Ossa Modifications

[mattylad]

I want to improve an Ossa MAR to run in the pre 76 class run over here.

I will fit it with a Gripper 310cc engine and would like to fit a first oversize piston.

Does anyone know of a source?

Have you modified them in any way?

For the frame this has to be the heaviest thing known to man.

Does anyone sell a lighter swinging arm? Or recommend an alternative?

What about footrest position?

Lower and rearward, but by how much?

Is laying down the shocks necessary as done on the later models?

In short any advise would be welcome.

Cheers.

Any other mods to improve reliability or rideability would be welcome

[fivemeister]

This collection of websites should keep you occupied for a while both for information and parts.

Pretty much everything you need for an OSSA. If you can

[woody]

Where's over here?

Surely fitting a Gripper engine and laying down the rear shocks puts it out of the Pre76 class? Pre76 Ossa is MK1 or MK2 250 MAR or the first 310 MAR. Gripper engine was 1980 and laid down shocks were MK3 MAR from '76 on. (I'm not bothered what you do as it's none of my business but modding a Pre76 Ossa with Post 76 parts to ride in a Pre76 class seems a bit of a contradiction - you may as well enter on a Gripper...)

If you fit a Gripper engine what will you do for an exhaust, Gripper exhaust is expansion chamber which won't fit a MAR frame, will it work properly with MAR type exhaust?

I have a '75 310 MAR and apart from lowered footrests, a skimmed head and Mikuni carb it is standard. Being perfectly honest, if I was to use it in a Pre76 class there is nothing else I would do to it. In standard form any MAR is as good as anything else Pre76. It's a nice handling bike, quick steering (in its era) good suspension at the front and with a good set of shocks plenty of feel at the back. Weight isn't an issue, or at least doesn't bother me anyway. If you want the engine to pick up a little quicker, 250 or 310, you could skim the flywheel. These weigh about 7.5 - 8 pounds. I've tried an enduro flywheel on a bike some time ago, these weigh about 6 pounds. I found it a bit light and although the engine would pick up quicker it would also stall a lot easier than with the standard flywheel. Skimming the original flywheel to somewhere inbetween the two should improve engine pick-up without sacrificing the low speed plonk which the Ossa is so good at. If your carb and ignition are good they are almost impossible to stall.

A reed valve also improves torque and engine pick-up on the 250 (never tried one on the 310) and I think they should be ok for Pre76 as Ossa UK were using Harpower reed valves on their bikes then (Roger Holden could confirm this as he has just started posting on here) A TY250 reed block could also be used as this was another conversion done at the time by Romat.

[mattylad]

Woody, thanks for the reply.

The reason I want to fit a Gripper engine is that I have one and not got anything like a complete MAR. There are too many bits missing to make it a viable rebuild

I think they are basically very similar with crankcases being the same.

The cylinder and crank are different but in my opinion not such a massive change, the original MAR head and outer cases will also be fitted.

Externally the only difference will be the number of fins on the barrel

I have modded the footrest to be 1" lower and 1" rearward.

I will leave the shocks as original.

I will try the MAR style exhaust but fit an alloy middle box and a different rear box, probably a WES Fantic 300 (again in stock).

I'll see how it goes and if I can pick up a MAR I can rebuild and fit it later.

Cheers,

Matt

[brian h]

Woody,

Any info still about on Harpower / Romat reed valves for the Mk 1 250 Ossa MAR - are they still available and if so, do you know what is involved in fitting a reed valve to the Ossa cylinder ??

Brian H

[woody]

Yes, the centre crankcase castings are exactly the same on the Gripper/MAR so the Gripper engine will go straight into the MAR frame.

I think you're right about the crank being different on the Gripper, it's the same assembly but the woodruff key position may be in a different place.

