Reddit mentions of Suzuki GS400-450 Twins Chain Drive, 1977-1987: Service, Repair, Performance

You are looking for part #24 on this fiche: http://www.bikebandit.com/oem-parts/1981-suzuki-gs450l/o/m21417#sch263854

While you are at it, I would replace the clutch cable, and all of the screws/bolts holding that in and in the area. I hope you have good metric tools, or be prepared to buy some new wrenches!

Here is the Clymer book for your bike, it is truly an invaluable resource.
http://www.amazon.com/Suzuki-GS400-450-Twins-Chain-1977-1987/dp/0892872373/ref=sr_1_fkmr0_1?ie=UTF8&qid=1413214095&sr=8-1-fkmr0&keywords=clymer+81+gs450

If you have more questions, head on over to www.gsresources.com , that is a great forum for these bikes. There you will find a member by the name of bikecliff, he has a great website/guide for these bikes, located at http://members.dslextreme.com/users/bikecliff/

I have used his guides for everything I have done on my bike and it hasn't led me wrong yet. It is always better to do your own work!!

I am also moderator of a small subreddit called /r/Suzuki_gs . we would love to have you over there!

Where are you located?