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#853472 07/11/21 7:56 am
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Z900 Offline OP
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This nut is mounted on my clutch (B33 - 1957/58):

[Linked Image from i.gyazo.com]

The original part no should be 42-3267 (5/8" - 20tpi).......... it is a hex nut,
but it doesn't fitt.

On the internet I never came accross to see my "square" clutch nut.

Any info ?

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The DPO is a term you need to know and remember (Dumb Previous Owner) or maybe not so Dumb as this one got a 60 year old bike running again with the original part not available. I take it the proper nut is for a larger diameter, so the DPO tuned the worn thread off, cut a new smaller diameter thread that matched a nut he had lying round. So your choice is new gearbox mainshaft so the proper nut will fit or reuse the odd nut on the modified gearbox mainshaft.

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That was for the crankshaft nut.

My question is: has someone ever seen this nut used on a British bike ?

What should have caused to use this special nut ? The shaft looks original.

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Looks rather like the front wheel spindle sleeve adjusting nut from a girder fork bike
# 24-6867 if it was on a WM20


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Yes finally, you are right Trevor:

https://degroot-bsa.nl/nl/product/voor-achterwiel-moer-m-model-nl/

mmmm, im checking it !

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Oh Dear
Used up my "right" quota and we are not 1/2 way through the month yet.
Two types of "restorers"
If it fits then it is the right part
or
Only the right part will fit


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Originally Posted by BSA_WM20
Two types of "restorers"
If it fits then it is the right part
or
Only the right part will fit

Depends if the PO was restoring the bike or needed to get the bike on the road so he/she could get to work the next day and used what they had in their draw of bits.
Once it’s fitted, if it’s doing it’s job satisfactory then it will seldom get swapped out again. If it kept coming loose etc then maybe the PO would have changed it for the right one.

One of a million possibilities and reasons.


Now let’s all have a beer beerchug

68’ A65 Lightning “clubman”
71’ A65 823 Thunderbolt (undergoing restoration)
67’ D10 sportsman (undergoing restoration)
68’ D14 trials (undergoing transformation)

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Guys, this nut (on the foto) was originally the one on my B33, which I bought a couple of month ago.

But I wanted a real HEX nut to secure it mechanically.

For some reason (I got 4 alternatives) I cant get the right one.

When I measure the thread of the shaft its 16mm, I wonder if this is original the right distance?

Looks a bit thick for a 5/8"- 20tpi nut to put on........

Last edited by Z900; 07/14/21 11:43 am.
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Threads are always under the nominal size so a 5/8" nut is never going onto a 16mm threaded shaft even with the same TPI. What TPI is the shaft, if it is 20 TPI then run a 5/8" 20TPI die over it or get some one with a lathe that can be set to 20 TPI to cut the thread to the proper OD. If its not 20 TPI then that old odd nut looks very attractive and you can make the circular section a hex.

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kommando (thanks!): the nut is a 5/8"-20 and fits perfectly on the shaft. Maybe this nut is tapped a bit oversized, due to the (not standard) oversized shaft ?

Also, the replacement nut (42-3267) that doesnt fit, I heard about that the size isnt always correct, there are problems noted at the A7/A10 forum.

I ordered a 5/8"- 20 tap to check how this replacement nut is gonna behave. I'm curious to the outcome !

Last edited by Z900; 07/14/21 2:43 pm.
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Put Dykem (bluing) in the shaft and check where the nut is touching. If the peak of the thread is sharp what feels like a good fit might be only the peak touching or possibly the thread angle is wrong and touching the corner of the V. The peak is commonly either flat or rounded to height/8 on National threads and height/6 on Whitworth.
The triangular height for a 20 TPI thread (0.050" pitch) are:
National - 0.86603 * pitch = 0.043315"
Whitworth - 0.960491 * pitch = 0.048025"

Subtract half that from the basic major diameter to get the pitch diameter.
National - 0.60335"
Whitworth - 0.60099"

The theoretical O.D. is
National 0.625 - 0.043315 / 8 * 2 = 0.61417"
Whiworth 0.625 - 0.048025 / 6 * 2 = 0.60899"

Was the shaft cut oversize or to a metric dimension? 20 TPI is 1.27mm. If the thread is actually metric a 20 TPI nut will fit if the depth is cut far enough and the length of engagement is short enough.

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