HA,
OK. Thanks. But I run DOWNLOADS, and most (but not all) of the Atlas/Craftsman manuals and drawings in there came directly or indirectly from me. So I have MP-2, MP-2A, MMB-1A, MMB-3, MMB-4 and MMB-5. I'm still looking for MP-1, MMB-1, and MMB-2.
The motor pulley on the M1, MF, MH, M1A, MFA and MHA is M6-428 for 1/2" motor shafts (or M6-429 for 5/8" motor shafts, but that isn't shown on the original parts drawings or lists - I got it from Clausing when I bought one recently). The 2-step countershaft pulley is M6-427 on the base models and M1-427 on the "A" models.
But the reason that I asked what model mill you have is that someone else with an MFC had reported that his belts were 4L350 motor and 5L280 spindle. Did you by any chance get your numbers off of original belts or did you put everything together and measure required length? Unfortunately, in the case of the other reported belt numbers, I don't know the answer to that question. But also, the Atlas part number shown for the motor belt on the M*B and M*C is L3-125 which is also the motor belt on all 14 models of the Craftsman 3/8" bed 12" lathes. And dozens of people have reported it as 4L350.
I recently spent about an hour on the phone with someone in Tech Support at Clausing over the belt issue. And about all that I learned is that they've lost all records of what the industry part numbers for all of the belts in all of the mills were.
Incidentally, although the fonts used in a lot of Atlas documentation (and most places that it appears on the Internet) makes it look otherwise, the second character in the model ID of the manual cross-feed models is actually a numeral "one", not an uppercase letter "eye". So the first mill was the M1, not the MI. And most of the original part numbers begin with "M1-".