Thread on a Campag Gran Sport Front Derailleur?

[rrarider]

I have a mid 80s bike, fitted with a Gran Sport front derailleur. The screw connecting the two sides of the cage at the rear is stuck and the slotted head is damaged. I may not be able to free it without irreparably damaging it. Does anyone know what diameter and pitch the thread might be. The screw passes through what I presume must be a hollow cylinder as a shroud and engages with a nut on the other cage plate. I can't free the nut as it's semi captive in the other plate.

I call it a 'screw' as I'm guessing that it is threaded for its whole length. Some might call it a 'bolt' as it engages a nut but I only call a bolt a part which is only threaded at the end. Either way I would like to replace the screw and the nut.

 

[GilesM]

It will almost definitely be a metric screw, I would guess at M3, can you see enough of the thread in the nut to measure it, M3 would be 3mm diameter of the threaded part of the screw, good place to find quality screws of all sizes is here:
http://www.westfieldfasteners.co.uk/

 

[rrarider]

Unfortunately I can't measure the thread in a reliable manner, as hardly any of the screw protrudes from the nut. I also guess that it will be about M3, or maybe M4. If I knew for certain that the cylinder was a hollow part, then I would be more confident of drilling out the screw from the nut end. In that respect, an exploded diagram of the Campagnolo Gran Sport front mechanism would be useful and maybe reassuring. It's not vital for me to undo the screw and nut but it would give me a bit more slack to play with when I drive the rivet back into the chain after I've cleaned it.

 

[Ajax Bay]

I have been out to check my old bike (ALAN) and pretty sure my (replacement for a Nuovo Record) FD is one of these. With a little trepidation, I will remove the screw which meets the description in the quote below and try to determine its diameter and pitch. It certainly looks like the screw goes through a hollow cyclinder but I cannot remember dismantling it.

The screw passes through what I presume must be a hollow cylinder as a shroud and engages with a nut on the other cage plate.

 

[Ajax Bay]

The bolt is just over 1/2" long, the pitch is 30tpi and the outside diameter is 3.45mm (apologies for the mixed dimensions).
The exam question is actually whether the bolt has a standard thread (along its complete length - screw/bolt!). It has, and I've tried a random pair of bolts and they are 'right' but I don't know my M3s from my M4s (may I be thoroughly stricken down as an ignoramus).
The hollow cylinder has the same OD and ID of the ones in a normal Shimano RD jockey wheel (if you lose the current one on the garage floor) but maybe 2mm shorter.
HTH
Mine now refitted with some threadlock. Btw, surprised you would want to take this apart each time you split and clean the chain. Creates a mode of failure, imo, for no significant benefit.

 

[midlife]

Likewise, I'm just curious why you want to take it apart ?

 

[rrarider]

The bolt is just over 1/2" long, the pitch is 30tpi and the outside diameter is 3.45mm (apologies for the mixed dimensions).
The exam question is actually whether the bolt has a standard thread (along its complete length - screw/bolt!). It has, and I've tried a random pair of bolts and they are 'right' but I don't know my M3s from my M4s (may I be thoroughly stricken down as an ignoramus).
The hollow cylinder has the same OD and ID of the ones in a normal Shimano RD jockey wheel (if you lose the current one on the garage floor) but maybe 2mm shorter.
HTH
Mine now refitted with some threadlock. Btw, surprised you would want to take this apart each time you split and clean the chain. Creates a mode of failure, imo, for no significant benefit.

Thank you for taking the trouble to dismantle your front clanger. I was particularly wanting to know that the cylinder, is indeed, hollow, as mine is stuck and can't be rotated with snipe nosed pliers. I've got a box of mainly M4 dome headed, Phillips screws and nuts which look like they would fit. Their OD, as measured by my Vernier caliper is 4.08mm.

I want to dismantle my clanger mainly because I can see the head is damaged and I can't undo the screw. Having a screw which I couldn't undo when I wanted to, just plain offends me. As far as advantages when refitting the chain go - well I could just thread the chain through the chainstay prior to driving the rivet back and that might make it a tiny bit easier.

 

[winjim]

Btw, surprised you would want to take this apart each time you split and clean the chain. Creates a mode of failure, imo, for no significant benefit.

The idea is that you can remove this bolt and dissassemble the cage on the rear mech in order to take the chain off without splitting and rejoining. So you've removed a potential mode of failure at the chain itself.

 

[rrarider]

The idea is that you can remove this bolt and dissassemble the cage on the rear mech in order to take the chain off without splitting and rejoining. So you've removed a potential mode of failure at the chain itself.

You couldn't remove the chain from the bike entirely, as it would still be circling the chainstay.

 

[winjim]

You couldn't remove the chain from the bike entirely, as it would still be circling the chainstay.

The presence of a chain doesn't impede access to the chainstay in the same way as it does the mechs.

 

[Ajax Bay]

when I drive the rivet back into the chain after I've cleaned it

The idea is that you can remove this bolt and dissassemble the cage on the rear mech in order to take the chain off without splitting and rejoining.

Whose idea @winjim? Not the OP's, who is clearly splitting his chain to clean it (see quote). Read the thread (third post) with a little more attention to detail. But I agree that splitting a chain and rejoining by driving the pin back in would create a mode of failure, unnecessarily. A quicklink does the job for me, if I want to remove the chain to give it a thorough clean (perhaps once in a chain's useful life if it's been out in the weather).

 

[winjim]

Whose idea @winjim? Not the OP's, who is clearly splitting his chain to clean it (see quote). Read the thread (third post) with a little more attention to detail. But I agree that splitting a chain and rejoining by driving the pin back in would create a mode of failure, unnecessarily. A quicklink does the job for me, if I want to remove the chain to give it a thorough clean (perhaps once in a chain's useful life if it's been out in the weather).

Campagnolo's idea. They're not keen on splitting and rejoining chains, which in characteristic understatement they warn may have lethal consequences. They'd much rather you split the cage of the mech. But then they also specifically forbid you from removing the chain from the bike for cleaning.