Gearbox Mainshaft Bush on C15

[Barnsey]

I am in the process of replacing the kickstart stub shaft on my C15 but I notice that the bush on the end of the main shaft is worn and the skirt is wearing thin to the point of splitting. This one is open at the end to expose the pin which operates the clutch.   I ordered a replacement but, when it arrived, it was the wrong one, closed off at the end. I don't want to get it wrong again.  Could some kind person please tell me what to order?    The photos on Ebay are all unhelpful as they do not show the end of the bush.  BTW this is an early bike of around 1963 (A plate) vintage. 

  Thank you and regards,

            Ian

[Spaceman]

The parts diagrams below show the numbers for the various bushes.

[Spaceman]

I assume you were referring to part 76-3017 but, to me, this is confusingly referred to by most of the venders as a layshaft bush. Since the engine is basically a re-engineered Tiger Cub engine, I referred back to the parts diagram for this with the corresponding part, T1147, referred to correctly as the mainshaft bush. This is the same part as BSA part 40-0223 which was the version of 76-3017 used on the earliest C15's confirming that 76-3017 is a mainshaft bush.

It would be interesting to know what part you wrongly ordered?

[Barnsey]

Thank you so much for your help.   The diagrams really help.  I think the bush which I have is from the opposite end of the shaft and blanked off.

     Regards,

          Ian

[Spaceman]

I think that would be part 40-0026 which is the drive side layshaft bush!

[Barnsey]

OK,  I ordered 40-0223 and it turned up quickly.  However it wouldn't fit in the hole in the gearbox inner cover.  I measured it with my micrometer and it had an OD of 20.57mm, vs the original which is 19.65mm.   Of course I couldn't measure the hole with the micrometer, but my vernier suggested that it is almost exactly 20mm.  So the original bush is too loose and the new one is too big to even be an interference fit.  I also note that it is substantially shorter than the old one and a loose fit on the shaft with around 0.5 mm of play.   Hence sent it back.

  However, I then looked on the Feked off website:
https://www.feked.com/triumph-bsa-gearbox-mainshaft-bush-t20-tiger-cub-models-c15-and-b40-models.html

And they say that this part should be 20.4mm OD.     WHat is going on !??

   

[Barnsey]

This is the one which I ordered:

https://www.classicmotorcyclespares.com/index.php/default/layshaft-bush-bsa-c15-b40-76-3017-uk-made.html

Which claimed to be 19.96 mm OD but was more like 20.57mm

[chaz]

probably not a bad thing seeing as the cases are 50 odd years old and may have worn/sman or crept over the years.

[Spaceman]

I have just pressed the bush out of an early C15 inner timing cover that I had knocking around and the bush dimensions are (+ or - 0.05 mm):

Length: 19.1 mm
ID:        15.6 mm
OD:       19.8 mm

It is likely that the bush started life with a larger OD giving a good interference fit - I would have thought a starting diameter of close to 20.0 mm would have been about right. Mine has probably squeezed down to the dia of the hole in the timing cover over time.

[Spaceman]

A couple of sentences got missed off my posting when the internet went down:

I assume the suppliers are quoting the length minus the width of the flange; in other words, they are quoting the depth of the hole in the timing cover?

If you take into account the bush needs to be reamed out to give the correct clearance at the main shaft, my ID is probably compatible with the ID's quoted by the two suppliers. It would be useful to know what the mainshaft diameter actually is? I assume it is not 5/8" or 15.875 mm?

[Barnsey]

Thanks for checking.     19.8mm fits with the size of the hole as I am measuring it.    20.4mm seems massively oversize to me.  I am struggling to imagine an extra 0.6mm of diameter squeezing down that much.  I'm not putting a cork in a wine bottle.     

[Spaceman]

Off the top of my head, I would have thought about 5 thou of interference was about right giving about 20.00 mm OD at most - 20.4 mm would give about 24 thou of interference which is way too much.

That said, I'm slightly confused about the OD of the bush you bought from CMS as they quote an OD of 19.96 mm whereas you measured 20.57 mm which is not far off what Feked are quoting. Unfortunately, this may be another case of a motorcycle part that has been machined to the wrong size.

[Barnsey]

Yep,  I agree with all the above. I might just have to ring up and get them to check the size before it is shipped out. 
That doesn't really explain the difference in length though. Of course.

[Barnsey]

Here is a snap of the old and new bush side by side.  The original one is the longer of the two

[JulianS]

This site lists both bushes with dimensions. Whether correct or not I dont know.

https://www.bsaunitsingles.com/ItemList.wws?cpubcode=BSA&Category=Gearbox&Page=2