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Thread Gauges

Submitted by roger_johnson on Tue, 01/03/2022
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Heavy Twins

Sorry chaps and Chapesses but I still don't understand how to use the thread gauge.

Using my thread gauge on the clutch spring stud the thread gauge shows 26G - I am confident this is correct. Using this on my nice new shiny Zeus this says it is a 1/4 BSF with 26 treads per inch [I still don't know what the G means]. Armed with this I ordered from ACME Stainless 1/4 BSF stainless nuts, yes I did oil the threads, [among other bits] BUT they do not fit the stud.

What am I doing wrong?

This is holding me back as I need to fit a new clutch centre from NOC.

And that stops me putting the alternator back together because I don't remember which way round the crankshaft sprocket fits - I first stripped this down last summer but have had more than a few distractions in that time.

The 1/4 BSF nut winds on to a metric bolt but obviously incorrect due to too much rock.

Thanks Roger

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Submitted by dan_field on Tue, 01/03/2022

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26 TPI is also cycle thread, although I thought they were interchangeable with BSF at that size but the thread angle is slightly different. 

Submitted by peter_shand on Tue, 01/03/2022

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A photo of the stud would help try and id the problem. 26 T.P.I. is correct for 1/4" BSF it is also the T.P.I. used for the range of C.E.I. threads that are found on Brit bikes of the era. You can have a bigger diameter stud or bolt and it will have 26 T.P.I. threads. Cylinder head studs for example, 3/8" BSF one end and 3/8 CEI at the other end. Measure the dia of the stud.

The MK3 Commandos have almost the worlds complete range of thread forms on their fasteners:- Whitworth, BSF, UNF, UNC, Metric, BA, CEI, BSP

 

Submitted by andrew_sharp on Tue, 01/03/2022

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Hello you don't say what year your clutch is, and I understand later cluthches used 1/4" unf thread, which is 28tpi. for me using a thread gauge to tell the difference between 26and 28 tpi needs a magnifying glass. If they are unf the studs the nuts  will start and do a couple ofturns before sticking.

Hope this helps

Andrew

Submitted by robert_tuck on Tue, 01/03/2022

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The nuts are a bit special  so you would be better advised to get the correct parts.  Secondhand if need be. And I am still  of the opinion that they are BSC  ( cycle) thread.  RGM stock them £2.50 each  part no  040356.  Order today and they will be with you in a day or so.  Normal for RGM. I have never used a thread gauge , too slow  ,just keep a selection of nuts and bolts.

Submitted by mikael_ridderstad on Tue, 01/03/2022

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Usually 1/4" BSF (26 tpi 55 degrees) and 1/4" BSC (26 tpi 60 degrees) are interchangeable. But in some instances not. Depending on manufacturing tolerances and different top and bottom configuration. Happened to me on a BSA rear brake adjustment recently. Norton was mainly using cycle threads. Vincent and Velocette preferred  BSF. BSA seems to have been a bit confused using both BSF and BSC. Later on the Commando, as British Standard turned to UN threads, Norton gradually moved to UNC and UNF.

My guess why Norton preferred Cycle threads. It takes time to change gears on a lathe for different threads per inch. So keeping same tpi for different screw diameters makes sense.

I have to defend BSA's use of different threads on their Motorbikes.

The cycle parts used cycle engineers institute (BS Cycle or CEI) and if  truth be known they probably invented it.

And used BSF on the engine / gearbox iron and steel parts and BSW on the aluminium parts.

At least they did on my WD M20.

I honestly believe this is the most logical use of different threads on English bikes.

Mikael, I'm not sure I agree that Veloce preferred BSF; the engine and gearbox mounting bolts are all CEI, including the one that screws into a lug on the frame and which one thus has to manoeuvre through a lug, two gearbox plates and a boss on the crankcase before (hopefully) getting it to engage correctly with the thread into which it screws. 

Submitted by robert_tuck on Wed, 02/03/2022

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Once you get the clutch  fitted  it  will be easier to sort out the engine sprocket as the chain  run will show if you have it wrong.  

Submitted by philip_hannam1 on Wed, 02/03/2022

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The clutch studs fitted to Dominators appear to have used the same Cycle thread at both ends. The early versions had internal threads with bolts  at the pressure plate end whereas the later studs all used external threads and adjustable nuts.  

The early version p/n being D2/401 and the last 650/750 and Hybrids using 04 3441. The Commando had a diaphragm  clutch with a big single spring and no studs.

The chances are that as AMC made the Dominator clutches and sold them to Norton the studs threads did not change threads at all. So UNF or AF threads at not likely......unlike the Mercury and Commando.steering heads.

 

Submitted by roger_johnson on Wed, 02/03/2022

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First thanks for all your replies, I now have another piece of information.

I now understand why I have this problem, the clutch has 2 1/4 BSF studs and one cycle thread!

I have just ordered the nuts, springs cups adjusters and studs from the club's store so even if I damage the old parts to remove them I can rebuild with all similar items.

Again thanks for all your help. Roger

Submitted by mikael_ridderstad on Thu, 03/03/2022

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Never had a M20 but my more modern B40WD has a mix of CEI, BSW, BSF and BA. And of course BSP. Plus some Metric, added by a previous owner. Seems the same on my B44VSs. Though I think the Cycle threads I made in the top yoke on a CBR600RR for a smartphone holder is a bit unusual.

Julian, I believe you're correct. In my defence I wrote it on hearsay as I've been lucky enough just riding the Venom without much spanner work.

 



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