Classic & Vintage - Headset for 82 Motobecane Mirage sport

I need to get a new headset for my mirage sport, and am a little confused about sizing. The top cup that presses into the frame is loose and the bearings are in rough shape. So I likely just need new bearings and the top cup but how do determine the size of the frame cup?

That 26.4 vs. 27 number is throwing me:

http://www.niagaracycle.com/product_info.php?products_id=11201

or
http://www.niagaracycle.com/product_info.php?products_id=5237

I will resuse the rest of the old parts to not worry about french threading.

thanks!

Well, I guess I'll give it a go..........I am probably wrong but I would assume that the numbers correspond to the quill diameter of the stem. You should measure yours to make sure the headset is correct.
As far as the headset replacement is concerned. Have you tried just properly reinstalling the top cup? It's usually a really tight fit that requires a tool to do the job correctly. Maybe someone replaced it at some point and didn't get that sucker in there all the way. Coupled with an improperly adjusted headset it is possible that it would adjust itself. My Schwinn Sprint adjusted itself till I got sick of it and took it in because I didn't have the proper spanners to tighten it all up properly. Another thought on the self adjusting is maybe someone repacked the old bearings. Bearings get slightly out of round with age and get extremely hard to get good bearing adjustments till they figure themselves out. It takes a few adjustments to get them to spin smoothly........if they ever will again. Maybe they just adjusted it once with old misalighned bearings and sold it to you........ Is it pitted? If not..... just replace the bearings with a copius amount of grease.

Thats all I got. Hope it helps.
Kelly D