Fiat 124-131 Transmission Swap FAQ

FAQ
Project.

124/131 Transmission Swap

Author: Matt Webb

Email: MattWebb502@yahoo.com

Date: 10/07/2005

Subject: Fiat 124, Installing a 131/Brava 5 speed transmission.

Resources: Mirafiori forum members and first hand experience.

Forward:

This write up covers my personal experience with installing a 131 transmission into my 1975 124 Spider. The information here may not pertain to other types and years of the 124. All I can offer is that this information probably holds true for all CS1 and up chassis. As always, YMMV.

Intro to 124 and 131 transmissions:

The 124 5 speed gearbox is notoriously weak and sensitive. When upgrading your power, the stock gearbox is the first weak link in the chain. The gears and sleeves are known to fail under elevated power conditions.

They also contain brass faced shift forks (debatable) and brass synchro rings. These were ok when new, but after 20+ years of service they tend to be worn. To make matters worse, common EP gear oils are corrosive to these parts and quite often have been run in these transmissions for long periods of time. The result is a poor shifting (grinding, hard engagement) transmission that can refuse to stay in gear. Even after a rebuild you can expect to have problems unless every component related to the problem area has been replaced and all specs have been checked.

This is frustrating. If you decide you need a replacement transmission, the affordable route is to get a used one -- this is a big gamble. It has been my experience that used 124 transmissions are a waste of time and money. It seems the only reliable solution is to do a FULL rebuild or to buy a new replacement transmission, both are expensive options.

There is another option however, the Fiat 131/Brava transmission. To start, these transmissions have been known to stand up to 400 wheel horsepower! They come with steel synchro rings, and no brass bits. They have a reputation of being stout, reliable, and maintenance free. Change the oil periodically and go.

There is one common failure mode I’ve heard of, and that is with the reverse idler gear. Apparently they can crack and make noise when going in reverse. I’ve been told that this can be avoided by coming to a full stop before putting the car into reverse.

There are two common types of 131 transmissions. The later type, which uses a “guibo” or rubber flex joint like the 124, and the early type which uses a splined slip yoke on the output shaft. This document will only cover the late style flex joint 131/Brava transmission. These are rumored to be ’76 and up.

I dug this picture up that Jon Ranney posted to the forum some time ago. Odd 131 shifter.

The differences:

- slightly different ratios

- no interchangeable internal parts

- different clutch forks

- slightly different bell housing size

- different shift housings, different shifter placement

- different overall length

- 131 gasket set is cheap!

- 131 has steel synchro rings, 124 are brass

The similarities:

- same bell housing bolt pattern, bolts right up to block

- same input shaft splines

- same pilot bearings

- same throw out bearings

- same starter provision in bell housing

- uses same size rubber flex joint “guibo”

Drive Shafts:

Because the 131 transmission is longer than the 124 unit, the driveshaft will need to be addressed. The front section will need to be shortened to fit. If you obtain an automatic transmission 124 drive shaft you can use it. The front section is the correct length for a 131 transmission swap, and the rear section is also the correct length if you are running a late style rear end. This is no myth, I have done it myself. If you are running an early style rear end, you can marry the front half of the auto trans driveshaft with the rear half of the early driveshaft. See my 124 rear end swap faq for details.

The 131 transmissions yoke extends 95mm further than the 124 transmission yoke. The124 automatic transmission driveshaft has a front section that is 100mm shorter than a standard 124 driveshaft front section. If you cannot locate an automatic transmission driveshaft you will need to make measurements and have your driveshaft shortened. The easiest way to do this would be to set the 124 and 131 transmissions side by side and measure their lengths. Subtracting the 124 length from the 131 length will give a very exact number. For good measure you could shorten it a little bit more, and let the splined section on the front shaft take up the slack.

Thanks Curly for this shot.

Here is a driveshaft that has been shortened by the required 100mm. Looks just like an auto trans 124 front section.

If you wind up with balance problems with the driveshaft, Allen Lofland had good luck and fair prices @ http://www.drivelineshop.com/intro.htm

I have a sneaking suspicion that they could do the modifications for you too.

