Ceriani forks for a '76 Jota

Your description doesn't really help us understand what's worn out. Stanchions can be replated. Springs replaced. Internals overhauled or upgraded. Maxton in the UK can do the latter.
 
If the problem is stiction in the forks, those old Cerianis tend to get tight as they get old. It's just the crappy pot metal (die cast) piston expanding with age and jamming inside the fork tube. Sometimes they get so tight that you can only get them out with a hammer and drift. It's easily fixed by making new pistons from billet aluminium. I've done it several times.
What part of the world are you in? Your location of "Sutton" isn't much help. There are Suttons all over the place.
 
Hi everyone. Thanks for responding.
I'm in Sutton in South London, England.
You are quite correct in suggesting that "forks shagged" is not a detailed description. The bike is a 1976 Jota that was a racer, but I have no idea who ran it. I know that Phil Todd did some experimental work on the head where I believe he was trying to sink the valves deeper and used larger, lighter valves. It went like the clappers and could easily outpace my later 1980 Jota and my RGS.
To try and get over the problem of fork flexing under heavy breaking [mud guard could hit the centre exhaust pipe], the stanchions have been extended about an inch with spacers that screw to the tops. there were also slugs in the tubes compressing the spring that made the forks spring-bound. With all this abuse, the forks either rattle about at idle because there is so much wear, or they seize solid, which is what they are at the moment. I've known this bike for 40 years and I'm fed up with these bloody forks.
I believe Maxton is making Ceriani replicas, but they are extraordinarily expensive.
 
I had one seized piston in my 77 ceri forks, the whole bike was pretty much a basket case. Had them both tickled up on a lathe , new seals etc and fork stanchions , new springs pending but probably a lot cheaper than new forks complete. I doubt I'll ever get my forks to flex that much driving over here .
Paul
 
Taking a bit off the original die-cast pistons with a lathe is an option if you can get them out without damaging them (bashing them out with a hammer doesn't do them much good). But you could be back to the same problem later if they keep expanding. That's why I prefer to turn up new pistons from Aluminium billet. They'd be more dimensionally stable.

I've never found an explanation as to why they grow over time. I presume some chemical change is going on within the crystal structure of the metal. Perhaps some chemical from the fork oil leaches into the metal. I have noticed a "crazed" appearance on the surface of old Ceriani pistons. They have very fine web of cracks all over them. It appears to only be skin-deep because it disappears if you skim the surface off in the lathe. I presume the expanding metal causes cracking within the oxide layer on the surface. Without knowing the exact make up of the die cast alloy, it would be difficult to figure out a plausible explanation as to what's going on.
 
Ducati 860GT steel dampers are much better than the Laverda pot metal rubbish.

They are a bit longer than my 750 dampers but may suit triples. I should measure them to work out fork travel.
 
All my Ceri damper pistons are fine with no sign of this in two sets of forks. I check them closely especially after this problem was raised. How common is it?
 
Dellortoman said:
Taking a bit off the original die-cast pistons with a lathe is an option if you can get them out without damaging them (bashing them out with a hammer doesn't do them much good). But you could be back to the same problem later if they keep expanding. That's why I prefer to turn up new pistons from Aluminium billet. They'd be more dimensionally stable.

I've never found an explanation as to why they grow over time. I presume some chemical change is going on within the crystal structure of the metal. Perhaps some chemical from the fork oil leaches into the metal. I have noticed a "crazed" appearance on the surface of old Ceriani pistons. They have very fine web of cracks all over them. It appears to only be skin-deep because it disappears if you skim the surface off in the lathe. I presume the expanding metal causes cracking within the oxide layer on the surface. Without knowing the exact make up of the die cast alloy, it would be difficult to figure out a plausible explanation as to what's going on.
Click to expand...
Ahhh, Cam. hope not! (keep expanding)
When recommissioning the AE, one fork leg was frozen solid and I had to drive out the piston.
I ordered a new one however when it arrived it also was too tight (presume it had "grown" on the shelf!!) I do not have a lathe so took one along to my regular engineering shop and he turned it down to the same size as the good one from the other leg - which fitted both legs.
Currently I am not happy with fork performance however I am very close to needing a new front tyre so was waiting till I took the front wheel off for that, I was going to dismantle the forks, replace oil and make sure all was ok. I am sure (hopeful) that won't be the issue as surely they wouldn't continue to "grow" in a bike used regularly?
 
As for the changing pistons, I couldn't work it out. Without a lathe I was reduced to filing the bloody things down but soon after fitting them the same problem returned. I went and bought 2 new ones from Phil but they were miles too big so I gave up.
What about the Maxton replicas, anyone know anything of them other than the £2500 price tag [or so I believe]
 
Personally, I don't think it's worth spending £2500 to fit fancy Maxton internal cartridges to old 38mm forks. They'll still be bendy 38mm forks on a 40+ year old tractor of a bike. If you're going to spend that sort of money, you may as well fit a modern set of USD forks. The most I'd spend on a set of old Ceriani or Marzocchi forks would be a couple of hundred to fit a set of gold valve emulators. They'll improve the damping and ride comfort without over-capitalising.

The quality control that came out of the Ceriani factory in the 70's was a bit haphazard. But with a bit of tweaking, you can get them to work reasonably well.

One issue was the unreliable fit of the original pistons, which results in supposedly identical forks having different damping characteristics.

I've found that the ID of Ceriani fork legs is somewhat variable. If there's too much clearance the oil will bypass the damper valve. Unlike Marzocchi forks, Cerianis have no piston rings so the only way to minimise leakage past the piston is to make them a very close fit. If you're using gold valve emulators you could be chasing your tail trying to get reliable damping control if the pistons are letting a lot of the oil leak past.

When I make Ceriani pistons I aim for a clearance of 0.05 - 0.1mm. That'll allow them slide quite freely and seal reasonably well. However, there's more variance than that in the tube ID between forks, so the pistons need to be individually matched to each fork leg.

For the best results, I'd suggest you take the fork legs to your machinist and ask him to turn the pistons down so they have 0.05 - 0.1mm (2 - 4 thou) clearance in the bore of the fork tube. Make sure he's aware that he needs to match each piston to its leg and label them so you don't mix them up (although you might be lucky and have 2 forks with the same ID).

But as I mentioned before, with that close a fit, it may not be too long before the pistons grow enough to bind again. So better still, get your guy to make new pistons out of 6000 series aluminium billet (he can copy your old pistons or use the attached drawing).

You don't have to be as fussy with Marzocchi pistons. They can tolerate greater clearance because they have a nylon piston ring to seal against the bore of the fork tube.

I have thought about making ring grooves in the Ceriani pistons and turning up some plastic rings (nylon or delrin or some such), but it may not be worth the effort for a small % gain in damping performance. I haven't had another set of Cerianis come across my work bench recently to try it with anyway.
 

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Mine have Teflon rings, I first heard about this from Mr Racetech himself who we ran into at Wakefield Park, he does Suspension class in Oz and we were watching Crispin racing. He turned up in Chrispins pit and started talking about adding a Teflon ring to pistons. I had Terry Hays do this approx a year later. It took that long for the bits to arrive in Oz. BTW it took 2 goes to get this right, his first attempt was too tight so it went back into his lathe to trim a bit more off the Teflon.
 
Tippie said:
All my Ceri damper pistons are fine with no sign of this in two sets of forks. I check them closely especially after this problem was raised. How common is it?
Click to expand...
Took three or four Ceriani forks to bits over the last year. Two had stuck pistons.

Paul
 
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