A report on using a BernzOmatic mapp/oxygen unit for brazing a lug

[Doug Fattic]

My local hardware store had one of those BernzOmatic mapp gas/oxygen units on a clearance sale so I thought I would try one out to see if it would work for an amateur wanting to build a bicycle frame. The quick answer is yes it can but…

The unit comes with one each disposable mapp gas and oxygen canister. The 4’ hoses have brass connectors with adjustor valves that attach to the bottles on one end and the plastic torch handle is holding the brass neck and tip on the other. These adjustors can regulate the flow but not the pressure. The tip orifice size is about the same diameter I would use for making frames on a normal torch. It came with a simple wire doodad to kind of maybe hold the tanks.

I was able to successfully braze a practice seat lug to a long length of seat tube and a short section of top tube. The flame had a fine enough point so I could nicely clean up the shorelines. While it is possible to use this torch I do not recommend it. Here are the issues that make it less than ideal.

The hoses are only 4’ long, which limited my range moving around the lug. One time I pulled the portable cylinders off their little wire stand trying to get to the back side of the lug. This accidentally snuffed out my flame and seemed like a dangerous moment. If I was going to use this unit for real I would add hose length and ditch the cheap wire holder and make something that held the bottles secure and upright.

I barely turned the mapp gas control knob before it became too much (maybe only a 32nd of a turn) and the flame detached from the tip. Just a slight turn in either direction changed the flame more than I wanted. One the other hand the oxygen knob had to turn almost 2 full turns before any oxygen came out. This turn discrepancy made it difficult make adjustments to get a neutral flame or to make it have more or less output.

The oxygen bottle lasted12 minutes and that was just long enough to do one lug. I could probably cut that time down when I got more used to it. I used a Nikko lug that has much more surface area than a Henry James. When I braze with my real outfit it takes me between 3 and 5 minutes because I can turn up the flame easily when needed. An average student takes at least 15 minutes to do one lug. If their skills are below average it can take them a lot longer. This would make using disposable oxygen bottles expensive. I would estimate it would take at least10 bottles costing $10 each to make one frame and fork.

If I was to seriously consider making a frame with this unit I would also buy a 2nd similar single canister mapp gas (or cooler propane) heater for preheating. They have a much bigger but cooler flame (because it doesn’t have the forced oxygen accelerant) that lights with just a pull of its trigger. These canisters last a lot longer than the oxygen one in a dual unit and could provide the broad soaking heat that works for preheating before switching units to begin the specific brazing pattern for a particular joint.

Conclusion: It can work modestly okay if one knows already how to braze but will use a lot of disposable oxygen cylinders to make a complete frame. And if one adds in all the practice joints necessary for a beginner to catch on, that will add significantly to the final cost. It is of course in my self interest to say this but the most practical way to make a 1st frame is to take a framebuilding course like the ones I teach. You will have access to all the right tools and instruction and the teacher can always take over when something starts to go wrong.

[false_aesthetic]

Dude,

Your willingness to share info like this is effin' awesome!

Thanks

[Ben Hudson]

This sounds like a similar setup to the one recommended by Marc-Andre Chimonas in his book. It's really not a very good book overall but it does have the odd useful tidbit. The real problem is that the only advantage of this setup is the low initial cost, which is almost instantly outweighed by the higher consumable cost. Especially if you have to buy extra parts to make the whole setup safe. Add in the fact that it's likely a novice is likely to run out of gas mid lug and then have to heat it all up again to finish it off (then probably run out again half way through the next one) and it seems like a recipe for a lot of frustration at a point when you're trying to work out what you're doing anyway...