Tank shape, emblem size & tank construction

What makes a 'correct' tank for each of the Z1 / KZ900 models?  The answer is much more than the paint.  The two tanks to the right are both in the correct colors for a 1973 Kawasaki Z1 in brown and orange paint.  However, there are considerable differences between them, and only one is a 'true' 1973 tank.  But which one is the correct one?

There are essentially three different size / shape tanks used on Kawasaki Z1's & KZ900's.  The changes are fairly subtle and easily missed at first glance.  Genuine 1973 Z1 tanks are recognizable in a number of ways. 

The first major difference is the size of the tank emblem.  The screw holes in the emblem are 122mm apart on the Z1 tanks, and 140mm apart on later Z1-A, Z1-B and KZ900 tanks.  The emblems are different sizes - the small 1973 emblem is approximately 22mm high and 164mm long versus 25mm high and 189mm long for the later version.

The second major difference is the tank construction.  Underneath the tank are the joins from welding / crimping the different metal panels together.  The early Z1, Z1-A, and Z1-B tanks have a weld underneath near the center area (right tank).  The later 1976 KZ900-A4 & 1977 KZ900-A5 tanks have the weld closer to the sides of the tank (left tank) and much more visible from the side giving the impression of a larger tank. 

The throat at the front of the KZ900 style tank (left tank) is wider (allowing the wiring harness more clearance) and more of a 'U' shape, whereas the earlier Z1 tank (right) narrows in more of a 'V' shape.

Other differences include the hook at the back of the tank.  The hook is hidden underneath the tank on the KZ900 design when viewed from directly above (left picture), but much further back on the Z1 design (right picture).

Why the differences?  For years after the original models were sold, Kawasaki continued to sell replacement tanks.  These tanks used the same paint as the original tanks, but the tanks themselves were from later production runs where the tooling had been switched to produce the KZ900 tanks.  So frequently you will see a 1973 Z1 with a correct color tank on it, but because the tank is a replacement, you will see the larger emblems and often the KZ900 style tank construction.

To summarize this:

• Original 1973 Z1 tanks use small 122mm emblems and have the underneath weld to the center of the tank.
• Original 1974 Z1-A & 1975 Z1-B tanks use the larger 140mm emblems and also have the weld to the center.
• All 1976 & 1977 KZ900 style tanks use the larger 140mm emblems and have the weld to the outside of the tank.  Tanks used on the KZ900 motorcycles also use a locking fuel cap with a different latch mount on the tank.

• Replacement tanks for the 1973 Z1 may have large emblems only, or both the large emblems and the weld to the outside of the tank.
• Replacement tanks for the 1974 Z1-A & 1975 Z1-B will always have the larger emblem and may have the weld to the outside of the tank.

And which one was the genuine 1973 Z1 tank?  The one on the right...