For anyone looking at this because of interest in my original query (if you've managed to be as stupid as me and put a hole in a steel frame) I've covered quite a few miles on this bike since the 'incident', and it's still doing ok. Although it hasn't had daily use, I must have done about a couple of hundred miles on mostly decent surfaces (and a bit of off-road, including bumping over tree roots on forest tracks) and I can report that there is no sign of further deterioration. Other than sanding off the edge of the hole as advised, I didn't do anything (need to put some enamel paint on to prevent rust when I have a moment). I'll still keep my eye out for another stowaway to replace the frame though.

Pontification first, reading some other time:

Just to clarify for anyone reading this (who might be wanting to do similar work on a raleigh stowaway) the hole which I accidentally made (not the small drilled holes made by the manufacturer) was indeed made as I described in my original post, so do be careful sawing off the mount for the stand if you choose to do so. Also, I can recommend using a hack saw to remove a few mm of the bottom bracket shell. After this the edge can be neatened up using a file and a right angle tool (not sure of the name - Square Try?). Just be careful with how much you remove - you can remove, but you can't add! I wouldn't do it on an expensive frame, but for this purpose it's fine.

Brazing metal is about 3/4 the strength of mild steel and about half the strength of cromoly, made up for by the thickness of the fillet. So it's pretty strong. Certainly better than epoxy. It also sticks to the steel 100% unlike epoxy which comes off easily unless everything is perfectly cleaned and prepped etc etc. If you filled the hole with braze and sanded it smooth it would be a pretty good repair close to the strength of the original and would stop any cracks spreading.

It looks like this frame might actually just be mild steel which makes a weld or braze repair particularly easy, but it's hard to be sure.

:thumb:

Based on what I see in the #11 post ( 05-16-19 11:36 AM) photograph showing the hole.
You could possibly access that hole in the tube by removing the bottom bracket.
If it is possible to thread ordinary mono-filament fishing line from that hole in the tube to the empty bottom bracket opening:
YOU CAN THEN TIE THE FISHING LINE TO THE END OF A BOLT (something with a round head large enough to be significantly wider than the hole).
After determining that access is possible, and purchasing or otherwise obtaining a suitable Bolt and corresponding nut and washer.....
PLACE SOME JB WELD just beneath the bolt head on the flat portion that will contact the inside wall of the frame tube......
Do that only after you have tied and securely affixed the fishing line to the End of the Bolt.
Push and Pull the Fishing line so that it pulls the BOLT end through the Large Hole you wish to stabilize.
You want to keep the Line taut such that the bolt head is firmly set against the inside tube behind the hole and the threaded portion sticks out!!
Keep the Fishing Line pulled taut (or tight...you know as in that it HOLDS the bolt firmly there against the HOLE ,UNTIL THE JB WELD SETS...)
Wait 24 Hours, until fully cured.....with FISHING LINE PULLED TAUT, before then installing a washer with a tiny amount of JB WELD on it and then
the Nut with a very tiny amount of JB WELD in the donut hole of the nut (center portion of the nut)
TIGHTEN IT UNTIL FIRMLY AFFIXED and then ONCE THAT sets......you can cut off any remaining length of bolt that extends beyond the nut.
Hey, that gives you all the strength and reinforcement that you'd ever need, as you have the clamping force of the bolt head on the inside and the
nut/washer on the exterior along with the JB WELD Epoxy bond......
THIS METHOD WILL FUNCTION PERFECTLY.
Another method would be to go by any muffler shop, or automotive shop/garage where they build dirt track race cars/old car restorations, or business that builds custom iron fences/gates , or just someone that makes art out of scrap metal........ AND HAVE THEM WELD IT CLOSED.
-----Either method will stabilize and give you more than enough strength. WELDING IS THE BEST APPROACH.

Good info. And welding rod can be nearly as strong as chrome-moly. A medium- to low-temperature approach would be to use brazing metal to plug the hole. Higher temp would be to fill with welding rod (but this might distort the frame). I think your idea of braze metal fill might be the most practical.

If this were a vintage hand-made Italian frame I'd possibly think about carefully TIG welding in a circular coupon that was bent to conform to the curve of the stay to cover the hole. If this bike had high emotional value perhaps one could cover the hole with a coupon of metal , and braze it in place.

In any case I'd hand file and emery paper the edges of the hole very smooth to minimize stress risers before any epoxy, paint, braze or welding was applied.

Yes indeed, although for bike frames you often use a mild steel rod, which is a bit weaker but more ductile. A bit weaker is OK so long as you have a little bit of thickness in the fillet.

Trying to fill a hole that big with welding rod would be a challenge because of overheating as you say but also you might just end up with a huge steel ball wanting to go everywhere except across the gap. It's easier to span a gap with a brazing rod, but you would still get quite a bit of a blob falling through the back of the hole. I would use a fairly rough-fitting coupon, wedged in about flush, then TIG braze over the top of it all and sand it smooth.

You could cut the coupon out of a bit of old chainstay or offcut. The problem is if it was a vintage frame the alloy might not be weldable anyway. And this would be a tricky repair to do because the coupon would keep wanting to melt away completely.

Argh, yes, what was I thinking? I think was was lulled into thinking about welding looking at the pic of the OPs (welded) bike. But 531, for example, can't be welded without losing some of the strength of the tubing. So welding is out for many vintage frames.

Your post is informative, btw. It reads like you have welding experience. Ever weld bike frames?

Yes no welding on 531 and I assume the same for Columbus alloys of the period, but don't know. Yes, I'm getting quite handy with the TIG and have made three frames all of which are holding up so far :)

Awesome! TIG is a tough discipline. My neighbor was a welder (he owns a welding supply company now) and tells me stories of TIG welding jet engine test probes for GE. He had to hold his breath for about 60 seconds to ensure a good weld bead with no irregularities.

Three bikes is awesome. What did you make your bikes out of?

Two were Cromoly (Columbus Cromor and Zona), and the most recent was Reynolds 631. Definitely going to make some more! Bike frames are quite tricky (round tubes, some tight angles, very thin wall, thin to thick, fairly safety critical, somewhat exacting alignment requirements). But it's super-fun and, like anything, with a bit of practice you can do an acceptable job.