How tight should the damper piston be in the fork?

A

andym

Junior member
Location
Auckland
As part of my 3CL restoration I completely stripped my 38mm Ceriani forks.  First surprise came as I tried to remove the damper piston/rods -- one side came out reasonably easily and the other needed "significant" help with a pipe hammered in from the top to push it out the bottom.

A few weeks later it's time to reassemble and discovered that one piston is a 'loose' fit on one fork (i.e. fit the piston/washer/circlip to rod and can insert from the bottom easily) and is a rather tight fit in the other fork (goes in with some gentle hammer persuasion) -- and other piston is nearly 1mm larger in diameter and will need a hell of a lot of encouragement to go in.

As it turns out one of the pistons has a crack so I've ordered 2 new ones from Wolfgang (don't have access to a lathe or I might have turned a couple here) and while I wait for them thought I'd ask for some advice.

Clearly I have a difference in fork internal diameter -- not sure if this is common due to manufacturing issues or perhaps one of mine is out of round (not obvious looking at it) but just how tight a fit is the damper piston meant to be ????

Thanks
Andy
 
Hi Andy,

Came across exactly the same problem last week, one piston seized solid, the other quite tight.

As the stanchions were knackered due to pitted/flaking chrome, they were to be replaced anyway.  Surprise #1!... the new stanchions had different internal diameters and were not straight/parallel all the way through.  Have a few pistons in stock, all the same size, about 0.3mm too big.  Machined the pistons for a sort-of-satifactory fit with about 0.3mm clearance within the stroke of the piston.  Stuffed it all back together with 10W oil, seems to be ok.  Need to assemble the bike this week, can report how the forks perform if you like.

HTH,
piet
 
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