dksix

I bought a used frameset with headset but no cap on the expansion plug in the carbon fiber steering tube. I have to buy a cap but can't find one for an expansion plug alone and I'm wondering if the ones for star plugs are the same as the ones used on expansion plugs? I can find ones that are listed as just "stem caps" but I'm not sure if they will work or if I should just buy a complete expansion plug set and swap the plug in the steering tube so everything matches.

wschruba

Some plugs use their own caps. Post a picture.

dksix

@wschruba

McBTC

You are talking about what will be the stem top cap, right? I believe those should be pretty much interchangeable.

HillRider

They often are but not always. Expander plugs that use a center 6 mm bolt for headset adjustment use the same cap as a starnut.

However some expanders (Look and FSA that I know of) have a center post with male threads the sticks up and use a top cap with a cylindrical projection on the under side that has female threads. This cap threads onto the center post and has a hole in it for as 6 mm allen key to make the adjustments. This cap is not interchangeable with a starnut cap.

fietsbob

in your picture the hex expands the piece into your steerer Tube once that is tight then you screw in a Bolt
to the inner threaded part ..

a standard bolt should be 6 x 1 thread per mm ..

But It appears the needs to be a bit more spacer under the stem (or above It) because the highest has to be the top edge of the stem

(or steerer tube spacer ) by 5mm ish or there is No headset Adjustment ..

You loosen the stem bolts on the back , then the top cap pushes down everything beneath it to adjust the bearings of the headset.

Then you tighten the stem bolts to keep the adjustment.

Your First Threadless Headset bike?

wschruba

As the above poster[s] (all three) said, that specific expansion plug looks like it takes a normal M6 bolt through a normal top cap. As long as it fits your spacers, go nuts (also, with CF steer tubes, you generally want the stem entirely on it, which will require you to put a spacer above
the stem).

Measure the steer tube to see whether that means you need a 1.125", 1.25", or [very uncommonly] 1.5" top cap.

dksix

@fietsbob & @wschruba

No, 3rd threadless bike. I thought it needed another spacer (I have extra) but the diameter of the rim of the plug going into the steering tube is slightly larger than the diameter of steering tube itself. With that I wasn't sure that the stem wasn't to sit below the rim and some kind of a cap that was relieved for the rim's larger diameter. It looks like the OD of the steering tube and the OD or the rim of the expansion plug should be the same, if not the stem is going to clamp the rim of the plug before clamping on the steering tube, is that the way it should be?

dksix

@fietsbob & @wschruba

I went back out to measure and take better pictures but the diameter difference was an illusion. I added another 5MM spacer and used the stem cap from my MTB and it worked fine. Seems that it does use a standard stem cap, at least an OEM Fuji was the same. Thanks all.

dr_lha

Your steerer is 1.125 inch. The hint is in the number of millimeters printed on the compression plug! 28.6mm = 1.125 inch.

Basically any standard road bike top cap will do. They're a commodity item. You can get ones with cool / custom designs.

fietsbob

Even Stem CAPtain Products ? StemCAPtain