Wondering if it is a reasonable solution to insert a piece of tubing as a sleeve to reduce the I.D of a lug?
I have a design that I had to switch bb shells on and the down tube I am using is 31.8 but the lug is sized for 35mm. Seems doable proving I can get the right thickness tubing. Then brass braze it in and use silver for the actual down tube seat tube etc. once I have cleaned the bb up.
Yes it has been done and I haven’t heard of any failures from it. It’s not the weight-weenie solution, there’s extra metal there, but I can live with that if you can 
Especially at the chainstays, there’s a long tradition in England of using 3/4" stays in a 7/8" socket by sleeving it down. Here’s a MacLean of London. 1950s by my guess but it could also be older:
The sleeve can probably be made from .058" wall tube but that’s a slightly loose fit for silver, so make the sleeve kinda long so you at least have more wetted area. Possibly better, make the sleeve from .065" and bore it on the lathe to a perfect chef’s kiss fit for silver. But that’s a PITA and .058 is good enough.
'Nother option, use a lugless shell and fillet-braze it (or TIG if you’re so inclined). Probably not the answer you wanted though!
I messed up a seattube with an adjustable reamer. I decided to replace the seattube but go down a size from the original 31.8 (if I remember correctly) to 28.6. You can see in the picture, I left a little of the original tube in the BB shell, then turned a sleeve to make up the difference to the 28.6 OD of the new tube. I did the same at the ST/TT junction. I still ride this after 10 years - it’s a CX bike.
Awesome! I got a piece of tubing and have ground it to the correct thickness. Looks like it will work. I’ll post to pics once it get it done.
