Bent downtube?
 

I found a 58cm Centurion lemans with tange 900 tubes at a thrift store. The bike is in very good shape ,wheels, fork , bars , no signs of a crash. I started to break it down for rebuild and found a bulge just behind the head tube lug on the underside of the downtube. It almost looks like it was made that way
maybe to tweak the geometry, or maybe the bike ran into a wall .But like I say all of the components are in great shape and the fork is not even scratched. Anyone seen a frame with a similar bend ?

Yup. I suspect the bike has been run into a wall. How does the underside of the top tube look?

A bulge like you desrcibe is a very typical fsign or frame damage. They don't design frame like that.

I suspected that it was probably crashed, the tubes are otherwise straight and the drops are aligned. Maybe it was repaired. thanks for the info

If there is a steel builder in your area they probably could replace the tube if you think the bike is worth it. Take a flash light (one of the bright LED with a small twist end) and a dentist mirror and check the area around the inside to check for cracking, excessive rust, or the lug pulling away from the tube. May be ok. A good LBS with knowledge of frame building would be a good source.

I ran over a guy that had crashed in front of me in a race and had exactly the same frame damage. I rode the bike like that for quite a while. Very dangerous. Eventually the bulge will start a crack which eventually works its way around down tube. One day I was riding it and it felt really loose. Quick check, no flats, headset ok. Then I looked at the down tube and it was holding on by a thead! I was out about 20 miles and I called a friend to give me a ride home. Watch out!

At least the parts are worth something. Older Suntour components, Dia Compe aero brakes ,Araya wheels w/Suzue hubs. Not bad for $25.00.

Was that a steel frame or aluminum? I wouldnt think a steel frame would be weakened that much to crack.

Depends on the bulge. Steel has a yield limit and can bend some before it starts to give way. The yield limit could be exceeded by an attempt to straighten the frame.
If the frame is Al it should be discarded. Steel if not crazed or cracked might well last for sometime. It will start 'acting funny' when cracking begins, but once started the crack will propagate over a short time all the way around the tube, at which point the bike will be very squirrelly, not good at 25+ mph on curvy downhill. If usage is just short rides in non challenging circumstances it would probably be ok to use the frame.
Steve

It was steel. Had a nice bulge in it. Guys told me not to ride it but I had just bought it, probably raced it for about 2 months before it happened, so I was going to get my money's worth out of it.

Steel will slowly tear - not crack. Aluminum, on the other hand, will crack and then fail catastrophically. When you see a tear starting in the steel, then it is time to retire the frame. Until that time, (and so long as the frame is straight) ride without worry.