Going to attempt to lower the stem on a friends bike today. It has a carbon steerer, and an expandable bung pre-load device.

Just want to check the bung doesn’t need to be moved to keep in alignment with the stem? If my understanding is correct, the bung really has nothing to do with the stem, and is purely for pre-loading the headset bearings?

Cheers in advance.

Correct.

Yes, you are right. Leave the bung alone. It’s just like a star nut – fit and forget.

Nice one. Ta all.

Actually, thinking about it, one other question. Thread lock on the stem clamp bolts?

No – but observe the torque recommended by the Stem manufacturer.

It’s usually printed on the Stem near the bolts.

No, never needed to, but I’d always stick a bit of carbon anti-seize on the inner surface and tighten up to the stem specs (normally 5-6nm, but check for yours)

If my understanding is correct, the bung really has nothing to do with the stem, and is purely for pre-loading the headset bearings?

It’s not as simple as that. Many manufacturers require the part of the steerer enclosed by the stem clamp to have the bung inserted to the FULL depth/height of the stem, i.e. the bung serves a structural function to strengthen the steerer where it is clamped and resist any crushing forces on the steerer.

Given the relatively short depth/height of most steerer bungs, that usually means that only 5mm-10m at most of spacers can be placed above the steerer.

In practice very many people ignore this and get away with it, but it’s against many manufacturers’ advice.

I’d always stick a bit of carbon anti-seize on the inner surface

Both Trek and Specialiazed advise not to use carbon paste on carbon steerers, because of the risk of the paste migrating below the stem, and the inevitable small movements of the spacers causing the paste to act as an adbrasive on the steerer.

what slowster said – I’d be wary of clamping on a carbon steerer where there’s no bung inside to prevent crushing.

Oh, this thread started off so clean cut, and now there is a whole world of uncertainty!

Good points too ^. There are lots of cheap, but taller bungs that increase the available clamp area. Use one of these. Also no to threadlocking stem bolts, unless you really have a problem with them undoing. And yes to carbon paste on the steerer/stem interface as it will reduce slippage.

Both Trek and Specialiazed advise not to use carbon paste on carbon steerers, because of the risk of the paste migrating below the stem, and the inevitable small movements of the spacers causing the paste to act as an adbrasive on the steerer.

Use only ‘just enough’ ? I suppose you could start without and add a tiny bit if it slips?

Both Trek and Specialiazed advise not to use carbon paste on carbon steerers, because of the risk of the paste migrating below the stem, and the inevitable small movements of the spacers causing the paste to act as an adbrasive on the steerer.

As someone who has experienced a pretty rotten time with a seized seatpost in a carbon frame, I’m pretty sensitive to mixed-material seizing these days. Your point is quite valid though, but a bit of care to apply just enough mitigates this risk more or less entirely.

As a bonus, anti-seize also improves the joint between the two materials, so for a given torque, the grip will be increased. This is particularly true with seatposts, but will apply to stems and handlebars too, I’m quite sure.

I’d be wary of clamping on a carbon steerer where there’s no bung inside to prevent crushing.

As a note, because you are clamping a circular stem onto a circular post, I’d expect the clamping forces to be largely radial, rather than linear – i.e. it’s crushing inwards on all points on the steerer at the same time – and thus it doesn’t really matter whether you have a bung or not, it shouldn’t break under the relatively low forces involved.

Frankly, if you need the bung to maintain integrity, you’re either clamping it too tight, or your steerer is way too thin! 🙂

Use only ‘just enough’ ? I suppose you could start without and add a tiny bit if it slips?

a bit of care to apply just enough mitigates this risk more or less entirely

it doesn’t really matter whether you have a bung or not, it shouldn’t break under the relatively low forces involved.

Trek and Specialiazed are unequivocal in saying no carbon paste on steerers, and other manufacturers also state that the bung must be in the same plane as the steerer clamp (I think if the bung is only half as deep as the steerer, then there might be the potential for a stress riser).

Carbon steerer breakages are rare, but they do happen and the consequences are potentially fatal. Trek and Specialiazed are two of the biggest bike manufacturers in the world, and given the potential financial consequences of getting this wrong, especially in the litigious USA, I would expect their advice to be the best.

It’s also worth noting that they don’t give a hoot about seized parts, because that will always be someone else’s problem, not theirs.

Still your point is well made, thanks.

OK, so I’m not going to use thread lock, and not going to use carbon paste. I’m comfortable with that.

But what about the bung, jury still seems out!

You aren’t going to ride it….

Just crack on…

But what about the bung, jury still seems out!

The jury is not out as far as the manufacturers are concerned: I don’t know of any that say otherwise than that the bung should line up with the stem clamp area. The leaflets supplied with stems usually all have a diagram and instructions similar to this one

If bung does not have a lip at the top, it may be possible to undo it and re-position it lower down inside the steerer. If so be careful when doing this because it is easy for the bung to slide down further than you want, and to have to fiddle with bits of bent wire etc. to pull it back up. It’s also probably not something that was intended to be fitted and re-fitted multiple times in the same fork, and I would be concerned that doing so too often might cause damage (scoring) to the inner surface of the steerer tube as a result of the rotation of the bung before it sticks and starts to grip.

If the bung does have a lip at the top, then unless it is long enough to accomodate the stem being lowered you will either need to cut the steerer or replace it with a longer bung. Obviously cutting the steerer is irreversible. Longer bungs may or may not be available for the steerer, depending upon what the internal diameter is. The longest I have seen is this one, which is 90mm long, and which I have fitted to one of my bikes to allow me to change the stem height within a wide range.

Pull the bung out to check it’s height and whether it will support the steerer in the new stem position. If it’s too high, then cut the steerer to suit. You might want to cut the steerer anyway for a neater job.