I had to look mine up recently and found it difficult, you have to keep going at it, there are numerous sites, do searches for yamaha outboard spare parts diagram, eventually you may find the correct one.
There should be one or two letters before the 40 which is the engine type, then the 40 which is obviously the horse power, then a letter which is the engine generation 'y' then the E would be electric start, then the T would be power trim and tilt, then there should be a letter for the outboard transom height/shaft height I think.
The following link is the best ive used, needs adobe flash to run properely, scroll down and click on the online parts link.
Yamaha Online Parts Catalogue - Yamaha Motor UK
select from fields drop downs, outboard, model 40Y 2007