Fitting a Hewland MK4 gearbox to the Fiat Abarth GT 750 Coupe Zagato

OK, here it is, finally. A first attempt at a “how-to” for fitting a Hewland MK4 gearbox to the Fiat Abarth 750. Keep in mind that there is a lot of measuring and custom fitting to be done, but it can be made to work well. Before going any further, I’ll make a full confession, as you’ll probably notice the adapter picture and throw your hands up in horror! YES, I did design the adapter plate to fit the Hewland to both the Abarth motor and also to my spare 16V twin cam engine from the 93 Suzuki Swift GT.

Will the GT Swift engine fit? You bet it will. Will I ever actually put it in the car? Not a chance, but the option is always there and the thought of a modern 110 horsepower motor pushing along a thousand pound car is always fun to think about. For now, the OT1000 Abarth engine is doing a fine job, so in the spirit of “if it ain’t broke, don’t fix it” the Abarth motor stays. End of story.

Since you'll probably be designing for a single engine, your template should be a whole lot easier to make.

lexan model of the engine adapter plate

A piece of Lexan, a marking pen, a ruler, a few ideas and a lot of optimism. That's all you need to get into a crazy project like this.

In my particular installation, I used the Fiat clutch and pressure plate, but I kept the VW throwout bearing. This meant that I had to do some surgery on the input shaft since the VW shaft splines and the Fiat clutch splines are incompatible. I used a piece of the Fiat shaft pinned and welded to the cut-off VW shaft after it came through the transmission seal.

Getting the lengths correct took a lot of careful measuring. Do whatever you can to reinforce it, if you go that way. To determine the correct length, I used a straightedge across the bell housing and carefully measured the distance to the face of the fully retracted TO bearing. After installing a fresh clutch, I measured the distance from the clutch fingers ( the part where the TO bearing contacts ) to the part of the engine block where the bell housing attaches. This was kind of a tough measurement but critical.

Given those two dimensions you can calculate the distance between the bell housing and the block (the adapter thickness), allowing for a small clearance space between the TO bearing and the clutch. A bit tedious, but not all that bad once you get into it. I used a sheet of clear Lexan plastic and started with a small hole in the center that represented the center line of the transaxle input shaft. (see photo) Once lined up on the face of the bell housing, it was sort of easy to outline the bell housing shape and locate the bolt holes.

For the Fiat side, I did the same thing using the Fiat bell housing. When this was done, I used the plastic part as an inside and outside template for the adapter. Worked well. As the Abarth starter motor bolts in from the engine side, the bolt holes for the starter can be incorporated into the adapter. The template is good for that purpose as well. I found that to be the most difficult part of the job, but it came out ok.

Well, almost the most difficult part . :>) The doggone shift linkage gave me fits, but eventually (with a little tweak here and there) a straight rod did the job. Keep in mind, the VW box is going to be tight where it fits between the rear suspension arms. I actually had to trim the filler plug and do a very small bit of filing on the A arms. It IS a close fit, but (at least in my case), doable.

underside view of mounted Hewland transaxle

Not a lot of room to spare, but it does fit.

My adapter ended up being 1 inch thick, but I would highly encourage you to make those measurements on your own since there are a lot of variables to consider.

The axles are custom made and have VW /CV joints inboard and Suzuki Swift front CV joints outboard. Seems to work well. Double CV joints are needed because the axles do not exactly line up as the they did with the Fiat transaxle. Just a little off, but enough to require the double joints. Be fore-warned — I melted a set of Fiat rubber rear wheel couplers finding that out!! If there is any interest in how the custom axles were done, email me and I’ll try to detail the setup.

Custom made double CV Joint rear axle

Power to the wheels: This is what the custom double CV joint axles used to connect the Hewland axles to the Fiat Abarth hubs look like on the car.

custom fixture to hold the Hewland transaxle in place

Rear mounting bracket for the Hewland courtesy of my good friend and intrepid machinist Larry Bourgeois. Larry also did most of the work on the adapter plate. Thanks bud, you did good!