One other option I have heard mention of is mixing and matching 131 and 124 driveshaft sections. I don’t know the truth behind this.

What about the shifter?

The stock 131/Brava shifter and housing will put your knuckles into the radio console when selecting 1^st, 3^rd, and 5^th gears. With some creative bending it appears that this could be resolved. Really this is unexplored territory for me, but I do know that others have done it with success.

One interesting idea that came up a while back was to graft a 131 shifter and a 124 shifter together. To my knowledge this has never been done, but it certainly seems like a possibility. You’d better be good with cutting and welding though! This arrangement does have a drawback. All the gears in the shift pattern would be reversed! In other words, 1^st would be where 2^nd is supposed to be.

The other solution is to obtain the rare “Abarth” shift extension from outside the US. Pick a country where the “131 Racing” (or in the UK “Mirafiori Sport”) was sold. These came standard with the Abarth shift housing fitted. This special shift housing is an extension; much like the ones fitted to the 124, and will bolt right on to the 131 transmission. Not all have Abarth cast into them.

In a 124, this will place the shifter at the ashtray, which is pretty close to stock. Stock shifters, keep in mind, came out in front of the ashtray but were bent backwards some.

Here’s a snapshot of my 131 remote shifter. The shift lever does not appear to be stock.

File written by Adobe Photoshop® 4.0

Here’s another shot of one of these “Abarth” extensions. Notice that this one doesn’t have “Abarth” cast into it, and has a slightly different design as well. Thanks Curly for the pic.

If you can locate one of these, you’ll likely have to buy the transmission that it is attached to. Most people seem to be unwilling to sell them separately, but I’m sure they’d be glad to sell you both and only ship you the shifter!

It is possible to shorten the Abarth shift extension to cause the shifter to protrude in the stock 124 location. However, it is not possible to simply cut the appropriate amount and weld back together without forfeiting the shift rod seal. I talked to someone once who modified theirs to maintain stock shifter location and the shift rod seal, and they said that basically they had to make a whole new front section from scratch – apparently this was very labor intensive.

I’ve also heard rumor that the Fiat 132/Argenta has a 131 box and a shift extension much like the Abarth one.

For further details, Scott O’Brien posted some pics and measurements I took of

my Abarth 131 shift extension.

http://www.mirafiori.com/~sobrien/abarth/Abarth.htm

The transmission mount:

This is easy. Use the stock 124 mount cross member, and fit a stock pre 1983 131/Brava manual transmission mount. The cross member will bolt to the rear set of studs on the body. A 124 mount WILL NOT FIT. This is a rumor!

Thanks Curly for this snapshot of a stock pre ’83 131 manual transmission mount mated to a 124 cross member.

In the case that the mount is not available, you can cut, grind, weld, and drill a 124 mount to fit. It’s not that hard to do. Also you will need to grind a little of the 131 case for a proper fit. I would advise doing the measurements for this with the transmission in the car and supported by the engine and a jack.

I’m embarrassed of this one. Here’s my Franken mount. It’s a cut and welded 124 mount. It’s uglier and stronger than it looks from this angle. Afterwards I learned that early 131 trans mounts bolt in. They are NLA with some US vendors, but can still be found. Cross member is stock 124, and it lines up perfect with the rear set of studs on the body. Hey my cross member is in backwards! Oops.

The flywheel, clutch, clutch fork and cable:

The 131 clutch fork must be used. It will fit the car without problems. It will accept a 124 throw out bearing.

The stock 124 clutch cable is used. Well, that’s what I’ve been told. I’m having a slight issue with mine. It’s not long enough to fit with some play between the throw out bearing and the clutch. Not sure what the solution will be.

The late style (large) flywheel and 215mm clutch are used. This will require the appropriate starter as well. If your car has the later clutch and flywheel already, your starter will work.

For a period up until somewhere with the 1756cc engines, the large flywheels came with 10mm bolts. Later versions (late 1756cc and up) moved to 12mm bolts. So if you do need to convert to a large flywheel, pick the flywheel that will bolt to your crank.

A 124 bell housing WILL NOT bolt to the 131 box. This rumor has floated around in the past as a way to maintain the smaller clutch and as a way to help avoid tunnel modifications. Same goes for the idea of putting 131 gearbox internals into a 124 box --- this is NOT a possibility.

