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Ossa Mar Forks


basketcase
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Hi, just stripped the first fork leg ready for new seals.....the `oil` that came out was disgusting, thin, watery and smelly!!

Is the screw always a pig to remove? (still haven`t got it out yet but haven`t tried with a vengance)

The spring measures 17", is this the correct length? And how much new oil goes in, what grade?

Sorry but the bike is a Mk2, 1974ish.

Cheers, basketcase.

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I'll check spring length tomorrow. From memory I think they come to just where the fork cap threads are in the tube and they had one spacer as standard. Oil is 180cc and 10W works fine.

As you're dismantling completely check the fibre 'piston ring' on the bush at the top of the damper rod (earlier forks didn't have one but yours should I think, depends on what parts they used) If this is worn it will let oil past which reduces damping effect. As a guide, see how loose the damper rod is in the stanchion before you remove it. Push it up into the stanchion and with the stanchion upright, let it go. If it drops back out with no resistance, the fibre ring is probably worn. You can replace these with PTFE card, available from ebay as a sheet in varying thickness (same as is used in Rockshocks) Just measure the thickness of the existing ring. Cut strips from the card using the original as patterns for length and renew. Refit the damper rod (it will be a tighter fit this time and need a bit of persuasion) then repeat the 'test' from before by pushing the damper rod into the stanchion and then letting it go. It should now hold its position due to the tighter fitting bush, or at worst, lower itself slowly instead of the previous 'freefall'.

A bit fiddly but worth it as the damping qualities of older forks need all the help they can get after 40 years use and wear, although the Ossa had the best from that era (in my opinion)

EDIT: Is the difficulty to remove the allen bolt in the bottom of the fork or the drain plug?

If the allen bolt that holds the fork bottom onto the tube, fit the spring and some extra spacers and replace the fork cap, the extra spacers will preload the spring more as this sits on the top of the damper rod. When you turn the allen bolt, if it is tight it will just spin the damper rod inside the fork, as it is the damper rod it screws into. The extra preload on the rod should stop it from turning. However, if it is really tight, it will need shocking free and that is best done with an air ratchet.

The other way is remove the spring. The damper rod has a slot in the top of it which will take a blade, such as a flat blade screwdriver blade. Push the fork leg up as far as it will go and shine a pencil torch down inside the stanchion, you should see the top of the damper rod and see the slots (some rods don't have them but Ossas usually do) Check that the splash guard isn't obscuring the top of the damper rod. This is a U shaped piece of metal that is a press fit into the bottom of the spring and comes out still in situ. If it ihas lost its tension it may have dropped out of the spring and be sitting on top of the damper rod. It will just slide out if you tip the leg up.

The slot in the damper rod is thin, too thin for most screwdriver sized blades. Dealers had a special tool. You can grind a bade and lengthen it, or make something suitable that will run down inside the fork tube and locate in the slot, this will hold the damper rod and enable the allen bolt to be undone.

If it hasn't been apart for some time, it will probably be tight.

Edited by woody
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Many thanks woody, a bit more complicated than the usual forks that I`ve dealt with!! Although the ones on my Matchless were a pain to sort out.

It was the drain plug that I have found to be tight. I daren`t use too much force i.e. an impact driver as I`m afraid for the alloy fork bottom. I can probably pinch the screw in the vice and loosen it that way, there`s enough showing then clean it up with a fine file or emery cloth .

The allen bolt came undone quite easily, too easily really and there was oil all around it. I can`t say if it was leaking past the washer or what had leaked down the leg from the shot fork seals but there wasn`t much oil left in the leg!! Is there a fibre or copper washer between the bolt and bottom of the leg? I found 2 washers on the bolt and can`t tell.

There is an alloy spacer at the top of the spring. Also there`s a screw in the bottom of the top cap, what`s it for? Can the forks be adapted for pre-load? The top cap seems to have a mark as if it could be drilled through.

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Your springs are the correct length - 17"

I've used an impact driver on drain plugs, it's worked without me using too much force.

The screws in the bottom of the caps are some kind of valve to prevent fork pump up due to air pressure. You should see a small hole in the top of the fork cap somewhere which I believe was to let air out and prevent pressurising. I've never understood how these are supposed to work and on Bultacos in particular have only ever served the purpose of squirting oil in my face when the forks compressed. I've always sealed up the hole and never noticed any problem of pressurising in the forks.

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Sorry, forgot - the allen bolt that holds the fork leg on sounds as though it still has the original washers, as they were fitted with a flat washer as a seal and a locking washer.

I just clean and inspect the mating surface of the leg to ensure it hasn't scored or whatever and fit a new fibre or copper sealing washer. Tighten with an air ratchet if you have one, using either of the methods in earlier post to hold the damper rod.

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