Installing a Fiat 126 engine and gearbox into a Fiat 500

The FIAT 500, last manufactured in 1975, was a superb town car. It was cheap, easy to maintain, mechanically robust and very economical. But the engine, for today’s roads does lack some power and for the last 2 decades 500 owners have been installing the larger 600 or 650cc engine and often the accompanying gearbox from the air cooled 126 cars.

The engine train from a 126 it is generally very similar in concept, but differs in number minor details e.g. larger carburettor, different starter motor and often came with an alternator and importantly a synchromesh gearbox.

The main advantage of upgrading to a 126 engine train is the increase in power from a std 500 engine 499cc (18 bhp /22torque) to either the 594cc (23 bhp/29 torque) or the 652cc (24 bhp/31 torque) from a 126. This gives a dramatic increase in performance; much improved open-road ability and coupled with the ease of use of the synchromesh gearbox.

Either the 600cc engine or the 650cc unit can be transplanted by the same steps as explained below.

Ensure that when sourcing and engine/gearbox that you obtain all the essential ancillaries like the carburettor, starter motor, alternator, distributor, fan cowlings, exhaust, including the charging circuit wires from the alternator and the under car steel heater box if possible.

Conversion now follows the sequence below:  

(It has been assumed that all parts of the new engine have been inspected, overhauled and serviced as required, including the ancillaries)


  • Different settings and specifications are needed for the bigger engine – plugs, tappet clearances, gearbox oil etc. check the workshop manual
  • 126 engines still require leaded petrol so additive is still needed with unleaded fuel

A. Engine

Replace the 126 engine mounting pins, screwed into the timing case, with the longer ones of the original 500.

B. Fuel

A 126 carb has a return feed out from the back of the float chamber, this MUST be blanked off.  A simple solution is to take a piece of NEW rubber fuel pipe, cut a length to about 40mm, then take a small M8 bolt and “screw” it into the hose, secure with a jubilee clip and then fit the blanked off hose onto the brass ferule, again using a jubilee clip.

C. Electrics

a. Main Battery +ve (positive) lead

Re-route this main black lead, to meet the re- positioned starter motor. It is preferable to pull it through the central trunking the 3″ – 4″ needed and then re-anchor it to the outside of the offside suspension pillar.

b. Main lead to Regulator box

Replace or extend a few inches, the heavy brown lead from the Voltage Regulator box, (terminal 30) to allow for the repositioned starter motor.

c. Charging circuit (there are 2 options now!)

(i) Dynamo models

There’s no change to wiring, straight replacement

(ii) Alternator models with integral diode (most common

You need to disconnect:

  • Brown lead
  • Thin black lead from Regulator box
  • Terminals 51and 67 respectively and tape up (old wires from Dynamo)
  • The old wires from the Dynamo (at RHS front) need to be taped up and secured out of the way


  • If your alternator comes with the loom then connect the thicker brown wire to terminal 30 of the regulator box. Then connect the thin black/Mauve cable to the terminal 51 to join the green wire or use a connector block.
  • If your alternator comes without wires then you will need a new length of thick brown wire (35 amp) and connect as above, plus you will need a new thin wire as above, OR you can use the old thin black Dynamo wire if that suits.

The regulator box is no longer functional, but just used as a connector point! You can remove the box completely and just use connector blocks if that suits

D. Engine Bay / underside or car

(i) Starter pull cable

  • With the 126 engine having the starter on the side of the engine, then the cable needs to be longer
  • Replace with a 126 one
  • It is possible to re-route a std 500c cable, but it’s very tight (if too tight the “dog” catches onto the flywheel!) OR you can elongate the cable to fit

(ii) Steel heater ducting under the car

This needs repositioning and/or changing for a 126 one. Remove the unit and the flexible pipe to the car heater duct. It is fiddly and to line up correctly best to do after engine and gearbox have been fitted. Existing rubber pipes can be used at both ends and then juggling the position of the steel box to fit.

Some people choose to buy different flexible pipe and cut to appropriate lengths to aid fitting.

Whatever solution is adopted, make tight all connections (zip ties very handy!) and secure the steel box well.

(iii) Black steel tray to right and above the distributor

This needs to be removed and will need cutting / bending to fit the new 126 exhaust position. It is best to do this after engine is in place to get correct sizing.

E. Gearbox

The 126 engine cannot be fitted on to the existing 500 gearbox due to the starter motor being at the side where as the 500 is at the top.

As a result, it’s easiest to use the 126 gearbox unit as the bell-housing has the correct starter motor positioning. If required it is possible to modify a 500 gearbox to fit by fitting the bell-housing from a 126 gearbox, but most people opt for using a 126 box thus getting the benefits of the synchromesh.

Changing the driveshafts:

The 126 gearbox has driveshafts that are longer than a 500, so must be changed. To do this the 126 gearbox bell-housing needs to be removed and the differential split. See workshop manual for details.


  • It is best to use New 500 driveshafts and new couplings, as the chances of the splines on used ones being perfect is minimal
  • However the old ones from the 500 box can be used but you will have to strip them out of the gearbox!
  • The specification of oil for a 126 gearbox is different to a 500! Use SAE 80W/90 (NOT HypoidEP) see separate technical reference here

F. Engine Fitting

Having completed all the tasks above then it’s just a matter of refitting the engine and gearbox to the car.

It is far easier to assemble the gearbox onto the engine off the car as well as all other ancillaries e.g. starter. Then it’s an easy job to offer up the assembly as one, using 2 trolley jacks. (one under the engine, the other under the gearbox).

Locate the engine using the original 500 mountings at the front and the cross bracket under the gearbox, always carefully check that ALL the cables and wires are not fouled anywhere and check the gearbox flexible mountings for misalignment.

Check also the position of the gear lever in the car. It may need to be adjusted slightly, using the two bolts at the side of the central trunking in the car.

G. Commissioning

  • Connect all cables to the various units, clutch, carb, accelerator, starter etc…
  • Connect the fuel pipe to the pump and secure all electrical wires including the HT leads etc…
  • Heater pipes and connections need to be finalised
  • The plate above the distributor can be left off until engine is running well and all adjustments made
  • Start up and check that the generator circuit/alternator is functioning properly
  • Road test, watching for signs of trouble in any of the bits you have altered

Note: As you have changed the engine, the engine number will be different, you should report this change to the DVLA, and you should also inform your insurance company.