best shim for alu seatpost in alu frame?

[EggyBum6969]

im not sure which shim to choose

would an alu shim be ok for a
alu seatpost in a alu frame?

the one i need is for a 27.2 seatpost in a 30.9 frame

i was going to get a plastic one but ive read bad things about the use plastic shims, being a bad fit or slipping about :shock:

 

[02gf74]

I would vote al, same as for bb cups.

 

[RobMac]

Re:

I dont use shims on seat posts.

 

[velomaniac]

I do use shims but unlike Rob^^^ I will elaborate. Aluminium shim will work best, I too have found plastic ones slip. The oxide coating that forms on the surface of aluminium provides grip especially between aluminium surfaces. Important to coat post and or shim/seat tube with anti seize grease like copperslip as aluminium can effectively weld itself together via the oxide coating if left untreated.
Shims are sometimes viewed unfavourably in that they grip and thus support only a small portion of the total length of the seatpost. On a long seatpost forward and rearward movement of the post in use can weaken the post at a narrow pinch point caused by the shim and can work the post loose. Personally never had serious issue with this.
The correct post for a given frame is of course the ideal but decent posts are pricey and shimming a common size makes more economic sense.