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Bultaco M49 Help


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Does anyone know the thread size of the bottom yoke pinch bolt on the earlier Sherpas please. Looks as though they are 9mm but I've no idea what the pitch would be. 8mm is too small and 10mm too big. This is for the steel yoke with the single pinch bolt used up to the model 80.

I have to get some new bolts and also a tap to clean out the threads. I seem to have misplaced the original bolts...

And I thought it was only Ossa and Montesa that used stupid odd number sizes for their threads... First time I've come across anything other than even number sizes on a Bultaco

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Hullo Woody,I think that sometime in the past the thread has stripped & tapped out to a bigger size. You don't say whether it is just one or both with this problem? M9 is a standard size but not one I have come across very often. The pitch would be 1.25.If you could find a drill that fits with just a little clearance I can look up on my taps & dies chart to see what is it most likely to be. Either MM or Imperial will do, it could be MM or Whit or BSF or UNC or UNF. Hope this helps,John.

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Hi John, it's the same both sides

I've now found what appear to be the bolts, which are allen bolts. I'm pretty sure that they were usually hexagonal, they are looking at pictures from other M49 and M80 I've owned.

Putting the vernier across them, it reads 9mm. They screw in but they are tight as the yoke was powder coated and the threads have muck and dirt in them, as well as maybe some powder coat. They need cleaning out with a tap. At least if these are the bolts I can establish the correct thread now and get a tap.

If they have been stripped in the past and re-tapped, the mind boggles as to how you strip the thread of a fork pinch bolt - both sides...... and on a steel yoke too, not even alloy

More investigation tomorrow, I'm off out for a beer after a day of various Bultaco traumas.

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Hi Woody and everybody,

Just checked my M80 (same yokes), the size is M8 x 1.25 pitch (standard ISO metric coarse).

I still have the 'Bultaco' headed bolts.

M9 is an ISO non preferred size, so would be difficult to get replacements for.

However, looking in my metric tap set, the M9 tap has a 1.25 pitch.

Looking at the old German and French metric thread size charts, M9 has a 1.25 pitch.

So in all probability, if you have an M9 thread size, it will be 1.25 pitch.

Yes, the favourite on the Montesa is M7 (the bane of my life).

M7 x 1.0 is an ISO preferred size, but difficult to get replacements for.

Later Edit:- feetupfun- 3/8" is quite a bit larger than 9mm, Woody should have been able to tell the difference with a vernier, but, as you say, a possibility it has been tapped out to a 3/8" size.



Edited by sparks2
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Thread form and pitch gauges are made for this purpose. If you can't borrow or buy a gauge, use a rule to measure the pitch of the thread (count how many threads there are in one inch). There are thread tables on the internet, or you can post here and someone will look at their tables and identify it. I'm thinking its probably been retapped 3/8" UNC

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Hope i'm not talking breeze, an easy way to check, if 3/8 unc..a 9mm allen key probably wouldn't be a nice fit, would have to be an imperial 3/8ths allen key. Haven't checked just a surmise.

I suspect he was measuring the shank or the thread OD of the bolt, rather than the hex of the socket head

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To be honest, I haven't a clue on engineering matters when it comes to thread sizes etc. I can do drilling and tapping though which gets me out of most fixes....

I measured across the threads on the shank and when I measured it again this morning using the other bolt on which the threads are less damaged, it measured 9.5mm, so based on what's been said above I thought maybe it isn't metric... I was just assuming it was with the 9mm measurement.

Luckliy my grandfather was a top rate mechanical and electrical engineer with the old GEC and we inherited his tool set. Sadly I didn't inherit his skills.... so 99% of it I haven't a clue what to do with. There are thread gauges though which I found and it turns out that that the bolts are 1/4 whitworth.... But I've no idea what the threads inside the yoke are as the bolts won't freely screw back in. They were hard to remove I remember, but put that down to them being corroded in after 40 odd years. But trying to get them back in I could see that the tip of each thread was being flattened so I guess that when fitted they were persuaded into a thread they don't fit properly with a bit of additional elbow power. Friggin' butcher.

So back to what I can do, drilling and tapping. Problem solved by drilling out and tapping to 10mm ( not without snapping two drills... grandad would be proud...)

Thanks to all for the suggestions above

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I suspect he was measuring the shank or the thread OD of the bolt, rather than the hex of the socket head

Sorry, Didn't make myself clear, a normal 8mm allen bolt, usually has a 6mm (across flats) hex to accept a 6mm allen key. A 9mm allen bolt takes a larger across flats metric allen key. As 3/8 unc allen bolt is similar (but not the same) as 9mm, the hex in the top, where the allen key would be inserted is also similar (but not the same). I'm just trying to say the imperial hex in a 3/8 cap head is larger than the hex in a 9mm cap head, as maybe a way of determine wether the thread in question is either 9mm or 3/8th unc, but of course you would need either of the coresponding allen keys for comparison. Just a simple way, to maybe identify the bolt. Woody is your middle name axlerod ! Do you know what...I've never ever seen a M9 allen bolt...have you.

Edited by ask greeves
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Woody -

I wish I could have replied earlier; your own posts are invariably knowledgeable and helpful when responding to others in difficulty.

I know you have the matter sorted now, but I may be able to offer further info. re the "odd " old bolts you have, if you are still curious:-

9.5mm. dia. is very close to 3/8" imperial, which means they can't possibly be "1/4" whitworth (or 1/4" anything else for that matter, as this refers to the shank diameter).

If you arrived at that conclusion using a thread gauge - 20 t.p.i.- then I think your bolts/capscrews are actually 3/"8 BSF - ............unless anyone out there knows differently l

(Fettling old Brit. bikes for any length of time soon makes you aware of all these various Imperial standards - on just two bikes in the shed at the moment I have found BSW, BSF, BSC, BA, BSP, UNC, UNF threads - not to mention the odd spurious metric fasteners that shouldn't have been there.....)

Happy days !

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Hi Lorenzo, thanks for the reply

Funnily enough I was talking about this after yesterday's trial and my mate who is an engineer said the same thing. When I said that the thread gauge that fitted had 1/4 20 stamped on it and it was in the whitworth selection, he just laughed and said it doesn't mean it's a whitworth. He also said it was most likely the 3/8.

I didn't understand what the figures meant as I could see the diameter was much bigger than 1/4, so had no idea what 1/4 related to. All I know is I spent a while looking through 100 odd whitworth taps for one with 1/4 20 scribed on it. If I found one I was just going to check the thread in the same thread gauge to make sure it matched. Needless to say I didn't find one with 1/4 but I found plenty of 3/8 but obviously had no idea they were probably the correct size...

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