240z Transmission Swap
Putting a 5-speed transmission from a 280z or 280zx into a 240z poses a problems - you need to make certain you use the right collar for the release bearing. Here's how to figure it out.
Externally a 4-speed and 5-speed tranmission look pretty identical, but the clutch can lead to a problem.
For those of you who don't know how a clutch works, here's a simplified description:
When you step on the pedal, a hydraulic cylinder moves a pivoting arm that holds a release bearing. The bearing presses against the clutch's pressure plate, which "releases" the disc and disengages the transmission. Step off the pedal and the bearing moves away from the clutch, which pins the disc against the flywheel and off you go.
Swapping the transmission itself is pretty easy because the bolt patterns for all 240z/260z/280z/280zx engines are exactly the same. You don't have to change the starter motor or the flywheel, but with the transmission out it's a good time to get the flywheel resurfaced. It's also a good time to replace the engine seal because it's cheap and easy to do.
The main issue is the collar that holds the release bearing. If your "new" 5-speed transmission didn't come with one, or if you're changing to a different type of clutch, you might need a different collar because your existing one might not be the right length.
Nissan made a few different sizes - this picture shows the two most common ones. The difference is pretty clear. If you use one that's too short, the clutch won't disengage because the bearing won't reach it. If you use one that's too short, the clutch can't engage because the release bearing can't move back.
Start with the clutch. It doesn't matter if it's a stock or racing clutch because it's the height of the mechanism that determines which collar to use.
Take a straightedge and place it on top of the clutch. Measure from that point to the mounting flange of the engine. This will tell you how far the clutch extends into the transmission's bell housing.
Attach a collar to the throw-out arm, then use the straightedge to measure from the mounting flange of the transmission to the face of the release bearing.
As you can see the dimensions match, which means I've got the right collar and the clutch should work perfectly.
This part is very important:
When you change the transmission, you have to change the little pinion gear that connects to the speedometer cable. If you don't, your speedometer won't be accurate.
When I swapped the transmission in my '72 years ago I ended up getting a ticket for doing 85 mph when the speedometer said 75 (in a 65 mph zone). The cost of that ticket, plus the increase in my insurance, was a painful lesson.
All you need to do is remove the pinion in your existing transmission and install it in the new one. This page on the Classic Z Cars website shows how it's done.