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...
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...)
From what I can see of the airbox, the filter from the last model Sherpa looks as though it could fit as the filter and hose are a complete assembly, see link
The inlet hose rubber should be a standard part which are easily available. They come in different diameters depending on bike / carb combination, I'd imagine yours needs the hose that is 35mm ID both ends
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.
There are two effects of lightening the flywheel, one is that the motor will pick up quicker due to less mass, the second is that it will stall easier at very low revs if it is too light.
The former is good if you ride sections that need immediate response to sharp throttle openings. The latter means that you will have to ride the bike constantly on the clutch at low revs to overcome the possibility it will stall. With the standard flywheel the SWM will drop to a virtual standstill without using the clutch, plonk very smoothly at this speed and will pick up smoothly with no snatch. With a flywheel that it too light the bike will not run as slowly without the clutch and will jerk and snatch, meaning you have to slip the clutch to overcome the snatch and stop it from stalling. If you're used to modern bikes and don't mind clutching everywhere then it's probably not going to be an issue.
In the end, it's all subjective and personal preference
First thing you have to ask is do you really need to do it as if you're not riding sections that need the engine to pick up revs very quickly there is no point. An SWM should pick up revs quick enough to cope with any classic trial and probably any easy route in a modern club trial.
Get a spare flywheel and take half a pound off it and try it. If you want more take another quarter off it until you get something you're happy with.
First thing I'd do is ride it as it is, as you might find it's fine...
As regards the lighter flywheel making it grip less in slippery conditions, it won't affect the bikes ability to grip, what it will do is make the bike spin up and break grip easier but it will still grip, just requires more input from the rider.
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
They all had green frames so yours could have been stripped and repainted / powder coated. Unlikely a small firm like Italjet would change colours for different countries.
Yours looks to be the green coloured 250 but with a white tank, due to the black engine and hubs, but after 30 years you never know what's been changed. As the exhaust is already off, just lift the head and measure the bore. Around 70mm and it's a 250, 80mm and its a 325.
I wouldn't even know where to start looking for those. The pistons / bush have a fibre sealing ring around them and after 30 - 40 years they can be worn out so they don't seal as effectively any more. Early 70s Betors don't even have the fibre ring, just a plain piston / bush.
You can make new ones from PTFE card which is on ebay. Use the old fibre ring as a pattern and to get the thickness of card required, then cut new strips with a blade.
I'm no engineer but I've done it ok, just a bit fiddly
Moving footrests back and down is a common theme, what has changed since these bikes were designed ( by very bright people) to make moving the footrests beneficial?
Back in the 70s, which was the most prolonged run of twinshocks the bikes had the footrests set high to keep them clear of snagging obstacles etc as the bikes didn't have a huge amount of ground clearance. The bikes were also ridden differently with virtually no use of the clutch, the bike was kept moving and the positioning of the footrests enabled the rider to move around a lot on the bike to weight it in turns, keep balance over obstacles etc But the biggest difference was the bars which were a much higher rise than today's bars. Therefore although the footrests were higher set, with the higher bars the riding position wasn't uncomfortable. I'm 6' 3" and never had any issues riding the bikes then.
Later twinshocks and aircooled monos carried on with the higher footrest position as ground clearance increased but the bars got lower meaning you were pitched over the front more. No idea if this was by design or they just never thought about it but the style of riding these bikes differed from the 70s era with more emphasis on stopping and balancing and hopping the bike around. It took up to the mid to late 90s before the footrests got lower as with the riding style still evolving into what it is today, the need for the rider to be more upright, rather than hunched over grew. There was also no longer the need for the bikes to have 13 - 14" of ground clearance as modern riders began to ride more on the back wheel, keeping the front light or in the air, so grounding the bikes wasn't so much of a problem, plus being a lot lighter than older bikes they slide over stuff easier.
So the modern stance is to stand in the bike as they are designed to be ridden in straight lines now (my opinion) with riders hopping them around turns (better riders that is...) rather than the old fashioned way of steering them with both wheels on the ground. Standing upright on the modern bikes is the best position for using the suspension to move the front and rear of the bike around. The old style was to move around a lot on the bike to lean the bike where necessary when riding a section and the higher rests and riding position made this easier. It's actually quite hard to ride a modern bike in that old style - for me anyway...
The problem with the earlier bikes now, if you are tall, is that with the high rests and low bars, they can be pretty uncomfortable to ride, depending on your height. 40 years on, I can't bend as well as I did at 15... so on all my bikes I've lowered the rests and fit the highest bars I can get which is only 6", compared to 8 or 9" from years ago.
