Transmission

Teardown and rebuild

There are two transmissions, the original 4-speed that came with the 1300 cars and the 5-speed fitted to the 1500. Both are surprisingly easy to tear down and rebuild. Although the FIAT shop manual calls for a number of strange special tools, there are satisfactory ways of proceeding without them, described in the references below.

Procedures for the 5-speed are described in Steve Hoelscher's classic 5 Speed 101. There are numerous documented cases of total complete rank amateurs who had never even seen the inside of a transmission before, successfully dismantling and rebuilding their transmissions with the guidance of this invaluable text.

The 4-speed driveshafts and inboard CV joints are different, but otherwise the guts of the 4-speed are fairly similar to the 5-speed. Steve has written a 4 Speed supplement that covers the differences. (BTW, the magnet that catches metal shards in the oil is mounted in a different place in the 4-speed; that's about the only difference not covered in these two documents).

The detailed cross-section and blow-up diagrams in the shop manuals are also helpful.

Specifications

(It would be nice if someone could dig up the various gear ratios and final drive ratios available, and post them here)

5-speed 79-80: First 3.583 Second 2.235 Third 1.454 Fourth 1.042 Fifth 0.863 reverse 3.714

5-speed 81-82: First 3.583 Second 2.235 Third 1.461 Fourth 1.033 Fifth 0.863 Reverse 3.714

5-speed 1983+: First 4.090 Second 2.235 Third 1.469 Fourth 1.043 Fifth 0.863 Reverse 3.714

Differential

The differential is of course integrated with the transmission, so it is generally covered by the 5 Speed 101.

The one tricky thing about the differential is getting the bearing preload right. There's a procedure in the FIAT shop manual, but once again, Steve Hoelscher has a better simpler way. It's somewhere in the archives of the old xweb, and I'll post it if someone else doesn't get to it before I can).

Gearbox Oil

There are a number of recommended choices for gearbox lubrication:

• GL1 was the FIAT factory specification. It is still available, and if you just want to stick with the original recommendation, you can. However, lubricant technology has improved a fair amount in the intervening years, and many people are reporting better results with newer synthetics.
• RedLine MTL or MT90 is a well-regarded modern synthetic. Last I heard, Steve Hoelscher's warranty on rebuilt transmissions required that you use RedLine.
• Castrol 20w50 motor oil. This sounds wrong because the viscosity is all wrong, but motor oils use a different viscosity scale than gearbox oils, and in fact a 20w50 motor oil is about right.

The one thing that is known not to work is standard EP gear lube like you'll find at a chain auto parts store. The EP additives interfere with operation of the synchros and will eventually damage them.

Transmission swaps

A popular upgrade is to replace the 4-speed in an older car with a 5-speed. Now you'd think the going the other way, replacing a 5-speed with a 4-speed, doesn't happen as often... but that's basically the same as replacing a 1300 motor with a 1500 motor, so it's a sensible swap too. Both bellhousings and flywheels bolt up to both motors, so the easiest way to do these swaps is to keep an entire drivetrain together. If you're swapping a 5-speed in, use the 5-speed clutch, flywheel, starter, transmission, driveshafts, and rear A-arms. And likewise with a 4-speed swap; use a 4-speed clutch, flywheel, starter, transmission, driveshafts, and rear A-arms. If you go this way, remember:

• You can use the same clutch slave cylinder (it mounts differently but it's the same part).
• You can use the same struts.
• You cannot use a 4-speed shifter with a 5-speed; the 4-speed gate locks out fifth gear. So you'll need a 5-speed shifter if you're swapping in a 5-speed.
• There are two styles of 5-speed shift linkages (lollipop and metal arm - both illustrated here). You cannot mix and parts between them or with a 4-speed linkage; whatever linkage you use, use the whole thing.
• There is a metal bracket in the tunnel of a '74 that will interfere with the throw of a 5-speed shifter. Bend it down out of the way with a hammer and a punch if you're having trouble with reverse and fifth, or if a gremlin seems to be snatching the spring clip off the end of the shifter all the time.
• The 5-speed is very slightly longer than a 4-speed, so may rub the inside fender well on a 1300. One good whack at the fender well with a heavy hammer will indent it just enough to stop the contact.