It kept loosening when riding even only moderately rocky descents.

I removed it and replaced it with a new one; i cleaned the inside of the steerer tube with alcohol (removing any hint of grease).

Tightened the central bolt into the SFN but it still seems to only get to certain level of tightness. I am concerned it may simply self-extract if i cntinue to tighten more.

Any advice?

How tight should i be able to tighten it? Or, should it only be possible to tighten to a certain amount?

Any help gratefully received.

Do you have the right size for the internal diameter of the steerer. IIRC there were different sizes for thing steel vs thick ally steerers.

It is a SFN and not a connex/headdoctor/etc ?

Headset assembled correctly?

Enough space between top of steerer and top cap?

Stem bolts loose while adjusting?

Confusing brake pads rocking in caliper for headset play?

Have you put it in the wrong way up? I don’t see how a SFN can loosen unless it is a, the wrong size or b, put in the wrong way.

What headset is it? I used to have to crank down on the SFN really hard with my Hope(less…) headset due to there being too much friction between the headset parts and the steerer. ….And because I could never get the headset properly tight, it’d always work loose really quickly. Other headset only require gentle tightening of the SFN.

It shouldn’t matter if it does loosen – the stem is holding it together

no way it should be able to. the stem should be tight enough to hold it on.

you should be able to tighten headset, tighten stem and then remove top cap.

something wrong with the stem not the sfn imo.

It shouldn’t matter if it does loosen – the stem is holding it together

You’d be surprised how much leverage the fork puts on the headset, and in turn onto the stem. Even with a Raceface Diablous stem and the stem bolts done up stupidly tight, the headset would work loose with one run down Whistler Bike Park unless I took every last bit of play from the headset. I ended up having to hit the top of the stem with a hammer to take out the play, although the ultimate (and better) solution was to ditch the headset.

Not tightening stem before you tighten up the SFN are you? Probably a silly question…..

Walleater – if your stem was sliding up the steerer something is badly wrong – it should clamp on absolutely solid – after all its a very similar clamp to the way motorbike forks are clamped in the yokes and they don’t shift.

It kept loosening when riding even only moderately rocky descents.

It shouldn’t move really – the object of the SFN is to take up the tension in the headset and the the stem clamp should holid it in place – in theory you could get rid of the top cap and SFN once the stem is tightened as it doesnt do anything else. Are you sure the stem bolts are tight?

TJ, the problem was doing my head in and I assumed it was the stem that was at fault so swapped out the Thomson that was on it and bought the RF. The problem persisted. So that was a bit waste of money. The problem only stopped when I changed the headset. The fork in question was a 180mm 66 and Whistler BP is known for killing bikes.

NB once the stem is fitted you can discard the top cap and bolt and forget about the SFN until next time you need to adjust the stem. The stem bolts do the clamping, the SFN is only there to apply preload.

Walleater – I suspect the crown race was not bottomed out on the fork crown so that was moving down creating play – a mm or two is enough to create significant play. There is no way the stem should be sliding up the steerer

TJ – no, I’m a cycle mechanic and I installed the headset (and it’s replacement) myself using the correct tools. The issue was with the top seal on the headset clamping too tight against the steerer which stopped the SFN from working which in turn would never fully tighten the headset properly unless I used a big hammer….

Tj – how could that happen repeatedly?

Otherwise what SFB said (If everything is working correctly).

1. once fitted you do not need the SFN or top cap.
2. if your stem comes loose then two things could be happening. Steerer is sliding out of fork crown, stem is sliding up steerer. There are no other possibilities. Either your forks are shagged or your stem steerer interface is a bad choice, regardless of where you ride.

It can happen in very hard riding situations (and having seen Will ride he rides hard 😉 )
We have/had some vibratory equipment at work that used an almost identical system to transmit force to the next section and until we put a top cap system with locking nuts on it would work loose in 10 minutes.
Obviously there is a possibility that the odd steerer tube could be undersize which would magnify the problem.