# Help With Stem Cap and Explanation of Its Use



## digita6 (Sep 29, 2012)

Greetings, I've been wondering for a while what the exact function of the stem cap with expanding bolt is? Is it just to press the stem down to tighten it into place?

Below is a picture of my cap assembly. This has been really clunky to use, and is really heavy for what it does. Are there different types of these out there, and can someone recommend an alternative?

Thanks


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## DrSmile (Jul 22, 2006)

Theoretically you could ride it without the cap... but it's a nice feeling to know that if your stem bolts loosen you won't be holding on to your bar as your teeth dig into the asphalt!

The plug part of the cap also supports and protects the steerer tube from collapse.


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## newman (Nov 6, 2006)

Park Tool Co. » ParkTool Blog » Star Fangled Nut and Expansion Plug Installation

I'm pretty sure you would be grinding your face into the pavement if your stem bolts came loose,there shouldn't be that much tension on the top cover


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## Cinelli 82220 (Dec 2, 2010)

The stem cap is for adjusting the preload on your headset bearings.
It does NOT hold your stem or fork in place. Don't overtighten it or you risk ruining your headset. The plug supports the steerer tube so it is not crushed when you tighten your stem bolts.


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## digita6 (Sep 29, 2012)

Ok, so I should tighten just to the point that there is no play in the headset, but not wrench it down super tight then?

Is there a better design for this thing? The one that I have seems really clunky and awkward to tighten down.


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## eflayer2 (Feb 15, 2002)

*all different kinds*

if you have a carbon steer, then you need a design that tends to be more robust/complicated/heavy than the old star nut that is used on alloy steerers. The ones for carbon all tend to be a bit more clunky, more parts, heavier? than a star nut. 

a few options shown here:

Universal Cycles -- Headsets & Parts > Spacers & Parts > Compression Plugs, Rings & Star Nuts


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## Cinelli 82220 (Dec 2, 2010)

digita6 said:


> Ok, so I should tighten just to the point that there is no play in the headset, but not wrench it down super tight then?


Correct. Super tight=wrecked headset.

The plug is necessary as I said to support the steerer tube. A star-fangled nut needs to dig into the steerer to stay put. You don't want to be digging into a carbon steerer!


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## DrSmile (Jul 22, 2006)

If the stem bolts start to loosen, your handlebars start to slide around sideways if you have a cap on. If you don't have the cap on the stem and bar slide off the steerer. It has nothing to do with how tight the cap is on the stem. Even if there is absolutely no pressure on the stem, the cap will keep the stem from popping off the steerer tube.


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## jerrayy (Oct 27, 2012)

A little off-topic, but are there any possible cons of using one of the specialized headset caps that can let water, dirt, and debris in?


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## digita6 (Sep 29, 2012)

So how do I know how much to tighten the stem cap (i.e., how much preload to give the headset)? I use a torque wrench for almost everything to do with the carbon steerer tube, but have never seen any recommendations for torque on the stem bolt.


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## cxwrench (Nov 9, 2004)

depends on the headset. if it's an integrated headset you'll usually end up tightening it up a bit more than a regular press-in headset. basically you preload the bearings 'til there isn't any play. w/ an integrated headset maybe add 1/8-1/4 turn after the play is gone. there is NO TORQUE SPEC for any bearing adjustment.


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## Ullafk (May 19, 2012)

I find that putting the front brake on and rocking the bike back and forth works well to see how much play you have in the headset. If it knocks then it needs to be tightened up a bit more, only until it stops knocking and the movement is nice and smooth.

Don't forgot to leave the stem loose while doing this.


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## moskowe (Mar 14, 2011)

I absolutely hate expander plugs, for exactly that reason. I never know just how much I'm supposed to tighten it to preload the bearings, and half the time after tightening the stem to torque spec there is still play in the headset and I have to do it all over again...


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## cxwrench (Nov 9, 2004)

moskowe said:


> I absolutely hate expander plugs, for exactly that reason. I never know just how much I'm supposed to tighten it to preload the bearings, and half the time after tightening the stem to torque spec there is still play in the headset and I have to do it all over again...


your problem isn't expander plugs, it's not knowing how to install them and adjust the headset properly.


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## moskowe (Mar 14, 2011)

Yes, you're right. The problem is that there is little information is available online for people trying the DYI route with little mechanic experience.
I did figure out how properly do it in the end after looking for information on the internet and getting advice from my friendly LBS mechanic. Practice makes perfect, but every time I have to do it again I cringe a little bit.
I just find them to be a big pain in the *** compared to say, a starnut.


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## cxwrench (Nov 9, 2004)

moskowe said:


> Yes, you're right. The problem is that there is little information is available online for people trying the DYI route with little mechanic experience.
> I did figure out how properly do it in the end after looking for information on the internet and getting advice from my friendly LBS mechanic. Practice makes perfect, but every time I have to do it again I cringe a little bit.
> *I just find them to be a big pain in the *** compared to say, a starnut*.


i'm really having a hard time understanding this. you slide the expander into the steerer tube and tighten it up. no special installation tool, no pounding w/ a hammer. after doing this, everything else is exactly the same.


