5th gear mods
3YP does not immediately reveal itself. 3YF i think is the 1st XT660, air cooled 5 valve engine.
Considering how Yamaha use prefixes for basic models and then small changes to that prefix for certain countries, 3YP is probably just a variation of 3TF.
BUT; a new part takes on the prefix of the bike where it is 1st used.
Or that is what we are told. So you may find that there is a significant difference.
Your gear does not have the 5CH prefix of the bike, so was used on a 3TP bike. It has the correct middle part of number to be a 5th pinion. All 5th pinions anywhere are XXX 17151 XX XX. And it has 24 teeth. The last 2 sets of digits can be important. The 2nd last may indicate a mod of significance.
The very last 2 often omitted, usually a colour.
The best resource i have found: cmsnl has a discrption: Yamaha - frequently asked questions
Interesting to see that the late 4PT engines still used the old 34K 2nd gear. So the space was created another way.
I picked up my new 5th gears from my friendly Yamaha dealer yesterday 
Going to be a few days before i can get to work on the bike.
best,
p
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