La Brea Bike

I have now tightened a guy's FSA Carbon Fork Plug for him multiple times, and it lasts about a week before I have to do it again due to the tension bolt backing off. This is the only time I have seen this type of repetitive situation. Is this something that these things are known for? Is there a preferred plug that holds better? Thanks.

Litespud

the plug is only used to set headset preload when installing the stem. Once the stem is installed and the stem bolts tightened, the plug serves no purpose except to possibly provide some internal support to the carbon steerer. If the headset is loosening up over time, you should be looking at your stem bolt torque or considering some installation paste to stop the stem slipping. The stem plug isn’t the problem.

Andrew R Stewart 

There are a few differing designs of compression plugs (not including a basic star nut which is not to be used in a carbon steerer). One is glued in the steerer and tends to be fork specific. The two types taht are secured with an expanding section differ in what bolt does what job. The one bolt versions use the stem cap's draw bolt to both drive the expanding portion of the plug tight inside the steerer and also pull the stem cap down to preload the bearings. One bolt two jobs, often not the best choice. Style two has the expanding plug done by a hollow bolt at the top of the plug. The stem cap's draw bolt is then threaded into this hollow plug bolt. One step tightens the plug the second step/bolt preloads the bearings. Much easier to work with.

Either type of expanding plug usually needs to differentiate between the friction the wedges and "brake shoes" have and that of the bolt so applying grease to the proper surfaces allows better tightening and future removal. Carbon steerers have a max amount of plug tightening allowed before the steerer is at risk of fracture. Balancing all these factors is important to having the plug do it's two jobs, only one of which is needed after the stem is pinched in place. If the plug is coming loose and you can't understand and fix that then you owe the rider to find someone who can. His health and safety depends on his fork/stem remaining a solid item Andy

Barry2 

A metal steerer can use a star type plug to provide enough traction to preload the headset bearings.
A carbon steerer requires an expanding plug. This expanding plug helps stop the stem crushing the carbon steerer and provides traction to preload the headset bearings.

Do check that the plug is inside the stems clamp. Having the plug too low in the steerer, will stop it supporting the stem clamp.
Oh and don’t forget to use the correct torque on both the plugs expander and the stem clamp bolts.

Barry

PPS: Carbon Assembly Paste is your friend. The grit in the paste helps stop bits moving around. After all, with a carbon steerer you can’t just lean on the wrench !

La Brea Bike

Good call on the depth of the plug. I would have expected that the gaps would be different if it was too low, but that makes total sense. I'll verity! Thanks.

wesmamyke

Is it the style you have to secure with a larger allen? Usually 8mm allen to tighten the plug, and the cap uses the normal 5mm. I always find those loose for some reason. I have a Hope brand one that has always worked for me.

Andrew R Stewart 

I think the reason why the two bolts for the two functions plugs can sometimes loosen is that once in place and the stem cap draw bolt is in installed most riders (and shops) don't remove that cap and bolt after initial use to re torque the plug's bolt and take up the small, but there, slight compression that a carbon steerer sometimes have. Andy

shmuelyosef

It is widely believed that carbon paste is a bad idea in carbon steerers...the expanding plug should be done properly and torqued to the set specifications for both stem bolts (5-6 N-m) and preload (take out all the play, but no more). Make absolutely certain that your expanding plug fills all the space under the stem...for this reason, it is obvious that the minimum expander length must be 40mm (the most common stem clamp height). Generally should be a little more and should be (as well as can be done) somewhat centered under the stem clamp. It is good practice to use at least a 50mm expander...I tend to use 70mm or 100mm expanders for confidence...they weigh very little and I don't have to worry if I juggle my stem height a little.

shmuelyosef

Absolutely correct!!
The very last thing that you should do is slack off the stem clamp bolts, tighten up the top cap until you just barely feel friction on the steerer, then re-tighten the stem clamp. I do find this works best with snug fitting spacers (I use the Wheels Mfg aluminum alloy spacers...they are individually precision machined).

shmuelyosef

One last thing that I forgot to mention, along the lines of universal solution.
High quality, reasonably affordable and light compression plugs are available in lengths greater than 50mm. You can place these, leave them where they are and move spacers around knowing that you are covering the 40mm where your stem is clamped and not fretting.
The two that I use are:

Origin8 Fusion Expander Compression Plug - 90mm

Deda Elementi Expander Bung - 70mm

Camilo

Really? this is the first time I've ever heard that. Is there a source for this? Thanks.

Barry2 

Is this still the case if the Stem and steerer are both Carbon?

Barry

shmuelyosef

Check out this article
A tech bulletin from Trek regarding carbon assembly paste

shmuelyosef

As a notable example...I just opened up a Cannondale carbon steerer and found this:


This was not only much smaller than the stem compression zone, it was mounted offset (about 20mm lower than shown here)...providing little support for the steerer tube.