1972 T120R gearbox mainshaft sprocket nut thread - Triumph Forum: Triumph Rat Motorcycle Forums
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post #1 of 8 (permalink) Old 09-18-2015, 09:11 PM Thread Starter
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1971 T120R gearbox mainshaft sprocket nut thread

A dilemma!

In the process of re-assembling my 1971 T120 bitsa, I ordered a new gearbox sprocket nut as although the original was serviceable, it was a little butchered and I figured a few dollars for a new one wouldn't be a big deal.

It is (supposed) to be a 71 T120 (4 speed) and definitely is a 4 speed box (well in that it only has 4 gears!).

My parts book says it needs a 57-0440 nut (T440 in the book) which I believe is the standard one used by the 650s (up until the 5 speed boxes in 1972(?))

I went to screw the new nut on and after the first turn or two it started to bind - I didn't want to cross thread anything so I didn't try to force it.

I tried it with the old one which went on easily to the main shaft without any binding.

So it seems the main shaft thread I have is not suited to the 57-0440 I bought. I understand the 5 speed boxes have a different nut.

I am now wondering if the main shaft I have in my bike is from a later box (wouldn't surprise me as so many other bits were from wrong years).

Is that possible though?

The box I have has 4 gears. Were there different main shafts for 4 speed boxes, or could it be a 5 speed shaft with 4 speed bits?

If there are differences, how compatible are the bits?

And what else do I need to consider? (particularly bearings as I have replaced the bearing and oil seal on that side of the box using parts as specified for the 71 4 speed box).

If it all should just work fine I will simply buy a later nut and carry on but I am concerned if it is the wrong shaft I may encounter other issues and should pull it and get the right shaft.

Any thoughts?

Aunty Mabel

Last edited by jonkster; 09-19-2015 at 10:13 PM. Reason: title had wrong year model
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post #2 of 8 (permalink) Old 09-19-2015, 06:05 AM
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The five speed large socket is 1 7/8" if that helps identify 4/5 speed

The four speed large socket is 1 5/8

Have you inspected the internal threads to see if it's just a faulty part rather than the wrong one?

Last edited by DAVE M; 09-19-2015 at 06:08 AM.
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post #3 of 8 (permalink) Old 09-19-2015, 09:32 PM Thread Starter
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cheers Dave

both nuts are 1 5/8" across the flats.

A have a (SAE) thread guage that indicates both are 24 TPI threads. The only difference I can see is the new nut is slightly thicker and seems to include one extra wind of thread as a result.

I cannot pick any obvious things wrong with the internal threads on the new one but then my eyesight isn't as sharp as it used to be (I hate this growing old stuff ).

in both photos, the original one is on the right, the new (slightly thicker and rustier one) on the left.






I am thinking though because it is a 1 5/8" nut already fitted then I do have the right mainshaft parts for a 71 4 speed (my real concern) and it is just that the new nut is a bit dodgily tapped which puts my mind at ease.

Just wondering, is the thread on these UNF or is it something else? I do have a UN thread file I could use to tidy the threads up on the new one.

cheers

Jon

Aunty Mabel
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post #4 of 8 (permalink) Old 09-20-2015, 07:13 AM
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The info I have states: 57-0440 Nut Sprocket 4-speed twin
1 1/4 24 CEI.

Cycle thread.
The trouble is this nut has to be very tight, so both the old nut and shaft may have their theads stretched,
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post #5 of 8 (permalink) Old 09-20-2015, 09:19 AM
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Hi,

Quote:
Originally Posted by Caulky View Post
The info I have states:
Mmmm ... the "stainlessbits" list says that (too?).

But, if you do a wider internet search and look at online lists like Efunda and Maryland Metrics, there isn't any 1-1/4" Cycle, it's 1.29".

If that's correct, and your new nut has been carelessly-made, that 0.04" could be your problem.

I know it doesn't help your immediate problem but it's this sort of problem that some time ago persuaded me I had to invest in a micrometer or callipers. Luckily, here in GB, we have a chainstore that does all sorts of good-quality stuff at low prices, and they happened to be doing digital callipers the week I had enough spare cash.

Hth.

Regards,

Stuart
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post #6 of 8 (permalink) Old 09-20-2015, 10:07 AM
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Not Aldi /lidl but I recently bought a new digital set of callipers that also do depth for 20

These are stainless and do metric, decimal imperial, and fractional imperial
Very worth having
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post #7 of 8 (permalink) Old 09-20-2015, 11:49 AM
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There are some anomalies in the data on the stainless list.
It can`t be 1.29" cycle thread because its a left-handed type.
If it is really 1 1/4" 24tpi then its a Triumph special.
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post #8 of 8 (permalink) Old 10-04-2015, 09:10 PM Thread Starter
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Just to close this off - I spent a little while with a thread file and the nut started to fit fine.

Recently while buying some other parts a picked up another sprocket nut just to be sure it wasn't the threads on the shaft that were the problem and the newer nut screwed up fine.

Lesson? - I guess that sometimes brand new nuts aren't always tapped cleanly.

Aunty Mabel
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