I’m building a 1900’s style board track racer motorcycle using a Worksman industrial frame and I will need to increase the length of the top tube by 4” and modify the shape of the down tube, the lower top tube is fillet brazed it will be replaced.

There are two ways to accomplish this, one is to heat the lug/tube to soften the brass enough to free the tube and the second is to cut the tube and add a 4”. The problem with the first option is that the lugs will have to be heated to remove the tubes then heated again to braze in the new ones, the lugs will have been heated a total of 3 times, would the affect lugs structural integrity? The second would be to lengthen the tube by adding a section with a custom made tube that would be inserted inside the tubing extending past each end of the splice with plug welds (brazed) to add strength.

Which would be the best option?

Best would be to cut off the front triangle and build a new one as you'll never get brass brazed lugs apart without destroying some pieces and/or totally nuking the head tube. But, are worksman frames brazed? I thought they were all tig/mig.
On a welded frame, you could grind the old down tube welds off, do the tube extensions on the top tubes, and put in an entirely new down tube, better for safety and easier since it will be meeting the headtube at a different angle than the old one.

Worksman frames are brazed with brass, confirmed it with one of their builders.

Since the frame tubes are lugged at the seat, head and bottom bracket there's no way to lengthen the frame without completely removing the tubing or cutting and adding a section.

I haven't been able to find lugged head tubes or bottom bracket shells that are even close to the ones used on Worksman, anyone know where they might be found? The lugs are part of the head tube and bracket shell and are very strong.

Does this frame have curved tubes? If so, this might get a bit involved but, the best projects do that somehow...

Cut off front triangle as suggested above, source the right dimensioned 4130 tubing - you could even go thicker walls to give extra stiffness or whatever you're looking for. Build it from there. There are several sources for 4130 steel in many different dimensions. Error on the side of thicker walls so you have material to machine away to fit headsets and such.

Maybe just build a new frame with design based on the Workman but with 4130 (which the Workman might not be) and you'll probably end up with better results.

fun project!

those lugs are either made in-house or for them by a vendor, you aren't going to be able to get them. Re-heating will not hurt the lugs if the temperature is controlled in the process. You probably need to build some sort of hearth to get the tubes out though

One thing to keep in mind is that the angles that the top and down tubes make with the other tubes at each end will need to change a lot if you are going to add 4" to the top tube. This means that the down tube will need to be cold set down and if the top tube has any slope it will also need to be cold set..........and likewise the angles these tubes make with the head tube will change substantially.

In the end there is only one good way to do this and that it to remove and replace the top, down and head tubes. By doing removing the entire front end you can be sure that you won't load up the remainder of the top and down tubes on some funny way that will only lead to failure down the road.

It may seem like more work to replace the front end that it would be to graft some additional length onto the old one but in the end it will be the cleanest, most simple, and safest way to make it happen.


dave

maybe 'inspired by' and a fillet brazed replica is a better approach ?

motor bike? how about a plain steel filler wire TIG so you can braze over that?

Here's an image I captured from a video where a guy did several modifications to a steel Worksman frame, the tubing on these frames is about .083 wall thickness. He lengthened the 2 top tubes, replaced most of the down tube and modified the left seat/chain stays. It looks like he probably used the Tig welder in the pic to weld up the spliced in extensions.

I forgot to mention he completes the build with a cast iron Maytag washing machine motor, the motor weighs at least three times as much as my motor. He also added several steel levers and mechanisms it has to weigh over 100 lbs.

The Maytag Flyer - YouTube

The stock wheel set from my Worksman frame weighs more than my complete Specialized dual suspension MTB.

Here's the frame before and after the stretch. The top tube was stretched 5.5", the splice has a custom made tube inserted inside that extends past the each end of the 5.5" section by 2" and there are 8 plug welds. The lower top tube is a new section that's brazed in place.