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5.21 Frame Building Part 4: Plugging Vent Holes




Description

This article is from the Misc Bicycles FAQs, by various authors.

5.21 Frame Building Part 4: Plugging Vent Holes

The vent holes that had been drilled on the fork and chainstays for
earlier brazing needed to be filled in. One of the main reasons to fill
these in, besides preventing moisture from getting in there, is that
when the frames go to be painted, they get sand-blasted first. Sand
from the sand-blasting gets in the tubes and then leaks out during the
painting process and obviously can mess it up.

First the tube (chainstay or fork blade) was gently heated with the
brazing torch. Heating the tube would get rid of any moisture in the
form of steam from the inside of the tube. He put a little more heat at
the ends since moisture tends to concentrate in the crevices there. The
heat also creates some pressure inside the tube which would be used to
help seal the tube and prevent brass from dripping into the tube. To
plug the vent hole, concentrated heat was applied right at the vent
hole. When the tube was hot enough, a drop of brass was applied
directly to the hole. Since there's pressure inside the tube, pushing
outwards, the brass doesn't always end up on the hole, but may be pushed
off. When the temperature and pressure are right, or there's a lot of
brass around the hole, the brass does sit right on the hole and seems to
bubble up for a second, but once heat is removed, the pressure inside
the tube drops, pulling some of the brass in, and pulling it right
through the hole and sealing it without dripping any inside! This
process was done on the two holes on each fork blade and the one hole on
each chainstay.

 

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