woody

Don't forget Ross, Hamish is in France Incidentally, the 35mm measurement allows the use of upside down forks as they use 35mm stanchions.......

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.

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.

They were good bikes yes 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

I hope you're not suggesting that that is lovely to see.........

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.

From the factory, the engine and chassis numbers matched, so the engine number can be used to date it. Are you sure about that number though as the MAR serial numbers were nowhere near that high

When the 240 was current LeJeune was winning the world titles on his Honda, the bike Michaud was on in '85 was the mono For the SSDT,Michaud was on the 300 in '84 and the mono after that. Don't think Michaud ever rode the 240 as he was on the SWM then.

Yeah, I've had some fun with them too 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 Example of a 310 barrel in this ebay ad. http://www.ebay.co.uk/itm/OSSA-350-310-Mick-Andrews-Replica-crankcases-barrel-and-head-/321579683442?pt=UK_Motorcycle_Parts&hash=item4adfa48e72

Turned out it was the offset after all. 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.

I have the measurements I need now Thanks to John at Twinshockshop for scrabbling all over a MAR this morning with a tape measure

Well that's the strange thing as the chain seems to line up ok, I've also ridden the bike a couple of times with no chain issues. Although the wheel is noticeably set to the right it doesn't affect riding but I can't get the chainguard on and this weekend' s trials could be very muddy and to ride the Ossa, it will need a chainguard I need to have another look tomorrow and see if I'm missing something obvious. I've already had a go at machining the brake plate but things didn't go quite to plan... I absolutely hate loathe and detest that flat sprocket conversion. Original looks much nicer and I have a spindle a mate made me some time back from better material than the original, so no bending issues. I also have one of Keith Horsman's spacers with extra bearing that gave more support to the spindle to go with the dished sprocket but can't find it at the moment.

Having a few issues with rear wheel alignment on my MK1 MAR Does anyone know the length of the original spacer between the sprocket and swingarm please - this is the original dished sprocket with the longish spacer I've now fitted a new dished sprocket on a newly built wheel in place of the old flat conversion but the spacer is long gone. Normally I'd just measure the gap and make the spacer but I've an added problem. The wheel that came out of the bike was the original MK1 with thinner brake shoes but it had some damage. The wheel I've rebuilt is a MK2 with the wider shoes and the MK1 brake plate won't fit (I'd forgotten there were differences and the brake plate for this wheel is broken) When the wheel is tightened the brake plate lip fouls the hub as the boss on the inside isn't long enough. It needs spacing to stop it fouling when tight, but when the wheel is then fitted it is 10mm too far to the right and I can't get the chainguard on. The wheel offset is correct so either the spacer I made isn't long enough and the swinarm may have closed up, or there is still some sort of issue with the brake plate that is pushing the wheel over. I could do with knowing the original spacer length so I'm at least starting with the correct measurement on the sprocket side, so if anyone can help it will be appreciated

Probably a little more substantial than I was thinking....... but illustrates the point nicely about masking off the tyre Looks very much like a Bultaco guard

Physically yes, but I'd go for a 240 or 280 (the 280 is also referred to as 320 but it's actually 280cc) Older twinshock 125 engines are pretty gutless, especially if you're used to modern bikes. A 240 can be bored to 280. Older red/white models can be aggressive in the power delivery as they had lighter flywheels, the yellow bikes are smoother. Nice bikes.

I never knew that (never seen that parts diagram) So sorry Yellow_Cad, I gave you wrong info there. You can see how it fits as the top hoop must fit around the lug that the rear chainguard bolt mounts to. The bottom mount on the chain pad would just need a long enough bolt to attach to I used a brake shoe spring mounted like that some years ago on a bike that had a broken main spring, at a time when they weren't available to buy

It's pretty straightforward... if you ride in deep stodgy mud, the mud will stick to the tyre sidewall and get carried around as the tyre rotates. If the build up of mud is thick enough, and you haven't a chainguard, or one of the right depth, the chain will act as a scraper and slice the mud off the tyre which will then sit on the chain as it goes onto the sprocket. If the mud is thick enough or has stones or grit in it, expect trouble as it will unseat the chain Running the chain slack won't prevent this, it's impossible, as the stone or grit will still go between the chain and the sprocket. Running the chain slack will just increase the chances of it jumping off the sprocket if the suspension rebounds quickly On motocross / enduro bikes it's not such a problem as there is a lot more clearance between tyre and chain Bondy, I've no idea as I've never had an original guard fitted.

If you haven't got a chainguard, that is the reason. A chainguard needs to completely block off the tyre between the top and bottom chain run, otherwise the tyre just deposits mud on the top of the chain on the bottom run which then gets dragged between the chain and sprocket. The guard will also stop mud being carried onto the front sprocket. Slackening the chain won't stop mud building up If you have a guard fitted make sure it is effective and blocks off the tyre from the chain.

I think it was the green TR77 models a second spring was added. Don't know how they fit though as I never had a green model, only earlier MAR.

They do, as I've ridden two or three bikes with very effective rear cable brakes, one only yesterday. I can't make mine work though...... even with relined hub, longer arm etc.

They only had one spring, the one that fits onto the brake pedal spindle. The spring that is attached to the frame ? (swingarm?) has been added which some people did as the original spring isn't overly strong, but it does the job. So it's up to you whether you want to connect the second spring. Can't advise on the silicon spray question as not sure why you want to do it