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## stevesbike (Jun 3, 2002)

there are alternatives that act like a starnut but are carbon safe. You then use a standard top cap and tighten for preload. You can get a top cap that has some depth if you are worried about clamping forces damaging the steerer tube, but stem bolts don't need that much torque - 4 Nm should be sufficient. 


http://fairwheelbikes.com/extralite-ultrastar-2-expander-p-29.html


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## cxwrench (Nov 9, 2004)

stevesbike said:


> there are alternatives that act like a starnut but are carbon safe. You then use a standard top cap and tighten for preload. You can get a top cap that has some depth if you are worried about clamping forces damaging the steerer tube, but stem bolts don't need that much torque - 4 Nm should be sufficient.
> 
> 
> Extralite Ultrastar 2 Expander : Fairwheel Bikes, Cycling Boutique


ALL expanders act like a starnut...that's the whole point of their existence. this is just an expensive, lightweight version of what everyone already uses. i'd be pretty damn careful tightening to 15nm...i would definitely want the stem on the steerer tube when i did it.


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## stevesbike (Jun 3, 2002)

cxwrench said:


> ALL expanders act like a starnut...that's the whole point of their existence. this is just an expensive, lightweight version of what everyone already uses. i'd be pretty damn careful tightening to 15nm...i would definitely want the stem on the steerer tube when i did it.


I meant in the sense of two separate pieces like a starnut and a top cap (as opposed to the integrated plug like the OP showed). 15 Nm is definitely more than needed. Tune also makes one that uses a silicone bit so there's no metal to carbon contact.

Tune Gum Gum Expander Plug : Fairwheel Bikes, Cycling Boutique


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## cxwrench (Nov 9, 2004)

stevesbike said:


> I meant in the sense of two separate pieces like a starnut and a top cap (as opposed to the *integrated plug* like the OP showed). 15 Nm is definitely more than needed. Tune also makes one that uses a silicone bit so there's no metal to carbon contact.
> 
> Tune Gum Gum Expander Plug : Fairwheel Bikes, Cycling Boutique


that Tune piece is pretty cool, virtually no chance of damaging the steerer. 

the part the OP showed in his photo is no more 'integrated' than any of the other parts we've looked at. you remove the black 'top cap/steerer tube support' from the expander, install/tighten the expander, install the stem, then put the cap on and adjust the headset. you can do it all at once if you understand how it works and you don't get the top and bottom bits stuck together. pretty basic if you take it apart.


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## digita6 (Sep 29, 2012)

Thanks for all of your help and sage advice. This forum is filled with incredible, helpful people. I read many of your other posts before attempting what's below. I picked up a Ridley expansion plug at my LBS, which is longer and looks like it applies far more consistent "opposing pressure" to the inside of the steerer tube.

Unfortunately, at some point prior to figuring this all out, I managed to crack the steerer tube, even though I was using a torque wrench. It got to about 5Nm (Deda indicates 8Nm on the side of the stem!). It snapped a Ti bolt on the stem, and there was suddenly a burning smell. I looked down and was horrified to see a crack in the steerer.

Fortunately, the steerer was already a bit high for me, and I was able to cut it just below the crack, and then use the new expansion plug. I gently clamped the fork in my park workstand, and used painter's tape and a pipe clamp (gently) as a cutting guide. 32tpi hacksaw did the job well, then sanded with 150 grit sandpaper to remove rough edges. All seems to be fine now--just rode Sufferfest Revolver on the trainer (raining like crazy here, not going outside today!). Pics below.


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## cxwrench (Nov 9, 2004)

nice job! looks like a perfect cut, so everything should be fine. the expander you got is a good one, so you're all set. nice work!


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## nolight (Oct 12, 2012)

This is what I remember about headsets and stems.

That little gadget with the stem cap in the first picture, when tightened, pulls the steerer fork up and presses the headset stack down, compressing the system. In order for this compression to work, the stem bolt needs to be loosened first, or else this compression is prevented by the tight stem. After tightening the headset enough, then make sure the handlebar is center to wheel and tighten stem bolts to tighten the whole system.

Headset should not be too loose or tight. 

To check if headset is too loose:
- Grab front brake and try to push the handlebar forward and back, you should not see the headset move relative to the fork
- or Lift the handlebar to lift front wheel off the ground and drop it, it should not give loose vibrating sound, which you will hear if headset is too loose.

To check if headset is too tight: 
Lift the bike on your shoulder and turn the front wheel to left or right then release, the front wheel should be able to swing freely and not stay in its place (especially when bike is tilting down).


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## moskowe (Mar 14, 2011)

All I can say is I wish this thread was there a year and a half ago (you can laugh now, cxwrench). 
Good everything worked out for you OP, and sorry about that Zero100 stem.


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## NWS Alpine (Mar 16, 2012)

I think compression plugs make it extremely easy to install forks on integrated headsets. The FSA plug is really heavy. The tune gum gum is really light but can be a little touchy to install properly for some setups. Cannondale is shipping a new plug on their higher end Evos that is only like 12g vs the old 50g version. I'm waiting for cannondaleexperts to get them in stock to buy one. It's basically a "hollowgram" of compression plugs it seems.

http://forums.roadbikereview.com/attachment.php?attachmentid=269767&stc=1&d=1354628375


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## kinguin7 (Apr 28, 2011)

"are there any possible cons of using one of the specialized headset caps that can let water, dirt, and debris in? "

No. The bearings are on the outside of the steer tube, the specialized caps allow things to fall inside the steerer and out the bottom.


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