you really look ....OR .maybee one drop of red paint on bolt head and all around hole on underside of the fixed part of the slider ,,, won't be seen when assembled.

OR my other suggestion would be do all of them in teh 24 tpi ( its a better thread anyway) to avoid confusion

but do yourself a favor and get medium (en 8 .1040) or high tensile bolts .......im sure BSA made some bolts out of plastersine


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jj, It sounds like we're starting the discussion at step two; were you not able to get the remainder of the stud out of the fork leg? Did you drill it out already?

If you have to repair the thread in the fork leg, going with a different thread density will cross-thread and weaken the thread. Since it's only one of four studs doing the job, that could be acceptable, but tapping to the same thread density would be better.

You can get a 5/16" X 26 tpi tap from British Tools and Fasteners. Not a bad thing to have on hand anyway. I have taps and dies for 1/4" X 26 and 5/16" X 26 and have used them on multiple occasions.

BTW, British Tools and Fasteners can also supply a correct replacement stud and nut.


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Hi Jake, Some might disagree, but I have always run a simple SAE 5/16 fine bottom tap in those leg cap holes. Most times they have been abused in the past by the PO. Then I use grade 8 SAE bolts, cut down to leave the correct smooth portion. Tap work might not be as easy with fork still on the bike, but not really too tough. I reason that the caps just keep the axel up in the saddle, and even do the same with the handle bar caps. -26 to -24 TPI is only 2 threads in a whole inch, so 1 thread in +/- 1/2” right? Yes, it’s a BODGE, and I may take some flack here, but it’s been working for a long time.......
That said, if you go that way with all 4 bolts, I have a set I’ll send you for the postage. Photo shows them along side an original -26TPI.

CPBTS1.JPG CPBTS2.JPG
Last edited by KC in S.B.; 11/26/20 6:07 pm.

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I am doing this exact thing today on a Triumph fork. My bolts all came out (lost one) but are stripped.
They don't look cross threaded but like they've been smoothed down.
[Linked Image from i.imgur.com]
Bought the 5/16 XC 26 tap from British fasteners and found the bolts cheaper on eBay.

If they don't then feel secure I suppose a helicoil or other insert would be in order,

Last edited by Nick H; 11/26/20 6:36 pm.

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Another less invasive option is to use 8x1mm bolts. 25.4 TPI vs 26, 0.315" vs 0.3125". The slightly larger diameter will give more engagement to the fork leg.

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That larger Diameter makes good sense!


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Always cheers me up to have someone else admit to the sort of thing I maybe have or haven't done many times in the past.

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Originally Posted by DMadigan
Another less invasive option is to use 8x1mm bolts. 25.4 TPI vs 26, 0.315" vs 0.3125". The slightly larger diameter will give more engagement to the fork leg.

So you wouldn't tap for the metric bolt, just drive it in?


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Originally Posted by DMadigan
Another less invasive option is to use 8x1mm bolts. 25.4 TPI vs 26, 0.315" vs 0.3125". The slightly larger diameter will give more engagement to the fork leg.


I don’t follow Dave, 5/16” is equal to 8.3mm, which is bigger than the 8mm (M8) that you’re suggesting.


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Allan go to the bottom of the class for maths 5/16 x 25.4 =7.9375


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Originally Posted by Andy Higham
Allan go to the bottom of the class for maths 5/16 x 25.4 =7.9375
Whoops. My bad. It’s been a long week.

Now I have to figure out where I got the 8.3mm from and what it relates too... probably nothing bsa grin


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This may be a good place to put in a plug for replacing those bolts with STUDS and lock nuts, as Triumph did after 1967.

With the studs, no matter how many times you remove the axle caps you won't wear out the threads in the fork sliders, and if you wear out the threads of the studs (after XXX-times of removing the caps) you just replace the studs with new ones and start over again.

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"Whoops. My bad. It’s been a long week.

Now I have to figure out where I got the 8.3mm from and what it relates too... probably nothing bsa."


Could it be that one U.S pint of beer is egual to .83 UK pint and you had the decimal wrong. Maybe you drank 8.3 pints (U.K.) before posting?

Last edited by BAinLA; 11/27/20 2:21 am.

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Thanks for all the help everyone!

Just to clarify - I haven't removed the broken bolt yet. The left handed drill bits are coming on sunday. The thread is probably a little bodged. If the thread is bodged enough where I would need to shoot a helicoil in, my question was should I shoot a 5/16-26 CEI, or a 5/16-24 SAE up in there. I'm replacing all the other bolts - I got a new CEI set coming from an eBay vendor.

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If the bolt that is stuck up there is of the correct TPI then a clearance with a tap is likely going to be good enough. I wouldn’t chase the thread out with an insert unless it was required.


Originally Posted by BAinLA
"Whoops. My bad. It’s been a long week.

Now I have to figure out where I got the 8.3mm from and what it relates too... probably nothing bsa."


Could it be that one U.S pint of beer is egual to .83 UK pint and you had the decimal wrong. Maybe you drank 8.3 pints (U.K.) before posting?

Not last night, just finished working a few days away and was dog tired, infact I fell asleep very soon after sending that last post.

Just googled, 8.3 is 21/64” (next drill size up from 5/16).

Last edited by Allan G; 11/27/20 8:09 am.

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Originally Posted by Nick H
I am doing this exact thing today on a Triumph fork. My bolts all came out (lost one) but are stripped.
They don't look cross threaded but like they've been smoothed down.
[Linked Image from i.imgur.com]
Bought the 5/16 XC 26 tap from British fasteners and found the bolts cheaper on eBay.

If they don't then feel secure I suppose a helicoil or other insert would be in order,

Nick, those bolts you have there look to have too long of an unthreaded portion to them. So much so that the shoulder looks like it’s been bruising on the start of the threads in the fork leg, which in turn probably stuffed them up and destroyed the threads of the bolt as you removed them??? Perhaps.


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Originally Posted by Allan G
Nick, those bolts you have there look to have too long of an unthreaded portion to them. So much so that the shoulder looks like it’s been bruising on the start of the threads in the fork leg, which in turn probably stuffed them up and destroyed the threads of the bolt as you removed them??? Perhaps.

Could be. Surely not unusual to find incorrect hardware on a 50 year old machine. Threads may have been destroyed both going in and out. I'll compare them to new bolts when they arrive.


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Thanks - I have a complete set of CEI taps and dies, so fingers crossed on this one. I think I am going to replace them with studs as someone else has suggested - it makes a lot more sense.

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Never said anything about "just drive it in". That is a bodge too far.

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Originally Posted by DMadigan
Never said anything about "just drive it in". That is a bodge too far.

Ok, that's why I asked. They are quite close. The metric only .00246" larger and threads very close.
It would be better to tap it first for the M8. Don't suppose you'ld even want to drill it.

Last edited by Nick H; 11/27/20 7:10 pm.

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