Headtube length. After a while you can tell a frame size from afar. I mean the bearings in the bottom bracket must be checked/cleaned re-lubed . Don't neglect this French bottom brackets can be a bit finicky to adjust, but if yours is an 80's model it might have a standard (BSA) threaded BB which is huge, 'cause then you can find replacement cups and a spindle more easily. Not a bad idea to open it up and look inside at the grease at least. Get it re-packed before the grit in there destroys everything. This is the "heart" of any bike. Huge threads have been written on this forum about french bottom brackets. . try searching that or search Sheldon Brown on that. My spindle may be worn, has some pitting. BTW if anybody has one for a Super M. please PM me. Velo Orange sells sealed bearing units for 50.00, not bad and my bud said they spin great. But it will fit only IF you're is actually (pre 80's ) french thread. The other guys here know all the details on exactly what year they switched threading, I could go on for days on this... If you start hearing a knocking/clunking sound it's probably the dreaded bottom bracket. Make sure your headset stays tight. The headsets on these are fine, it's just standard procedure.