Calculating correct steerer length for new forks

'Minor' problem with the road bike, the stem has seized in the forks. It isn't going to come out, so it has been cut and the whole fork removed with stem part still in tube. (Yes, I could try to cut out the seized inner, I will, but I think I should plan for the worst.)

So now I need new forks.

The headset is also 20 years old and there is corrosion on a few of the ball bearings.

It is a 90's Koga Miyata, 58cm frame. The fork steerer measures about 160mm.

Unless there is someone on the forum with a nice, low cost, 1" threaded carbon fork with 160mm steerer, I must give in a convert to threadless.

I don't want to add risers, I want to keep the stem as low as it ever was, roughly what length of steerer should I expect to need on a threadless fork? 170? 180?

I understand the ideal is to cut down to size, but I was hoping for a helpful calculation, so if I buy an already cut fork, it isn't too short.

Thanks.

if it was 160 threaded, I'd say add about 40-45mm to that at a bare minimum. It depends on the current and future stack heights.

Measure what current headset adds to your headtube, double it.

Measure the depth of the stem you want to use, add a bit.

Add these together and then add to the length of your headtube and buy a fork with a steerer that long.

You'll have to either run a spacer or trim the steerer but either of those options beats having bought a fork with a very slightly too short steerer.

A machine shop with a milling machine would be be to mount the forks horizontally and then put either smaller milling cutter down the remains of the stem until its thin enough to break it up or if the have a cutter of the correct size mill the remains of the stem away.

If the stem is Aluminum then the caustic soda will work.

Thanks a lot. Any ideas of a location near elephant and castle that I could trust with the forks for the milling option?

Otherwise thanks a lot for the rough calculations.