With out knowing more I can’t tell but here are a few tips to go by
1981 Is The First Year Of The Watercooling on the YZ125, the rad was up in front of the bars
In 1982 yamaha put the first yamaha power valve system(ypvs) on the yz125 and yz250.
In 83 Rads came off the bars to either side of the tank on 125 & 250. Another new mono-x design!
Hope this helps
Dave
Parts for sale
85 KX250 Roller with engine
KDX motor
2004 E-Ton Viper 50 Needs work
Honda 87 Fat Cat Roller
Honda 200m motor
Honda 110. 80 frame 82 motor For parts. Runs.
Honda 84 ATC 70 Roller Cheap.
XR75 complete bike
XR80 parts
Yamaha 225dx parts.
Honda 84 200es parts Good bottom end. (quad conversion front end)
Kawasaki 93 KX80 Motor (needs piston & bore)
Yamaha 88 YZ125 cases
Suzuki 85-87 LT230s 2 complete parts bikes.
Honda 83-85 200x Parts
Honda 86-87 200x Parts