Orange Clockwork-sinking seatpost problem

[frank.youngman]

I have a 1992-1993 Orange Clockwork.Seatpost is a 26.8mm NOS DX,however I have this weird problem.The bike is used on road only and having set the seat height and marked it with a scratch mark I find that over the course of a typical ride,say 20 miles,the seatpost will gradually slip down inside the seat tube,reducing the saddle height by 10-12mm.

The post has been lightly greased with a smear of waterproof grease to avoid the dreaded post stuck in seat tube syndrome,but I do this to all my bikes and this is the only bike with this problem.

Seat clamps are new off eBay,but I have tried three different ones,all of which work on other bikes and none of which prevent slippage on this one.Seat post is a 300mm with approx 150mm inside the tube.

Any suggestions would be appreciated.

 

[jimo746]

Re:

Have you got some vernier calipers out and actually meadured the I.D. of the seat tube? I don't think there's any guarantee that all Clockworks had the exact same internal dimensions?
Does the post slide in quite easily? (oo-er!)

 

[frank.youngman]

Re:

Vernier measures 26.8mm but post slides in with a little pressure,but no great force required.Post itself is a precise 26.8mm.

My current thinking is that either cheap QR clamps don't generate enough clamping force or that the inside of the tube is slightly worn and I should go for a longer post in order to get to an unworn part of the tube.

 

[MatBH5]

Re:

I had this issue with a brand new Prestige in 1992. Eventually the shop swaped out the 'post for a slightly larger diameter one (I recollect 27.0 rather than 26. - it was also quite a nice Suntour 'xc' post to match the MD groupset, so I was pleased.
I have recently bought a '93 Clockwork and it came with a 27.0 'post, so there was certainly some variation around this time.

 

[hamster]

There is always some degree of heat distortion during welding. If you have an example which barely distorted then you might find a larger post fits. The simplest is to suck it and see if you can get a 27.0 to go.

 

[frank.youngman]

Re:

Interesting info regarding Prestige,I have tried a 27.2 but that just would not go.

Bike came with a 26.8,but I guess that I will have to get myself a 27.0.
The seller reckoned the Clockwork was a 1993 but while it has the sloping top tube it has the separate cable guides on the top tube and unfortunately the powder coat has filled in the serial no.

 

[Silvergoat]

Re:

I have an early Clockwork which takes a 26.8mm seat post. The seat tube (with rear-facing slot) should be 28.6mm. Had similar problems with post slippage. Considered getting a 27.0mm seat post, but didn't want to risk it seize in the tube. After measuring up with callipers, I ended up buying a 31.8mm seat post clamp (Superstarcomponents) to use in addition with the existing, in order to avoid over-tightening the braze on clamp on the frame. Nice and solid now, with a touch of colour coded shiny bling.