Refer to entry in the Workshop Manual.
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.
Transmission Installation Tips
These are the "tools" I use to change out a trans.
1. A 2 to 1 pully system and a BIKE HOOK to a rafter in the garage. The trans weighs about 91 pounds so installing a hook somewhere centered over the trans and HANGING from it will give ya a good idea if it will support the weight. I sometimes use 2 hooks and a rope sling between them to make a "gantry" of sorts and it gives me some side to side flexibility.
2. A CLUTCH PILOT TOOL, mine from Pep Boys, #14540 for Bertone/Fiat 3/4" x 17 splines made by Motormie Mfg. Colmar PA from TAIWAN. I understand Matt was selling them for a couple of bucks, mine cost $5.00.
3. A 3 inch long STUD made from a 12 x 1.25mm bolt with the head gorund ROUND and a SLOT cut into the end. (The slot allows a screwdriver to unscrew it from the engine when complete.) I install this stud in teh "2 o'clock" position of the bellhousing...
Then start installing the trans by locating the 4 o'clock STOCK stud, then this stud at 2 o'clock into the trans... and the INPUT SHAFT should line up perfectly.
See the Final Drive Speeds page.
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
Final Drive, All 5-speed: 13/53 or 4.077:1
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).
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.
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 gooseneck. 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.
This Fiat service letter is why there are two 4-speed clutch pressure plates, flywheels, and bell housings. media:tranny.pdf