Original fork 1'', new fork 1 1/8'' I put a picture of my original frame. The internal diameter of the straight cylinder is 31mm.
In this frame was installed an original fork of diameter 1''.
I would like to buy this fork : https://www.bike-components.de/fr/Surly/Fourche-Rigide-Straggler-Disc-28-700c-p38261/
but the diameter is 1 1/8''.
I wonder, since there is some space between the frame and the 1'' fork, if I can installed this 1 1/8'' fork ?
I will need also to buy a new stem, but it's ok, my problem is more about the compatibility frame/fork.
Your frame seems to have a non-standard head tube diameter or you mis-measured it.
Here you can see a list of headset standards: https://www.parktool.com/blog/repair-help/headset-standards
There is no headset standard where head tube internal diameter would be 31mm.
For 1+1/8&quot; forks and bicycles with external headset cups, the inner diameter of head tube should be 33.8-33.9 mm.
Only if you can find a headset that is compatible both with your frame and with the 1+1/8&quot; fork, you can install it.
I would measure the head tube internal diameter again. The external pressed-in headset cups might be affecting your measurement.
Do note that 1&quot; fork has 1&quot; = 25.4mm outer diameter. The inner diameter is 22.2mm
A 1+1/8&quot; fork has 28.6mm outer diameter and 25.4mm inner diameter.
So if your fork had 25.4mm inner diameter, it's a 1+1/8&quot; fork.