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I bought a secondhand suspension fork online and the steerer tube has been cut to 170mm which I thought was plenty from what I calculated, turns out not so much.

After doing a quick test install, the full stack height of the headset and stem brings the steerer tube exactly flush with the top of the stem.

I have some spacers coming tomorrow and plan to put a 5mm spacer underneath the stem to allow pre-load.

I am unsure if this is safe or alternatively if I should put the spacer on top of the stem. Could I risk the top bolt not clamping onto the steerer tube but putting the spacer under?

The third image is roughly what I plan to do with the spacer

Just reaching out for some advice as I only started learning about all this 2 weeks ago!

Is my plan sound?

(Click images for larger)

Stem is flush with the top of the steerer without spacers, from above

side-view of the same arrangement showing how close the heights are

this is roughly what I plan to do with the spacer

2 Answers 2

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I would look for a 2mm or 3mm spacer for that.

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  • Thanks for commenting! I actually have a spacer set coming with a 2.5, so I will give that one a go :)
    – Thierry 95
    Jun 5, 2023 at 17:36
  • It all depends on how much space actually gets sucked up when tensioning the top cap. Jun 5, 2023 at 18:13
  • Is there a way to know if there isn’t enough space? I gather I won’t be able to see, would there be okay on the fork, so if that’s the case I try 5mm?
    – Thierry 95
    Jun 5, 2023 at 18:59
  • Okay - play*** Sorry
    – Thierry 95
    Jun 5, 2023 at 19:00
  • Yes, if you bottom out you would expect there to be some play in the headset. Jun 5, 2023 at 19:01
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If the last photo is with a 5 mm spacer, IMO that's too much.

The top pinch bolt in that last photograph looks just about the same level as the top of the steer tube. That means the stem is both going to lose at least some of its "grip" on the steer tube and be under stresses it wasn't designed for. I suspect a torqued top pinch bolt with that spacer in place will be squeezing the stem where there's no steer tube underneath. I'd think the stem might then be prone to cracking in the area of the top pinch bolt.

I'd think it'd be best that the steer tube extend at least high enough to reach the same height as the top of the top pinch bolt's washer (or bolt head if there's no washer). That would ensure the bulk of the force from the top pinch bolt is only applied where there's a steer tube underneath to support the stem.

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  • That last pic is what I am thinking to try and do, there is no spacer but I just moved the stem up for the photo. Maybe 5mm will be small enough. Useful insight though, I will keep all this in mind when trying different spacers . Do you think I could potentially put a spacer on top of the stem or do spacers always have to at least be around the steerer? Thank you
    – Thierry 95
    Jun 5, 2023 at 19:04
  • @Thierry95 I don't see how a spacer on top of the stem would work with the steer tube that short. Jun 5, 2023 at 19:06
  • Ahh okay I thought that may have been the case. Thanks for confirming, I will give it a go with the spacers below when they arrive tomorrow
    – Thierry 95
    Jun 5, 2023 at 19:42

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