Montesa 348 Fork Issue.

[samwisemcg]

I have recently bought a 78 Montesa Cota 348. Everything is running smooth except for an issue with the forks. They were pretty seized and only moved under a lot of pressure.

I took one for to bits trying to locate the problem. It seemed like the damping rods were jammed in the stanchion. It took a few hard knocks to get it out and here is what I found.

http://i.imgur.com/d99wyGs.jpg

The piston ( part # 2135.023 ) on the damping rod is all cracked and wasn't sliding smoothly inside the stanchion. I located 2 more from spain.

Now I am trying to reassemble the forks and they just wont go back together. The new part just will not slide into the stanchion, it is as if it is fractionally too large.

http://i.imgur.com/x5H9uzl.jpg

I was wondering if anybody has any experience with rebuilding these forks and have you come up against this issue. The only solution I can think of is to slow take down the diameter with some wet and dry.

Any advice would be much appreciated, thanks.

Edited July 3, 2013 by samwisemcg

[feetupfun]

Have you tidied up the bore of the tubes? I had a set of seized 348 forks caused by being full of water for a long time, and I had to linish/hone the surface of the bores to get the pistons to fit and work nicely

[samwisemcg]

They seem clean and smooth inside but it's always worth me giving them a good clean.

It's almost as if the piston is ever so slightly too large, It just won't fit in at all.

I'll give both parts a clean and see what happens, thanks.

[paulc76]

This is a fairly common issue with these forks the solution is to turn up a new pair of damper pistons to suit the inside diameter of the fork stanchion. dont know what causes the problem though its as if the alloy piston swells up in the oil.

[samwisemcg]

The weird thing is I order 2 NOS parts from Spain. They were both still in the packaging, they both had the same tiny cracks even though they had never been exposed to oil, water, stress. I think they are cast and the casting isn't great.

Anyway I spent the afternoon cleaning the inside of the stanchion out pushing wire wool through with a rod, then a cloth with polish on it. Then I have filed the edge of one of the old piston dampers ever so slightly and cleaned it up with some wet and dry. It now fits and slides freely. So thank you for the help.

A couple of quick questions I have been told to use 190ml of oil for the forks, is this poured in the top before the air valves are screwed on? And does anyone know what is the correct air pressure?

Thanks once again.

[nelly1]

i was just going to ask if the valves where to release pressure or if air assisted what pressure to run at hope someone knows

[samwisemcg]

Ok so I had a dig around and it sounds like 12 psi is the magic number.

The air is there to assist the spring. Without it the springs aren't strong enough and will bottom out I believe.

[feetupfun]

Some 348 forks had soft springs that needed air assistance and some 348 forks had normal stiffness springs. Be aware that the springs in your forks may be either type so your forks may not need air assistance. I have both types of springs to choose from for my 348 and have found that normal stiffness springs and no air assistance provides a better action, most likely to having less friction at the seal lips.

[nelly1]

thankyou gents