How to measure stem size ?

[pedelpower]

Hi wanting to get a shorter stem for my road bike as I think it will make it mor comfortable on long rides not wanting to spend to much so looking on fleebay but not sure on how to measure my old one so I get the right size on bars and forks. Bike is standard Raleigh R100 that I use for winter but slowly upgrading the parts for a more custome fit.

 

[slowmotion]

Measure the diameter of the bars where the stem clamps on, and measure the diameter of the steerer tube ditto.

 

[andrew_s]

The R100 is threaded headset/quill stem, originally 7-speed rear.
The forks will be 1", but the bars need measurement with reasonable calipers as there are several similar sizes, and you really ought to get the right one (25.4, 25.8, 26.0, 26.4). A stem with a removable faceplate is both less fussy and easier to shim than a single bolt stem.
More recently both road bikes and MTBs have standardised on a 1 1/8" steerer and 31.8 handlebars. Stems for these are now much more common.

 

[pedelpower]

Thanks makes sense ill try measure tomorrow and see what I come up with