I think just lowering the rests is the best compromise but some people set them further back too. Depends on the bike and how far but if they are too far back it can put more weight on the front as the rider is again pitched forward to reach the bars and this can negate the reason for moving them back in the first place, which is to get more weight on the rear for grip...
Steepening the head angle is another of those subjective matters. It is better in terms of stopping the front pushing in turns in theory because there should be more weight on the front. A GasGas is much more planted on a full lock tight turn on a slippery surface than my Bultaco for instance. But I've ridden a few twinshocks with altered steering and the results have been mixed. Some felt ok, some were ruined as the front pushed everywhere when on lock. It doesn't always improve the bike
Besides, today's classic trials are nowhere near as hard as a centre trial or national from the late 70s early 80s, so there isn't really any need
BellaVista in Spain may have them. I needed a final drive sleeve gear for my 340 Sherpa which is different from the 5 speed sleeve gear, and was able to get one from them, through In Motion Bultaco UK
We sent the old one to them so that they could match it up as there is no parts diagram or part numbers for the 340. Worth keeping in mind if you can't find part numbers.
I think to make a 5 speed box fit the B cases, the layshaft bearing housings may need boring out to take the different bearings as the 6 speed uses small diameter needle rollers. You might also need to use the 5 speed selector pawl mechanism as I tried a 5 speed one in my 6 speed and it doesn't work due to it being machined differently on the inside where it engages with the drum
I wouldn't say a 'pup' either as you don't know it's history, who built it or whether it's been changed around in later years. It's not hard fact obviously, just an opinion, but based upon what little I know, I don't think it is a bike built for Ossa UK for one of their riders.
With regard to being a MAR frame, I can't see it, as based upon the number of changes that have been made to make it very different from a MAR, some of which would have been unecessary, why would they have? The Ossa UK bikes that were converted MARs were unaltered at the front half as there was no need to
It's all subjective Hamish, I know, but I've always thought of Marzocchis as having a better action than the Betors. I've ridden SWMs, Fantics, Armstrongs and always thought the front forks better than my Bultacos, not a huge difference but noticeable.
Maybe one thing to check on the Marzocchis is the fibre seal around the piston at the top of the damper rod. These can wear and / or go hard over time and they don't seal as they should. It's possible this may have happened in yours which allows the oil to flow outside of the piston rather than through the damper rod and this can reduce damping, making the forks soft.
If you take the forks apart, then insert just the damper rod into the tube, there should be some resistance to pushing the rod assembly up the tube. On some I've had, if I put the rod into the tube and then turn the tube upside down the rod just slides down with no resistance. When I've replaced the fibre seal (I make my own out of PTFE card) the rod needs to be pushed due to the resistance of the new seal, it no longer just free-falls down the tube - this is all my theory of course but it must help the damping by reducing the amount of oil that can bypass the piston
If I was having the yokes bored to take the 38mm forks, I'd want to be sure that it wouldn't affect the angle of the forks to the steering stem. As far as I know, the angle is built into the stanchion clamps, not the stem - ie: the stem is set at 90 degrees to the yokes but the stanchion clamps have (for argument's sake) say 2 degrees rake built in. If the clamps are just machined out to 38mm and are machined 'straight', you may lose the built in angle of the yokes and end up with parallel forks which would affect the steering. I'm assuming your engineer mate will know this (Stuart?) but worth considering
Personal opinion again, but I think I'd persevere with the Marzocchis and see if there is a damping issue before going to the trouble of fitting the Betors.
Incidentally, the 38mm forks were fitted to the last 240 SWMs as well as the 350
It looks as though it is from the picture. The Ossa top and bottom bearings are the same size, so the flange in the headstock is the same top and bottom. On your frame they're different sizes, plus there looks to be an additional flange above the bottom one.
The gusset plates are quite different too where the tank rubbers mount, and the coil mounting is in a different place. From the other photos, if I remember correctly, the rear of the top tube is very different from a MAR as well. On a MAR it flattens out to where the seat rails begin. It's also different where the rear of the bashplate mounts.
I wouldnt be surprised if this is john reynolds old bike, would that be possible?
It doesn't look like it, as it's completely different, although it's confusing which variation of the cantilever JR rode. I didn't think he rode the Cheney bike which riders like Geoff Guy and Geoff Chandler rode, although Don Morley's book said he did. However, from the picture of JR on a cantilever in that book, it isn't a Cheney, and that book has a few errors in its depiction of the Ossa models. There was another cantilever bike at that time but I can't remember who built it. Keith Horsman did tell me years ago but I've forgotten and it's that bike he told me that JR rode. It was the 'JR' frame that Keith made replicas of and someone rode one in the SSDT in the late 80s. JR's bike looked like this:
Cheney is said to have converted some MAR frames and also built his own cantilever frames. This is a Cheney bike (I was told) but I've also seen pictures of them without the exhaust modified in that way. It does appear to be a modified MAR frame though, although the picture isn't that clear when enlarged
The bike of this article definitely isn't a MAR frame, the headstock isn't MAR deign and even if you made some big alterations to a MAR frame during conversion, you wouldn't change the headstock. My guess would be that it is a bike that someone has built for themselves.
