Resizing damper pistons

P

paulincayman

Hero member
Location
Cayman Islands
Hi All,
I have now new fork stanchions from Wolfgang but now need to machine my damper pistons to fit correctly. I had to persuade them to exit the tubes. I may well upgrade  the internals at some point but for now I want to reuse the stock damping . Currently one piston is a tight fit that pinches and the other won't go in at all . I believe they are swollen as mentioned by Cam on a similar thread. 
Can anyone tell me or point me at the correct clearance I need to have between the piston and stanchion. Do I need to make allowance for the typically warm to hot weather here.?
Thanks
Paul
 
If I remember correctly, they have 0.3-0.4mm clearance inside the tube.  If heat expansion affects damping, you may be able to compensate with oil weight.

Instead of machining the originals, I'd knock up a new pair from a decent grade aluminium alloy.

piet
 
That sounds a bit sloppy to me Piet. They'd rattle around in the fork legs!  If the originals had that sort of clearance (before swelling up), perhaps it was to accommodate variations in fork stanchion ID, thereby avoiding the necessity to tailor fit each piston to each fork leg. I don't think the quality control in the Ceriani factory was as good as it could have been.

I did them with 0.05mm clearance in the forks I sold to Maurice. I forget which bike he put them on (he has too many to keep track of!) but he reckons they work very nicely.

Here's a link to the post I did about it. https://www.laverdaforum.com/forum/index.php/topic,96832.msg391443.html#msg391443

Cheers,
Cam
 
sfcpiet said:
If I remember correctly, they have 0.3-0.4mm clearance inside the tube.
Click to expand...

I just did some arithmetic. 0.4mm piston clearance results in a gap of nearly 17 mm2 between the face of the piston and the cylinder wall. That's equivalent to drilling a 4.6mm hole through the piston. I reckon that much area for oil to bypass the piston could significantly affect the damping characteristics of the fork.
 
Piston ring would certainly be possible. 38mm Marzocchis have plastic piston rings.

You may be able to buy Marzocchi rings and cut grooves in the Ceriani pistons for them (depending on piston wall thickness). But it might be a simpler job to just make new closer fitting pistons. They're not a complicated shape.
 
Hi Paul. Just be aware that with close fitting pistons, they may not be transferrable between fork legs.

I found that the IDs of the forks I had differed by more than the clearance I was working to. I couldn't do anything about the fork IDs, so I just matched up the pistons so the clearance was the same in each leg. I numbered the pistons and fork legs with a permanent marker so I didn't mix them up during assembly. Despite the name, a permanent marker is only temporary. If you want something more permanent you could engrave or punch some ID marks on them.
 
That makes sense as a precaution..I did measure both legs and they do appear the same although .05 is pretty tight.  The new tubes are 31.97 as best I can measure. None of other the o bits fit either..and being an idiot I can't now remember which bits to all the way into the narrow section ( the piston does for sure and the heavier damper short( rebound ?) spring.. But do the perforated washers stay in the slightly wider first section?? I have a photo of everything assembled on the damper rod but of course it's on the bench and not inside the stanchion.
Duh!
 
It just occurred to me that I was using the appreciation "ID" to mean two different things.

ID of the fork leg = Internal Diameter
ID marks on the pistons = IDentification marks.

Sorry for any confusion.
 
There is a mod Race Tech does where they attach a Teflon ring to the pistons to direct all the oil through the pistons., it took a few attempts to not have it bind.
 
Dellortoman said:
It just occurred to me that I was using the appreciation "ID" to mean two different things.

ID of the fork leg = Internal Diameter
ID marks on the pistons = IDentification marks.

Sorry for any confusion.
[/quote

I abreviate your concern ...but no confusion caused.
Click to expand...
 
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