How do i measure my steerer tube to find out what size fork it will hold?

Question

I have a fork in there now, i just want to upgrade it. im not sure how to get the correct measurement with the headset and fork still on. i ride the bike, therefore i dont want to remove the old fork until i have a new one. Its a Raleigh 410 road bike 70's - 80's. if that helps.

Answer 1

I assume you have a threaded fork. You can measure it while on the bike. Use a tape measure. Measure from the bottom of the head tube to the the top of the lock nut. I'd then deduct 1/8" as the measurement will be a little long due to the lock nut top.  

Answer 2

I think if you go NEW, There should be enough lenght for your bike unless it has an unusually long head tube.