Odd bulge in frame. 1980 XS400


XS400 Enthusiast
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Spearfish, South Dakota
So on the left hand side of the frame, below the engine, I found what appears to be an odd bulge in the frame. Any idea if it is normal for there to be a bulge in the frame there?

That would be the spot in question. Doesn't seem right to me. Figured I would check with you guys about it.

Also, yes, I know it is a mess... been working on cleaning it up/restoring it for a bit now.
What does it look like underneath? I can only think of 2 explanations, one of which would be fine the other not so much.
It seems to be less pronounced underneath than it is on the top and the sides. Doesn't appear to be dented on the bottom, but the bulge doesn't appear to come out quite as far. It's a little hard for me to see it very clearly though because it is currently up on a jack stand with no wheels on it. So I can't really get a really good look at it on the bottom at the moment.
I just looked at mine and no bulge like that. If there is no dent on the other side, I would suspect prior repair and maybe it was ground down to blend in? Could have been done at the factory too... My frame has some interesting areas that were obviously mistakes and then touched up at the Yamaha plant.

Rust can make areas bulge out a little, but if the rust was that bad from the inside out, the frame would be in two pieces at that point, so I would rule that out.

That's all I got, if its still together with the weight of the motor in it, I think you are ok :)
Based on the location, water probably got into the frame and froze. I've seen it before.
That was going to be my idea also. Being that side would be the low side leaning on sidestand. I've seen ice put a bulge in 3/4 to 1 inch steel plate, so the thin tubing of an XS400 would be simple work for ice!
Okay cool. Good to know. Thanks guys! Just wanted to make sure that it wasn't something that I really needed to worry too much about. Sounds like it'll be fine. My XS400 will never be stored outside now that I own it, so probably don't need to worry about that getting worse in the future. Thanks again! :)