Step 1.

There were a couple of reasons to try to convert the headlights (old type: BA20D) to a H4 fitting. First of all, the old type, seen below, isn't widely available. So in case of a popped headlight it would be difficult to find replacements. Next, I wanted to switch to yellow headlights for improved visibility, and I could only find yellow light with a H4 fitting.

So what do you need for this conversion: Two H4 fittings, two H4 bulbs, and some soldering stuff, shrink tube, etc.

First, remove the complete headlight assembly. The fittings can be reached from behind, so further disassembly isn't necessary.

Step 2.

Remove the rubber dustcaps from the old fittings, and remove the old fitting/bulb from the headlight casing. Be sure to keep the springs (one for each headlight) that keep the old fitting in place, they will be reused. Cut off the old fittings.

Step 3.

Make the H4's fit in the headlight. The old fitting, had a notch, which would slide in a sort of groove in the headlight. Then the spring would retain the fitting in the headlight.

We will do a similar thing, the spring will retain the H4 bulb in the headlight, but there needs to be a sort of hook, which sort of replaces the old notch. Bend one of the three "wings" on the H4 bulb upwards. Be sure that this is the "wing" that is opposite to the part of the rim where the small holes are (these will be used for the spring). Do this so that the "wing" touches the metal rim as you can see on the picture. Then bend a little hook on the wing.

The right one on the picture has the hook, the left yet to be done.

Step 4.

Now, with a little fiddling, the H4 bulb can be placed in the headlight, first the part with the hook, which hooks under the rim of the headlight, then put the retainer spring back. And Bob's your uncle, well, for this part.

Step 5.

Do this for the other side as well. Next, the electrics.

You'll need two H4 fittings with wires for this. Available at any reasonable equipped carpart supplier.

Step 6.

Here you can see a simplified bulb drawing. Looking at the rear end of the bulb, this is (if I documented it right) how the wires should be connected. Feel free to check this if in doubt...

Step 7.

After some soldering and shrink tube.

Step 8.

You can pull the rubber boot over the rim of the new bulb. So that it is reasonably waterproof.

Step 9.

Here the next one has been done as well. Use some tie wraps to nicely route the wires.

Step 10.

Ready for the test drive!