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Old July 14th 18, 04:24 AM posted to rec.bicycles.tech
James[_8_]
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Posts: 6,153
Default drill/tap in frames

On 14/07/18 00:41, 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.


Regardless, the wall thickness of the main tubes will not be sufficient
to drill and tap sufficient threaded area to hold a bolt.

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.


Even the chain and seat stays are unlikely to be much more than 1mm
thick. If they were 1.5mm thick, that is still not enough thickness to
drill and tap a hole for a 5mm bolt.

Have a frame builder braze on a tab with a 6mm hole, and use a 5mm bolt
and nyloc nut.

Or to attach to a chain or seat stay, form a metal band around the tube
with a pair of 6mm holes at the ends, such that when the bolt and nut is
passed through and tightened and holding the chainguard, it also clamps
onto the tube. Use a rubber strip between the metal band and tube to
prevent paint damage and increase the clamp friction.

--
JS
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