Vent Holes
Mon, June 22, 2020 
I am sometimes asked, why are there tiny holes drilled in certain parts of a bicycle frame, like the ones shown on the left?
These are vent holes. During the brazing process the air inside the tube expands as it is heated. The vent hole allows the air to escape as it warms up and allows for air to enter as it cools.
If the tube is totally enclosed, on cooling the air contracts sucking the molten brass inside the tube leaving a pin hole that is almost impossible to fill.
Worse still pressure can build up in an un-vented tube and hot brass can blow back in your face. Anyone not knowing this will soon learn the importance of vent holes after picking little globules of brass embedded in their face or finds little brass balls hanging like tiny Christmas Tree decorations from eyebrows, mustache, or other facial hair
Vent holes are only needed when a tube is closed both ends like the example shown above. The top tube is closed at both ends and is usually vented with holes into the seat tube and head tube. (When the bike is assembled these holes are hidden.)
Seatstays are enclosed with a fork dropout one end and the seatstay cap at the top. The front fork blades are also enclosed both ends between the fork crown and the fork tip or dropout.
Other tubes like the seat tube, down tube and the chainstays are open inside the bottom bracket shell. These tubes are not totally enclosed so do not need any additional vent holes; neither does the brake bridge because it has a brake bolt hole.
On some of my custom frames you won't see holes in the chainstay bridge like the one in the picture. They are hidden inside the bridge tube by drilling holes sideways through the left and right chainstay tube, before the bridge tube was put into place. Only one hole is needed for venting but often two holes are drilled for better drainage of moisture later.
The vent hole in the seatstays on my frames is on the inside up near the seat lug. You might have to turn the bike upside down to see it.
On my front fork blades I drilled one vent hole in each fork blade near the bottom, but after the fork was fully brazed and had cooled I went back and filled it by brazing a piece of wire in the hole. The heat generated in doing this was so small and the air space inside the fork blade was big enough that it did not cause a problem. This whole process only took a minute to complete.
I did this for two reasons. Front fork blades are highly stressed, so filling the hole eliminated a potential weakness at that point. Also rust needs oxygen, and with the fork blades completely enclosed and airtight, no corrosion inside is possible, even years down the road.
A small and probably unnecessary precaution, but one that took such a small amount of time, I always figured, why not?
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Reader Comments (7)
:-) thank you!
as always, a pleasure to read your blog (i'm not a frame builder).
best regards, from Bucharest, Romania!
A good set of vent holes is always reassuring.
Also, if you think you care about vent holes, talk to the hot dip galvanising people :) Mind you, wet is worse - nothing quite like a steam explosion in your vat of molten zinc to wake everyone up.
I like the brass Christmas tree ornament analogy.
Another informative post Dave.
When I built a frame under Dave Yates’ tutelage, he got me to fill in all the vent holes with wire as you describe and once the frame was finished painted, there was no sign of the holes.
There were some frame builders that hid all of the vent holes.
Of course today with so many frames TIG welded the venting issue is a bit different. The good builders are back-purging and they need access for that.
I have squirted Framesaver through the hole in an attempt to prevent corrosion. Dave, you mentioned closing the hole during frame construction. Is there any sense in sealing the hole with tape or caulk or something afterwards?
Did anyone ever ask you if you could fill those fork blades with helium before sealing them? Imaging how much faster your bikes would go if you reduced the weight by a few milligrams.
In answer to John Blum's question, you can use beeswax to fill the vent holes.