Hey Arran, I'm rebuilding an Alien SP at the moment and have found that the drive side BB shell has an angled in face (where a normal BB shell is flat). This suggests to me that an unusual BB was used originally (1990), something with a flange to mate to this machining. Do any of your ACXs (which have the same front triangle) feature this?
I can't comment on the b/b of the '90 ACX. I haven't had occasion to pull it out... it's NOS!
But... the b/b shell on the '91 does have a (for lack of a better term) counter sink on each side for some reason. Maybe Nishiki thought it'd be easier to start the b/b cup threading process or something equally as stupid...
* '89 Nishiki Alien 17/19"
* Mantis Valkyrie-EC 18"
* Titan 1/2 Trac 18" (Stay tuned to this channel...)'89 Titan 1/2 Trac, '90 Nishiki Alien ACX (NOS, M735), '90 Pearson Kea, '91 Nishiki Alien ACX (M900), '91 Haro Extreme Comp (M735), '91 Milazo Rock Comp II, '91 Mantis XCR-EC, '92 American Comp-Lite (XC-Pro), '12 Soma B-Side (650B), '14 Cannondale Evo Hi-Mod 1 DA9000, '15 Cannondale Trigger 27.5 Carbon 2 (XTR)