The fuel tank cap for the 78 KZ650 SR was 51048-1001 and it was used on the following models
KZ650-D1 (SR) (1978)
KZ650-D1A (SR) (1978)
KZ650-D2 (SR) (1979)
KZ650-E1 (LTD) (1980)
there is also a gasket part# 11009-1053 the the cap uses.
I am also looking for this type of gas cap. I bought a project bike from a guy and the gas cap was missing. Guy told we it was a kz650 tank from early 80's. Did my research and found a replacement cap (which I thought was 51048-1001). Long story short, gas cap came in over the weekend and it is too small. It measures about 1-1/2" diameter and my gas tank opening is 2". Which gas cap model should i be looking for? Please help.