Here you can see the 131 fork easily clears the body (on install I thought I was going to have problems). Also in this shot you can see the 124 clutch cable fitted. Note that in my case there is no jamb nut in place and at this adjustment level the throw out bearing is making contact with the pressure plate. Instead of a jamb nut I might try something like safety wire to keep the nut from backing off.

Other people report no problems.

A few words about clutch cables.

- The 131 clutch cable is of a different design with adjustment at the firewall and will not fit the 124.

- The clutch cable used is the 1971 and up 124 clutch cable. Coupes and Spiders seem to have the same part number all the way to the end of production.

Transmission tunnel modifications:

There is a rumor out there that you can take 131 internals and put them in the 124 box to solve the following issues. This is BS!

Under the heater core box are two large dents in the transmission tunnel sheet metal. From my experience, it is not imperative that you remove these. For optimal clearance, they can be removed, as the 131 bell housing gets awfully close to them. Simply cut out the area and weld new sheet metal in place. It will take some forming of the new metal to get a good fit, as the tunnel is curved. This will not interfere with the heater box.

Yeah the paint is a poor match. Here you can make out the new sheet metal that has been welded and seam sealed into place.

Curly’s work is starting to really make mine look awful!

The other major concern is proximity of the rubber flex joint “guibo” to the transmission tunnel on the passenger side. It is best to put the transmission (fitted with the guibo) in place to observe this first hand. Mark the area that will need to be modified, and remove the transmission.

This should be the only time you have to dummy install the transmission. I advise you do this without even a flywheel fitted, but with a pilot bearing in the crank. This will make the process easier. It is also a good idea to clean up the input shaft and lube it well. Same goes for the bell housing / block dowels. Considering that you’re putting a transmission into a car that it doesn’t belong in, you should be happy that you only have to do one mock up.

Once removed, you have two routes you can take to modify the transmission tunnel: sledge hammer, or cut and weld. I always thought I’d go the cut and weld route, but in the end I chose the sledge hammer route. The clearance you need to gain is not that major, and you can actually achieve clean results by pounding the crap out of the metal. It’s quick too!

This picture shows pretty well where the sledge hammer did its job.

This picture shows the trans tunnel and passenger floor pan. It’s obvious where the extra clearance was needed. For a reference point, off to the right you can see the two trans mount studs.

Here you can see the optional welding route for providing extra guibo clearance. Thanks again Curly.

If you are running a 131 Racing Abarth shift extension, you will need to make a few more mods. Grind the seam sealer off of the joints at the end plate of the transmission tunnel. This will expose the spot welds. Chisel them apart. Now you can bring the back plate to vertical. In my case, this left enough metal to just re-weld the joints.. no real fabrication required! Peer down in and at the back of the shifter tunnel you will see another piece of metal that is protruding out ¾” or so. Cut on each side, and hammer it flat.

The old stock shifter access panel can get tossed out. You will need to open up the top of the trans tunnel quite a bit, so you will also be fabricating a new cover plate. Cut out enough material to allow the shifter to be installed and removed from inside the car. Otherwise, you’ll have a heck of a time trying to fit the transmission and shifter in place as one unit. You’ve got to enlarge the opening anyways, may as well make servicing easier while you’re at it. See the pic above for an indication of how big you should make the new access hole.

Fabricate a new cover plate. I did this with the inner shift boot in place. Use a piece of paper to make a template. Copy to sheet metal and cut. This will take some creativity on your part.

Here’s the finished product. Back section brought back to vertical and welded, front area cut out and new steel welded in, new cover mounted in place. Black tarry/silicone type gunk was used to seal the cover plate, nothing permanent. Foam gasket tape probably would have worked fine.

Getting the Interior to look good:

Sorry, I haven’t crossed this bridge yet. Curly has though!

Here are pics.

Gear Ratios and Speedometers

Before you read this section, let me warn you that it doesn’t lead anywhere conclusive. Don’t expect an easy magic solution to your speedometer troubles, because there isn’t one. The good thing about this section is that it does bring together many important and hard to find details regarding gear ratios, rear end ratios, and speedometer drives. I believe everything I’ve put into this section to be accurate; if anyone has any corrections please email them to me.