If you're using tubeless tyres they will creep a bit because the bead is different where it seats on the rim. You can trim them to the same shape as a tube type with a blade, that may stop them creeping as much (not done this myself and never compared the two, so can't explain how it's done)
I'm assuming you're using tubeless as Michelin don't do a tube tyre any more
As for the security bolts, I've used 'normal' cheap ones without any problem in the past, so can't imagine which you use will make any difference.
There's a facebook group - hard to find twinshock trials and aircooled mono
There is a bit about the aircooled mono Gasser on there. The model was called an Airie (think that's the correct spelling) might help you find more info on them
They were using them from 74/75. Cheney converted some Ossa frames and also built some of his own. Keith Horsman also built some cantilever frames, for the later type MARs. Some Grippers were also converted.
This bike is a real mystery as the engine number doesn't tie up with Ossa's numbering system, although the number on one of my bikes that has matching numbers doesn't appear in Ossa's numbering system either. The frame doesn't resemble a MAR anywhere and definitely didn't start out as a Gripper frame, which is what the chassis number suggests.
I think you're only going to solve this mystery by finding someone who has owned the bike previously.
I think JR had started riding the SWM for Holdens by '78
Confusing with the frame number as the 76 range is for the 250 Gripper....
The frame itself I can't help you with. I'm not that familiar with the cantilever bikes but that to me, is definitely not a converted Ossa frame, there are too many differences. But the cantilever bikes that had their own frames built also didn't look like that and the swingarms were very different as well.
When I had my first MAR, at about 16, I was still very much 'learning' mechanics. The rear spindle had bent and I hadn't a clue how to deal with it. I had the bike on the stand in the garage, a lump hammer and a drift which I'd found in my grandfather's array of tools we'd inherited (he was a hightly trained electrical and mechanical engineer at GEC) It probably wan't a drift come to think about it, more likely some intricate tool for precision machining...
Anyway, I was knocking hell out of this drift, trying to drift the spindle through and out of the sprocket side. It was seriously stubborn but it was moving eventually, bit by bit as it was bludgeoned through the spacer. All of a sudden it gave, without warning. All resistance to the drift disappeared resulting in the hammer meeting my hand and pulverising it between hammer and swingarm. The spindle left the bike like an exocet and travelled as true as you like into a large, vertical roll of cardboard on the other side of the garage. There seemed to be some sort of noise coming from the cardboard roll. I went to retrieve the spindle and as I pulled it out, some dust and small fragments fell out of the carboard. The spindle had neatly harpooned half a dozen or so of the old man's supply of spare flourescent tubes and reduced them to powder...
Grandad was probably spinning in his grave at the mechanical butchery - and at the fate of his tools...
Definitely an earlier (as in pre-green model) 250 barrel. The early 250 barrels had solid fins like yours, the 310 has slots machined into the fins, so visually they are different
From the green bikes the 250 also had the slots machined into the fins so look the same as the 310, therefore not distinguishable externally. You can tell from the stud pattern though, as the studs are spaced wider apart on the 310, although I can't remember the measurements of either
Because of the stud pattern it isn't possible to put a 310 top end onto 250 crankcases, so another way of checking is from the engine number. If that identifies it as 250 then it can't have a 310 top end, so it has to be 250 - and vice versa. Also, the 250 barrel can't be bored out to 310.
This doesn't apply to the Grippers though as both 250 and 310 have same stud pattern
The original wheel that came out of the bike has the MK1 hub, flanged rim and is fitted with a flat sprocket. The wheel I had built is a MK2 hub with a dished sprocket and non-flanged rim. In trying to compensate for the different measurements of the different components, I got something wrong, so when checking the offset of the new wheel against the old it looked correct but as it turned out, wasn't.
Once I had had the measurements I needed from an original bike still fitted with dished sprocket, I dug out another original MK2 wheel and found an old dished sprocket to fit to it. Put this in the bike and checked the measurements against those I'd been given and they matched. Checked this wheel against my new wheel and the difference in offset was apparent. That's now been corrected, chainguard is fitted and ready to go, I hope, for the weekend.
Bultaco M49 Help
in Bultaco
Posted
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...