Minimum Insertion Point?
 

I’ve got a vintage Ambrosio stem and bars. Based on the component evidence from the Fiorelli they’re on, I’m guessing mid 1960’s. The stem doesn’t have a Minimum Insertion Mark. How would I determine the minimum safe point?

Stem Insertion Depth
 

3" or 75mm to 80mm - or less depending on how long the threaded section of the steerer is.

You don't want the bottom of the steerer expanding in the threads:

Note how thin the steel is in the threads.
https://cimg6.ibsrv.net/gimg/bikefor...f685ca1db0.jpg
https://cimg8.ibsrv.net/gimg/bikefor...4a7674deb2.jpg
https://cimg1.ibsrv.net/gimg/bikefor...e9e5bb3861.jpg

YOU DON'T WANT TO EXPERIENCE A "HEY WATCH THIS" NANOSECOND!

:injured:

verktyg :50:

Thank you for the very clear answer 👍

Thank you very much for this Chas-

I'd always assumed that minimum measurement had more to do with how much steerer the expander needed to hold on to.

Everything counts, starting with determining how far from the top of the steerer that the threading actually extends!

Plenty of bikes were made with a LOT of threaded length on the steerer, certainly enough to where the "safe limit" line on the stem quill would not assure that the the expander was in an unthreaded section of the steerer.

I think that the quality of the stem plays huge as well, as some stems are vulnerable to having their slot-bisected lower end literally crack and collapse (with complete loss of bolt tension and steering control).

Rider weight, stem/bar leverage/length, riding conditions, and length of service, again, can all play huge into the reliability equation!