Aldo asks:
>
> What are the repair options for a bottom bracket shell which
> has become slightly oversized and ovalized.
> [snip] The bike, a '52 Learco Guerra, [snip]
Since I assume it is already Italian, tapping it Italian won't help.
I have used two methods to fix these - both require an experienced hand. Have a framebuilder do it unless you're very handy with a hammer.
Method One is to simply hammer the shell back to round - this can bring the diameter back down too, but not much, it's mostly to fix an out-of-round condition. The side opposite where you're hammering should be supported by a V- or half-round block. The hammer can be a hard-plastic-faced mallet (the steel of the BB shell is rather soft), but a rubber mallet probably won't do. Many blows around the circumference will be needed as you slowly blacksmith it back to shape. Sometimes squeezing in a vise helps if the main problem is ovalization. This is all done cold, by the way. It will damage the paint, probably flaking it all off in the hammered area. You will almost certainly need to chase the threads after, with a BB tap. ________________________
The other method I have used can fix a larger discrepancy, and needn't hurt the paint on the top (visible side in use) of the BB shell. I call it a "cut'n'weld" - though I usually did it by brazing, TIG welding (aka GTAW) would also be an option and has some advantages. (Gas, stick or MIG welding would probably be too messy and too much heat, if trying to preserve the paint.)
Start by de-greasing - we're going to need it clean inside and out for welding. You cut through the shell underneath with a hacksaw, starting from the faces, inward from each side until just past the threaded part - not all the way across the shell. Now you hammer the shell as described above, but only hammer on the bottom half, with the top of the shell supported by a wood block, padded by rags to protect the paint. Hammer until you've closed the gap caused by the hacksaw blade, taking care to keep the shell as round as possible - though theoretically it won't be round anymore, the tap (after welding) will take care of that. Now the diameter is reduced enough that the tap will cut threads into new metal. (A gap or kerf of one hacksaw-blade thickness seems to be the magic amount, though I suppose a radically-stretched shell might need more diameter reduction.)
Remove a little bit of paint adjacent to the cut, apply a little paste flux inside and out if brazing, then braze or weld the cut VERY QUICKLY (to minimize the amount of burnt paint). Wet rags wrapped around the chainstays and downtube can help keep the heat from spreading to those areas. There is also a clay or putty-like material called "heat fence" that you can get through a welding supplier, which is better than wet rags, but I never found it necessary. It can be done with little enough heat that only the bottom of the shell gets its paint burnt.
Now it's time to tap, and facing it again is probably a good idea. Be sure to soak all the brazing flux off first, it's hell on taps.
I did maybe 10 cut'n'welds over the years, and to the best of my knowledge none of them ever had any trouble after that - I consider it good as new (probably _better_), except for the damage to the paint. Though cutting through the shell sounds radical, I believe this is a better repair than just laying some brass inside the shell and re-tapping, which I have seen other framebuilders do. For one thing, my method can be done quicker - an advantage in itself, but mainly good because of the paint saved. Also the threads are still all steel, stronger than threads in brass.
Mark Bulgier
Seattle WA USA