Author Topic: Seat post shim - mandrel size?  (Read 3551 times)

Seat post shim - mandrel size?
« on: 01 January, 2023, 07:27:43 pm »
I'm assembling the bits needed to do a DIY replacement of my Brompton seatpost shim. I've found a cheap second hand 32mm reamer on ebay, which I'm hoping should be good enough.

Does anyone know the exact diameter of the 'mandrel' that is used to clamp the bush in place while the glue is curing? I have a lathe, so I can turn something to the correct diameter.

LittleWheelsandBig

  • Whimsy Rider
Wheel meet again, don't know where, don't know when...

Re: Seat post shim - mandrel size?
« Reply #2 on: 01 January, 2023, 09:54:40 pm »
Thanks, very useful, I'll find/make a 31mm rod.

Out of interest, does the reamer have its size marked on it? I was wondering if it's actually 32 mm or 1 1/4" (31.75 mm), as Brompton seem to use imperial sizes for some things like this.

The one I found has a morse taper, so I will make some sort of guide to keep it straight as it goes in.

LittleWheelsandBig

  • Whimsy Rider
Re: Seat post shim - mandrel size?
« Reply #3 on: 01 January, 2023, 09:58:18 pm »
Brompton seatposts are nominally 31.8mm dia.
Wheel meet again, don't know where, don't know when...

rogerzilla

  • When n+1 gets out of hand
Re: Seat post shim - mandrel size?
« Reply #4 on: 01 January, 2023, 09:58:22 pm »
These big reamers can often be turned with two large adjustable spanners on opposite sides of the taper.  It will still need some kind of guide, but at least you're not applying any off-centre turning forces.
Hard work sometimes pays off in the end, but laziness ALWAYS pays off NOW.