Originally Posted by
prairiepedaler
...On frames as old as this Magnum, isn't there a risk that they could fatigue and snap? On the last bike I had (RIP), and the current one, I've used Rust Check sprayed into the frame for rust prevention. Works really well on cars, does so too on bikes I'd imagine. This fellow has used this bike for a fair bit of touring in the past and that weight over distance means stress. I've thought it would be kind of cool to put a bike frame in an xray machine to see a potential structural problem point before you buy it. I think it's safe to say there isn't "an app for that".
There's always a risk. However, it should be very small. Theoretically, steel will not fatigue provided it is not stressed beyond a certain level. This level is above what frames receive in normal use, even in loaded touring. However, a big impact, corrosion and overheating during manufacturing can all lower the stress limit.
The CrMo steel used in your Magnum ST is far more resistant to corrosion than the hi-tensile, CMn or MnMo steel alloys used in other bicycle tubing. The Japanese manufacturers of the mid-1980s also typically phosphated the frames to resist corrosion. Application of a rust inhibitor has lowered that risk even further. You should be in the lowest risk category. Personally, I'd have no qualms about riding the frame.