Looking for chain pitch and approximately how many links for a 1972 125sixdays
428 x 136?
That is what I start with, but you didn't mention how big of rear sprocket.
Ron Carbaugh
57 rear sprocket
Better to get one too long, easier to shorten one than to add to it.
Try a 428 X 132 link DID Chain. I've used several for my 125 Six Day Restorations. The 132 length accommodates pretty much all sprocket combinations. You should end up with three or four extra links with a 57 tooth rear sprocket.
75 250 MC
73 Cafe MX
72 Six Day (3)
70 Six Day