P100 Headlamp Mounting

Putting my headlights back on today. The stud is not long enough to engage the full thickness of the nut. Can this be right?

As an engineer, I am uncomfortable seeing a nut with less than full thread engagement.

How do you guys aim these headlights without the reflector and bulb in place? Do you just guess, or put the bucket lens more or less perpendicular to the ground, or do you leave the big nut a little bit loose, like loose enough to push the headlight around on its socket?

Hi Rob,

If memory serves me correctly I think the normal nut is thinner than yours. Are the nuts on both your lamps the same?


If more than 2/3s of the threads are engaged, it won’t fall off.

Both nuts are the same. The thread is 5/8"-14 BSF. Nut thickness is 3/8". I would call that a thin nut. But only about 1/4" is engaged, so that’s a little over 3 threads.

Do the studs unscrew out farther? Does anybody know what’s on the underside?

Maybe I remembered it wrongly but it does look thinner than yours.

Here’s mine.


Yes, yours is certainly thinner, and it looks like you have only about 3 threads engaged. Same spherical washer. Headlights aren’t all that heavy for a 5/8" nut, so I guess it must be ok, and if its held up for 82 years and 96,000 miles it will last another 82. Thanks Peter.

What about alignment? Ed?

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Is that nut brass? You could thin it up, on a large, pretty easily.

Yes , the originals weren’t brass.so seems t be a replacement. WE do it with one of us holding the lamp at what we think is the correct orientation and the other tightening the nut with a bit bar on a socket I recalla star washer being a participant
You are thrice fortunate to have pre war P100s. The post war won’t focus both beams if you use double dipping globes.
But because of a different socket , and so different bulb, The pre war do
WE just point the car at the garage wall and turn on the lights. And keep adjusting. Looking from the side one would lke to see them with both flat fronts at the same angle. My brother insists that the RH h one should face slightly left but that won’t help much in LHD countries
The studs are cast in.

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Thanks, Ed, I won’t attempt to unscrew the studs. I’ll leave the nuts as is since being taller the wrench fits them better. I used a machinist’s level to set them. I’ve changed the left hand light to dip straight down rather than to the left. The right hand still just goes off for DIP.

Forgot to ask, what is this little clip on the pivoting frame? It looks like for a fuse or light, but it is grounded on both ends. Is it for a spare fuse for the solenoid?

Your lamps have a very different interior to mine but I would guess it’s spare fuse holder.


p.s. Perhaps this one still has a spare fuse.

Yes, it is a spare fuse holder, definitely fitted to post-war P100Ls, and probably pre-war ones as well. Annoyingly, the fuse is not the standard size, so might be difficult to source them now.

Rob, That should make for interesting times driving at night.

The P100s on my 'rent’s PIII Rolls were “Americanized,” such that both dipped.

So, it takes one of those old pointy ended fuses, probably a 35 or 50 amp. Searched ebay, no luck. I have a couple of blown ones. I wonder if they can be restored? Just solder in the right size wire. :grin:
So far at night I’ve only driven around my neighborhood, but nobody flashed their brights at me. Even on high those lights aren’t anywhere near as bright as everybody else on the road.


I was thinking of the one that isn’t lit.
I hope Pauals parents P100s didn’t have those awful sealed beam inserts.
I have my pre war P100s with double dipping bulbs and they work quite well , Both high and low beam are a good focus.
This only works on pre war P100s which have a smaller bulb holder and take bulbs with the 2 filaments close together.
I wonder whether its worth all that trouble with the fuse, The fus eis for the dipping mech not the bulb.

Nope: they were “Flamethrowers!” Dad had the reflectors resilvered, and he reported they worked pretty good.

Ok sorry, I didn’t think we were talking about a twin filament bulb arrangement. I can see where it would mean that one filament was not in the focal point of the reflector. Which is how it is in Mark V headlights.
With my pivoting reflector the filament is always at the focal point. If I wanted two dipped beams I would look for another pivoting reflector from a wrecked P100.
What a pity some PO didn’t find a second one decades ago when it would have been more available. Probably what Paul’s had, unless the original destination country was not the UK. I believe, and correct me if I’m wrong, that it was only the UK that had left-dips-left, right-goes-out.

Yes, it’s not difficult.


You don’t need another reflector, just another solenoid, which are also quite rare.
The non-dipping reflector is just locked into position with a screw attaching it to the rim. Remove the screw and it dips, assuming the spring is in place.