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Simple Seat Post/Tube question from a MTB newbie

(Please see attached photo- a pic is worth a 1000 words)

I have a Saracen 6061 Aluminium bike MTB frame(approx 4 or 5 years old) , i need to buy a seat post, however, i am confused about the correct size to buy, i keep getting slightly conflicting advice from bike "experts"?

The photo attached shows my seat post tube on the frame and i need to know what size i should buy which would fit it please?

I looked on Sheldons seatpost guide site but my british frame is not there.
Once again thanks to everyone who has helped out so far, the bike is slowly taking shape, from the frame up.

thanks regards,

I'm not sure you can determine the size from the photo. It is the inside diameter of the seat tube that matters. I suspect that 31.8 is the outside diameter of the tube which tells you nothing. Even measuring with a good caliper can be tricky as posts vary in 0.2mm steps and you have to have the right size.
If you can't find an online listing or a Saracen dealer who can tell you, you may need to bring it into a shop. Check first, but most shops have a tool for measuring that will tell you the right size.

Hi DaveM,
What can i say, many thanks!, once again bicycletutor has come up trumps but especially DaveM you are the topman (Alex is super as well)!
I was advised that if i got a seatpost within a + or - 2mm range that would suffice?
I think i will have to bring it to a shop, a few miles away, and hopefully they will have the seat-post measure tool i believe that is the best advice i can take, thank you.
I checked out Saracen before, n o-website or company at present, and they went into liquidation and then i think the original owners have bought the company and started Saracen Cycles or something similar.
DaveM if i had a cycle business, you would be numero uno in the company,
PS:- DaveM where, approx, are you situated in the world please?
- NewbieAM -England

Make that rather 0.2mm. With seatposts, a fraction of (ok, let's say 1/2) a mm can be the difference between "fits just right" and "stuck" (or "slightly too loose, moves when you sit down"). I had problems with a seatpost that was just slightly too small (.4mm). Go to the shop, they most probably have the sizing rod as well as a selection of seat posts (though seat posts come in an gazillion different sizes, brands, types, so they might have to order the right one). I did that, but only after slightly screwing up the tabs at the seat tube that form the clamp on older steel frames. Stupid me! (plus, I still wonder how the previous owner managed to ride that bike, though I have to admit, the post was stuck in place when I bought it).

I guess there might be some frames that you can get away with the wrong size post by 1 step (0.2mm). But they are probably lower end steel frames. On a decent aluminum frame, I definitely wouldn't recommend using anything but the exact size. Too big and it will seize in the frame if you even get it in. Too small and you may damage the clamp area or have slipping problems.
Los Angeles

Hi Guys,
Thanks for the help, if anyone is new to biking and seatpost fit issues it will certainly help a great deal to read all the sensible advice in the posts above,
many thanks,
thanks regards,

newbie did u find out what size it was ? 31.8 is a popular mtb post size and will be the size of the clamp when closed

Boardman Team Road/Race Bike

Felt 620 MTB


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