Note that the ratios here cover US market models, and ratios could be different for other markets.

Transmission gear ratio table:

Gear	124, to 1978	124, 1979 & 1980	124, 1981 on	131/Brava, 1975 to 1979	131/Brava, 1979 up
1^st	3.667	3.612	3.667	3.612	3.612
2^nd	2.100	2.045	2.100	2.045	2.045
3^rd	1.361	1.357	1.361	1.357	1.357
4^th	1	1	1	1	1
5^th	0.881	0.830	0.881	0.870	0.834
Reverse	3.244	3.244	3.244	?	?

        Just because transmission ratios are close (or in the case of ’79 and ’80, VERY CLOSE) doesn’t mean that the speedometer will be accurate. What’s real important is what the rear end ratio and tire size was for the vehicle that the transmission came out of.

        Below is a chart of 124 and 131 rear end ratios. It’s a little bit of an odd layout, but it keeps years, models, and ratios accurate.

Model Year	124 Manual	124 Automatic	131 / Brava Manual	131 / Brava Automatic
1967-1970	4.10:1	NA	NA	NA
1971-1974	4.30:1	NA	NA	NA
1975-1977	4.30:1	NA	4.44:1	4.44:1
1978	4.30:1	NA	4.44:1 AND 4.1:1	4.44:1
1978.5	4.30:1	NA	4.1:1	4.44:1
1979 up	3.90:1	3.58:1	3.58:1	3.58:1

Tire size:

What I’ve found indicates that tire sizes are the same for the 124 and 131. 165/80-13 (aka 165R-13). The alloy wheels ran a tire size of 185/60-14, which with all other things being equal would only account for a 1.9MPH difference in speedometer reading at 65 MPH. I highly doubt Fiat equipped the cars with alloy wheels with a different speedometer or speedometer drive. So, we can say that stock tire size for the 124 and 131 is 165/80-13.

Changing the Speedometer drive:
Contrary to popular belief, speedometer gear reduction is not handled inside the gear drives bolted to the side of the transmission. Simply taking one from a different rear end ratio car and bolting it to the transmission will do no good. These speedometer drives all have a 1:1 ratio.

Speedometer reduction is handled with the exposed gear on the speedometer drive unit and the worm gear that is internal to the transmission. Because of this, it will do harm to your transmission to try to swap over a speedo drive without also swapping over its matching internal worm gear in the transmission.

Taking these speedo drives to get “calibrated” is nonsense for our FIATs.

To the best of my knowledge 131 and 124 drives and gears are, unfortunately, not interchangeable. Someone PLEASE correct me if I’m wrong. I think the drives will bolt up, but I don’t think the trans gears are interchangeable.

The speedo drives are color coded, and this will indicate what rear end the transmission is set up for.

124:
Red = 4.3:1
Blue = 4.1:1
White = 3.9:1
??? = 3.58:1
131:
??? = 4.44:1
??? = 4.10:1
??? = 3.59:1

I have two questions that no one seems to know the answers to: - What are the color codes for the 131/Brava transmission speedo drives? - what are the speedo reduction ratios with the 124 and 131, and what are the intended final drive ratios to go along with them Testing your speed

My favorite method for this is to use the road side radar speed indicators. Every time I see one of these I hold steady throttle/speed and check what it says against my speedometer. I have found them to be very reliable.

Here are some options for solving speedometer issues:
http://www.speedometershop.com/ratio.html#ADAPT
http://www.speedometersolutions.com/

Another option is to adjust the speedo needle and/or scan and Photoshop the gauge face. This can be done on the bench.

Figure out what your actual speed is when your speedo indicates 50 MPH
disassemble the speedo, move and hold the mechanism in place so that it reads 50 MPH
adjust needle up or down to read what your previously recorded actual speed was.

Care must be taken while doing this. Don’t expect totally linear results here. That’s why I mention Photoshop. This can get you more in the ballpark with your speedo though if you pick a speed that you typically drive your car to calibrate against.