Ceriani pistons really stuck!

1200ts

1200ts

Hero member
Location
USA - Riverside CA
Trying to rebuild a set of 38mm Ceriani forks and the pistons have grown and are really stuck. Gentle bashing will not remove them. I have seen this before but never this bad. No signs of any corrosion and the oil was clean. Any other methods to remove them without destroying the damper rod?

Ceriani went cheap by choosing to use Zamak metal for the pistons which can grow over time if the zinc has any lead impurities in it. This was a problem up to the 1950's and if the zinc is properly refined is not supposed to happen. Whoever was making the pistons for Ceriani was buying very poor quality Zamak that was not inspected.
 
I can't offer any advice, but had to drive a piston out of one of my 35mm Cerianis with a lot of force.
Interestingly I purchased a replacement piston however it was also too large and wouldn't fit!
I ended up getting the new one turned down to the same size as the other piston which hadn't grown, so appears to me they don't have to be installed to grow........
 
If you, during a rebuild, replace the stanchions, you'll have to mill down all the internal parts.
The walls seem to be a little thicker now, at least at the 38s.

Ernesto
 
I'm not surprised that new pistons won't fit. They could be NOS and just as swollen as the ones you had to bash out.

Modern forks have a pretty rigid connection to the wheel spindle, so they can get away with different damping in each leg. Some bikes have compression damping in one leg and rebound damping in the other. But our old clunkers have small diameter axles and not massively rigid connections between fork leg and axle, so you want the damping performance of each leg to be pretty much the same so as not to impart bending loads on the axle as the forks compress and rebound.

38mm Ceriani fork legs are nominally 32mm inside diameter, but I've found that there's quite a bit of variation, so "off the shelf" pistons, even if they're new and well made to an exact dimension, can be a bit hit and miss as far as fit is concerned. The only way to ensure the damping is equal in each leg is to have pistons that fit each leg. I turn up new pistons from aluminium bar, matched to the ID of each fork leg with 0.05mm clearance.

Making the pistons is a simple job on a lathe. Any engineering shop can do it. I just uploaded a drawing of the piston to the library section.

180 triple - New pistons for 38mm Ceriani forks - History

laverdaforum.com laverdaforum.com

You can take the drawing to your local engineering machine shop (or your mate with a lathe) to have the pistons made. The machinist will either need access to the forks so they can measure the ID of each leg. Or, if that's not possible, you'll need to provide them with accurate ID measurements of each fork or a final diameter you want for each piston.

To highlight the importance of accuracy in measurement: while I don't recall the exact measurements, the last pair of forks I did had IDs sufficiently different that a piston with 0.05mm clearance in one leg was an interference fit in the other.

The piston swelling problem is characteristic to Ceriani forks only. Marzocchi forks are unaffected.
 
I don't care if I destroy the old pistons as I will be making new ones on my lathe. But the problem is on these forks they are really stuck and I don't want to destroy the damper rod. So I guess I will bash a little harder. At the moment they don't move at all. The bike was last ridden a year ago and the forks were fine then. Once I get it apart the tubes will be sent out to be hard chromed. I have a honing machine to clean up the inside of the tubes.
 
Some modest heat to the tube? Maybe helps expand it away from the piston? I expect the piston material will expand more than the tube, so be quick.
 
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