The positioning of the cylinder studs is different too. The 250 MAR models (I'm including the green models in this) are all the same. On the 310/350 (whatever you want to call it) MAR they are spaced wider apart (sideways only) than the 250 so a 250 cylinder won't go onto a 350 crankcase and vice-versa (unless you elongate the holes - messy). I think that all the Grippers have the studs spaced the same as the 310 MAR, so to fit a MAR head to 350 Gripper you'd need the 310 MAR head - which you'd need anyway to match the 350 Gripper bore size of 77mm.

You may need to check the compression ratio too as the 250 MAR heads differ, some have a deeper recess than others where they join to the top of the barrell, so you can get a higher compression ratio with some than others. Not sure whether there are differences with the 350 MAR heads but you'd need to check it is the same as the Gripper. You can achieve a lower ratio if needed with thicker head spacers but if you need a higher ratio and the MAR head has a deeper recess than the Gripper you may need to skim away some of the head casting around the combustion chamber, otherwise if you fit the thinner spacer the casting hits the barrell before the mating surface of the head touches the top of the liner and it won't seal.

A conventional exhaust should work ok as I was forgetting that one of the other mods that was one at the time was to fit a Phantom motocross barrell to the MAR with a reed valve fitted. This worked ok with a MAR exhaust so there is no reason the Gripper engine shouldn't.

MAR side cases will fit the Gripper engine no problem - do you want to sell the Gripper cases? (if they are the ones with the cloverleaf on them)

Gripper engine in a MAR is a mod I've seen done a few times by the Spanish - seen them on photos and videos from their classic trials. Maybe Mr Greeves knows one or two tips for the conversion.

[woody]

Woody,

Any info still about on Harpower / Romat reed valves for the Mk 1 250 Ossa MAR - are they still available and if so, do you know what is involved in fitting a reed valve to the Ossa cylinder ??

Brian H

Harpower reed conversions are no longer available on the Ossa - or more accurately the person that did them doesn't do them anymore. Bikes with the conversion come up for sale every now and then and sometimes converted barrells appear at autojumbles but probably not in your neck of the woods....

I don't know who did the Romat conversions but it's in the back of my mind it may have been something to do with Mick Andrews following his return to Ossa from Yamaha. The Harpower conversions were already in existence by then.

As far as fitting a reed valve is concerned it is pretty basic for a competent engineer (beyond me unfortunately as I'm not even an incompetent one) The fins are machined off the back of the barrell leaving a flat surface to mate the reed block to - either right accross the barrell or just in the area where the reed block will sit like a TY250 twinshock. Obviously the angle and height at which the reed block will sit needs to be considered here in order that the carb still sits in line with the airbox. The cylinder fins get thicker towards the centre of the barrell so where they have been machined off they can be drilled and tapped to enable the reed block to be bolted on. The inlet port is opened up to a larger diameter. On the cylinders I've seen this doesn't appear to be too scientific and the hole is rarely symmetrical.... No idea what the optimum size is. Sometimes vertical flutes are machined into the liner top and bottom of the inlet port but not always. When I had mine done it didn't have flutes and performed very well. The back of the piston is modified to allow more fuel in, either by cutting a big arch into the bottom of the skirt or by cutting a window into the skirt. The former means there is less bearing surface on the piston so it could wear out quicker, the latter retains the bearing surface as the bottom of the skirt is untouched.

As far as getting a reed block to use, you could use a TY250 assembly or one off a modern bike. Some people here have used GasGas reed blocks for recent conversions.

Nothing else I can think of at the moment, if there is I'll post it. Hope this is some use to you. It applies to any MAR cylinder by the way, not just MK1.

[brian h]

Woody,

Thanks for your comprehensive reply on reed valves and mention of Harpower and Romat versions - reed valves interest me, however, as you have noted previously, the 250 Ossa MAR has plenty of grunt for todays twinshock events, they start easy and handle well and grip well in mud and do not really require any serious mods.

Thanks

[ask greeves]

Hello, I thought the name was ROMAC and the gents name was Clive Miles, from memory a Norman Motorcycle 'works' rider. Hope he reads this, or somebody that knows him.

[andy m]

There is a Gasgas reed block and manifold listed on ebay tonight.