Hello all, Just purchased a 1978 Yamaha DT125 and was inspecting the frame and found a small hairline crack under the fuel tank. What would be the proper procedure for tig welding the crack? Filler? Do the same rules apply that are involved in cast iron? Drill holes at ends of crack, grind out and fill? Thanks for any help on this.
I think the DT was a basic on/off road bike so the frame is probably some mild steel that is weldable by a variety of methods. I'm pretty surprised the frame is cracked.
Can you describe the crack and location it more? That might have been one of the first "mono-shock" street bikes, so there may be a fairly complicated "structure" under the tank to support and house the stout single long shock. Is the crack in that area? Or straight across the top tube? Or........?
One pretty good rule is to NOT weld straight across a tube or beam loaded in bending or tension. If your (bike's) crack goes straight across the tube, even if it is in or near a factory weld, some thoughtful reinforcement with a diamond shaped plate might be a nice upgrade.
Sometimes a crack indicates a problem elsewhere, like broken or missing supports or bolts.
wrote: (clip) Sometimes a crack indicates a problem elsewhere, like broken or missing supports or bolts. ^^^^^^^^^^^^ This is an excellent point that many of us (me, in particular) might have overlooked. Thanks.
Drilling holes at the end of a crack is a way of stopping it from propagating. It drastically reduces the stress-raiser factor by increasing the radius of curvature. It is particularly effective on a crack which is not going to be welded. In cast iron, the repair is almost always done with a different material, so the ends of the crack could still act as stress raisers, and drilling would be beneficial. If you are welding up a crack in steel, presumably the stress raisers will be gone after the weld, so drilling would do no good (provided the weld is good.)