Hi Tony,
That is one sweet-lookin' Yo you've got there. I wish I had mine back. I built a special ultralight Yo for myself in '93. I used straight gauge .028 wall for the stays and had some Columbus tube that I threw in as well. I also used the machined 17-4 PH Stainless Steel dropouts and massaged those quite a bit. This was another trial frame since it was the first frame to be completely welded with 309L MO stainless filler wire instead of the ER80S-D2 that I normally used on the CrMo frames. It came out really nice and I was very happy with the results. For everybody's info, we never used 4130 filler wire on our steel frame welds. The only time 4130 filler should EVER be used to weld CrMo is when the entire part is post-weld heat-treated. It is not necessary to heat-treat thin-wall tube fabrications even for mountain-bike (abusive) conditions if the correct filler wire is used. Most other fabricators used ER70S-2 or ER70S-6 (cheaper, easier to get) which are not as strong as the ER80 grade and do not have anywhere near the fracture toughness and fatigue resistance which are much more important characteristics than ultimate tensile strength. We actaully built a fatigue tester to test various welding materials, gusset designs, and other design elements and we obtained much useful information. To me, that was one of the really fun parts of the job. Getting paid to break stuff is the best!! It was especially important in developing the titanium frame.
Looking forward to getting back out on the trails this year
Best wishes
Scott Bengtson