let's see if I can explain this well but I am going to try and brak down what Yamaha part #'s mean.
all Yamaha part numbers are 12 digits but they can be grouped into 2 different ways. lets look at a 1998 wr/yz 400f top end
Piston 5BE-11631-00-00
base gasket 5BE 11351-00-00
piston pin circlips 93420-18082-00
rings 5BE-11603-00-00
1999 yz/wr 400
piston 5GR 11631 00 00
base gasket 5BE 11351 00 00
piston pin clips 93420 18082 00
rings 5BE 11603 00 00
now lets look at the numbers themselves for the piston. All Yahama pistons are ???-11631-??-?? the 1st 3 ??? denotes the model/s the 5BE being specific to the 98 and 5GR being specific to the 99. Now all other surrounding part's numbers are the same so I tend to believe that the pistons are interchangeable but something like the designed weight denotes the difference between the two model years. The last 4 ??-?? would be 00-00,10-00,20-00 for the different sized pistons for cylinders with boreable liners OR
01-0A,01-0B,01-0C,01-0D.... the 01 saying something important is following and the 0a,0b for the a,b,c,d cylinders. There is no o's in yamaha part numbers only zeros. The base gasket and rings are the same sothey keep the original model designation that they where produced for .
parts such as bearings,bolts etc follow a different grouping of numbers as the piston pin circlips- 93420-18082-00 93420 says it is a special type of circlip.18082 says its sizes. Snap rings will have a different 1st five numbers as do different circlips if they have the little ears you can grab on etc etc.
all roller ball bearings start with 93306 all needle bearings start with93310. and here is something that can be very useful with nuts and bolts.
91317-06014-00 is an allen head bolt
98706-04012-00 is a countersunk flat head,the 1st 5 numbers saying what kind of bolt it is and the following 5 stating its size If it is a standardized hardware 06014 is a 6mm bolt 14 mm long 04012 is a 4 mm bolt 12 mm long.Bolt # 90105-104A7-00 and bolt#90105-10323-00 are both 90105 washer based bolts both are 10mm diameter bolts the next digit says that it is special, the A7 being a lenght that is not standard the 23 is a standard length.
this is something that I am still observing /learning so it is not 100% accurate but I am heading down the right path understanding them.