On Fri, 13 Jul 2018 16:41:16 +0200, Emanuel Berg
wrote:

John B. Slocomb wrote:

And the mid-bottom part of the down tube?
The screws are interchangeable in size
and length.

Well that depends. Probably on the cost and
class of the bike. The down tube is pretty
thin on most better class road bikes.
Say Perhaps .6 - .7 mm in wall thickness.
Depending on whether it is butted, non
butted, etc,

These are not road bikes but ordinary standard
steel frames for commuting, shopping, and
everyday transport, say from the 80s.

Sorry but road bike data is all I have :-)

Yesterday BTW I saw a second bike with the
exact same configuration. The bracket is
bent so it is round along the tube surface,
but what holds it in its place seems to be
a single M5 screw with a flat PZ2 head.

Given that all the pedaling torque passes
through the bottom bracket I'd doubt if a new
bike would use a single screw to hold the BB
onto the frame tubes.

What is held are two stays that then hold
the chainguard.
--

Cheers,

John B.