I picked up a Guid stem at the SD Velodrome Swapmeet on Sunday to potentially replace the Ambrosio stem that came with my "new" PX-10. My calipers confirmed that the Ambrosio stem is around .1 - .2 mm thicker diameter than the French Guid stem, so that probably accounts at least somewhat for the difficulty in getting the Ambrosio removed.

So I'm planning to switch to the Guid stem, but there's no minimum insertion line on it. I know it's probably from an era when no one would bother with a min insertion line because it was expected that only a little bit of quill would be showing, but I prefer to ride with my bars around saddle height, so it's important to know how high I can set this stem. Does anyone know or have a guideline for this sort of situation?

Thanks!

From Sheldon:

Most stems have a "minimum insertion" mark that shows how high it may safely be set. If you can see this mark, the stem is too high. If your stem doesn't have such a mark, a good rule of thumb is that there should be at least 2 inches/50 mm of stem inside the fork.

I'll also add that if you have an extended steer tube that you built a lot of spacers into, the wedge should end up at or below where the top tube meets the head tube.

If there's no minimum insertion indicator, I'd always go with the 30% rule. or 1/3 of the stem shaft inserted into the steerer tube, minimum.
I've encountered this ratio as a safe baseline in a lot of structural designs in buildings. Less woud usually require a special design to keep the safety factor adequate...
You might also consider going for a bit more insertion length if the stem forward extension is unusually long like maybe something over 120mm....
JMO

Chombi

I like to make sure the stem inserts deeper than the stack of the headset proud of the headtube. Figure 2" to be safe.

A good rule-of-thumb is the insertion depth should be no less than 2-1/2 times the outside diameter of the part.

On a 22.2mm diameter stem, in the absence of a marked minimum insertion line, I use 2" (~50mm) minimum inserted into the steerer tube.

Hmm... alright, different suggestions, but with some consistency. I'll mess around and see what seems like a good blend of these ideas.

Thanks!