Metering Head Part Numbers

I have two metering heads for my EG code 1.6 engine, the first came fitted to the car and is:

Bosch 0 438 120 189
VW 049 133 353 AA
When I search those part numbers the most helpful information I find is the same units listed on Ebay as for a Mk1 1.8.

The second I bought from an EG car being broken, and its part numbers are:
Bosch 0 438 120 035
VW 049 133 353J
When I search those I seem to find Mk2 1.8 units.

Now I know these are the part numbers off the air flow meter section, and that the fuel distributor section has a seperate part number, but I'm assuming they have been swapped as a pair like they usually do.

Looking on VAGCAT I have found the part number 049 133 471 D which is for a 1980 EG GTi

So unless the above is wrong, I have the incorrect metering heads, my car has run from the AA head for the past 10 years without issues, although I assume it would over fuel.
Was intending to rebuild one of the metering heads to chase a stutter but would rather start with a unit for a 1.6. What is the technical internal difference between the 1.6 and 1.8 metering heads?

 

Ptrobaly no difference contact Martin Challice (grey Goose Restorations)

I put the question to chatGTP, interesting reply:


Key Internal Differences Affecting Flow Rate

1. Fuel Distributor Plunger

• Yes, the plunger is different between 1.6 and 1.8 units.
• The plunger diameter and the shape/profile of its metering slits are tailored to control how much fuel is metered for a given flap position.
• 1.8 units have plungers designed for higher fuel flow, meaning larger or differently shaped metering slots and sometimes a slightly larger plunger diameter.
• This affects how quickly the plunger moves within the fuel distributor to allow fuel flow.

2. Calibration Screws and Metering Valves

• Underneath the calibration screws (sometimes called control valves or flow valves), the valves themselves are different sizes or shapes.
• These valves limit maximum fuel flow and fine-tune the characteristic curve.
• For example, the valve orifice diameter in a 1.8 unit is often larger to allow greater fuel volume, while the 1.6 valve orifice is smaller for reduced flow.
• These valves also influence how much fuel flows at idle and part throttle.

3. Airflow Sensor Plate / Flap

• The shape and weight of the airflow flap and plate affect how the fuel distributor plunger moves — heavier or differently balanced flaps in the 1.8 unit cause different mechanical responses to airflow.
• However, this mostly affects sensitivity and is secondary to the actual fuel flow control inside the distributor.

4. Metering Slits and Slots

• Inside the fuel distributor, metering slits/slots in the plunger and distributor body differ in size and shape to control flow rate.
• These slits are precision-machined to calibrate the system to the engine’s displacement.

Summary Table Why Does This Matter?

• The combination of plunger and valve sizes controls the max fuel flow and fuel curve shape.
• If you fit a 1.8 metering head on a 1.6 engine, the metering head allows too much fuel for the given airflow, causing a rich mixture.
• Conversely, a 1.6 metering head on a 1.8 would starve the engine of fuel and cause lean running or hesitation.
  
  More Info:

ChatGPT - 1.6 vs 1.8 Metering Heads