stripped threads for fork drain plug

[seattle##gs]

The drain hole in the fork leg has stripped the threads. Is there enough material to tap for 1/4 - 28? Otherwise I will need to buy a helicoil kit to do the job. Any idea what the existing thread is?

 

[Time Warp]

Probably way off but the tapping drill size for 1/4 - 28 is around 7/32" / 5mm which is less than than the stock thread hole so would not work.
The stock thread looks to be 1/4 - 20 but would need verifying.

The fork is thread deep, the screw thread short in comparison.

 

[kommando]

1/4-20 it is, unf would be too fine even if there was enough metal.

 

[marinatlas]

as said by Kommando it is BSW thread , unless your slider is pre commando in that case it is BSF ......I have a mongrel fork with one side 1/4 bsw and the other BSF (same with mudguards mounts !!), you just need to remind when you dismantle your fork not to mix , that make my brain / memory working !!

 

[L.A.B.]

as said by Kommando it is BSW thread

kommando said 1/4-20 which, in this instance, is UNC.

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Motorcycle parts and spares for Andover Norton motorbikes: Andover Norton manufactures and supplies all Factory Spare Parts for Norton motorcycles built before 2002.

andover-norton.co.uk

 

[marinatlas]

Hi LAB , as usual , andover said UNC and CEI and rgm BSW and BSF ........reminds me the featherbed bottom yoke pinch stud thread

 

[Schwany]

I'd pull both tubes off and tap for 5/16-24 UNF and put a soft washer under some stainless button head socket cap screws to use as drain plugs. You'll be doing similar with a helicoil to reuse whatever bolt Norton used. The advantage of a helicoil is they are far more difficult to strip than soft alloys. If UNF is too scary there is always 5/16-18.

 

[seattle##gs]

It sounds like there is plenty of room to helicoil for 1/4 - 20 then.

 

[Schwany]

You'd have to verify based on the thread size required for the 1/4-20 helicoils. I don't have any 1/4-20 helicoils in my toolbox to verify how much meat you'll need around the drain hole on the fork tube. Somebody else round here probably knows though.

 

[texasSlick]

1/4 - 20 Helicoil tap drill is 0.262" dia.
The major diameter of a Helicoil insert is 0.322"

Slick

 

[marshg246]

It sounds like there is plenty of room to helicoil for 1/4 - 20 then.

IMHO there is plenty of diameter for a helicoil but not enough depth and even if you get the helicoil in, you'll probably have trouble getting a seal. If you have a machine shop willing, you could make a flat spot and use a 5/16 x 18 button head or hex head. The metal is very thin there - if I remember right only about 2.5 to 3 threads.

 

[seattle##gs]

Anybody with a spare fork slider, can you measure the depth of the hole? I know the screw is very short. Right now the screw is in place and holding oil but barely. I will leave it in until more info comes in.

 

[850commando]

i replace mine factory screw with 1/4"-unc x 1/2" lg socket head cap screw, push a vernier up the hole till contact, you will be surprised that there is still more thread in there, and you may not need to heli-coil it, i'd be very tempted to use a locktite make-a-thread kit to save the drain.

 

[Time Warp]

Removing the same screw I removed last time to get the thread size and pitch via pitch gauge I see maybe 7 mm.

 

[seattle##gs]

after a couple of small projects I will remove the screw, then I am committed to solving this problem. Thanks for any and all info

 

[MexicoMike]

I would drill/tap it for "a size larger" whatever thread you choose - metric, SAE, British, whatever...doesn't matter as long as it's a good seal. I'd probably just look at whatever appropriate screw/bolt I had around and drill tap for that. Helicoil is better, of course, but even if a HC will fit, it's not really necessary for this application. "Snug" as far as tightening the screw/fibre gasket is all that's required.

 

[Time Warp]

Just remember at the end of the hole is the damper unit, If you were to drill on location and to far you might alter the compression damping ?.

 

[kommando]

Just remember at the end of the hole is the damper unit, If you were to drill on location and to far you might alter the compression damping ?.

The damper unit is a solid at the point of contact if the drain bolt was too long, the cavity starts above, BSA used drain bolts with an extension which engaged a slot in their damper body to stop it spinning on tightening or loosening the bottom damper body retaining bolt. I have added the same slot to Norton damper bodies and used a pin up the drain hole when tightening the bottom bolt.

 

[chaztuna]

Anybody with a spare fork slider, can you measure the depth of the hole? I know the screw is very short. Right now the screw is in place and holding oil but barely. I will leave it in until more info comes in.

As it happens, I have both my Mk III lower sliders in my solvent cleaning tank at the moment. To those who say simply tap it out to 5/16", no can do. The counter-bore for the screw head is only .375". You would have no room for the screw head to seal. No big deal IF you have a Bridgeport mill.
I counted 6 threads on the "short" side of the tapped hole in my Mk III sliders. The "non caliper" side slider is the same part number for all disc brake front ends. They use the "oval" sliders, rather than the round sliders in the earlier drum brake units. I'm currently trying to figure out how to remove one of the drain plugs. Current plan is to drill a pilot hole, heat the slider and remove the plug with an EZ Out. If that fails, drill out the plug and install a HeliCoil. In the past, I've often had to shorten a HeliCoil, due to depth restrictions in a tapped hole. Shortening a HeliCoil is no big deal. Determine the max number of threads and cut the HeliCoil to match.
I'm considering replacing the stock plugs with 1/4" UNC stainless steel SHCS [socket head cap screws, aka Allen heads] Probably won't be able to do that, if I have to install a HeliCoil. I apply NeverSeize on any fastener going into aluminum.