At 05:22 PM 7/24/04 -0400, Fred Stankovich wrote:
".... I took the socket back to the mech .... he noticed an ever so slight ridge in the socket that looked like it would be enough to hold the threads of the bulb. ....".

The part is the screw base bulb holder for dash illumination and indicator lamps.
The bulb pops out of the socket, not held in place by the thread.
Pictures are presented here first to print all on one page. See report below.

New bulb holder with very shallow thread groove on left. - Source not yet verified.
Older bulb holder with deeper thread groove on right. - Moss Motors part from 1986.

The apparent problem is that the thread groove is not stamped deep enough in the sides of the new bulb holder, so the bulb is not held in place. There may also be a minor problem with the new bulbs, which should not be a problem at all if the bulb holders were made right.

Parts in question are:
the Bulb Holder - Moss part number 161-915 (but source not yet verified),
the Bulb - known to be a Moss sourced part 171-000 - (this part may be okay),
and associated assemblies, lamp holders and various wiring harnesses.

Hard to tell the bulbs apart, but the older bulb on the right
measures 0.010" larger OD across the crests of the threads.

This may be a wide spread problem. Follow this for a minute.

I received a message from a "fellow-1" (Fred Stankovich) with a fairly new MGA wiring harness. A restoration and paint shop "fellow-2" who procured the harness says it came from Moss, but not sure that anyone knows for certain just yet. But I also have additional supporting material.

See pictures above of bulb holders new and old. Fellow-1 says the new part on left is part of the MGA wiring harness, multiple pieces in the harness, to hold illumination bulbs for four MGA dash instruments plus high beam indicator, ignition light, map lamp and turn signal dash indicator lamp, a total of 8 bulbs in the MGA dash.

I also have in my posession a Moss part number 142-400 lamp ass'y (MGA turn signal dash lamp) recently procured. The bulb holder is identified in the Moss MGA 0402 catalog as part number 161-915, used with the MGA map lamp (not part of the lamp ass'y). The Moss web site identifies it as also used in MGB, Sprite/Midget, and TR2-4A. This bulb holder is likely to be used in as many different wiring harnesses and lamps as the length of the bulb application list. The bulb is part number 171-000 used in many applications, AH 100 & 3000, Jaguar XK, MG TC TD TF MGA MGB Sprite/Midget.

The bulb holder has a very small internal ridge, slightly angled in two places on opposite sides of the bulb holder, supposed to serve as the female thread for the screw in bulb socket. Put the bulb in the socket, and it pops back out, no grip for the mating threads. Either the (partial) female thread in the socket is too large in diameter, or the male thread on the bulb is too small in diameter, or quite possibly BOTH PROBLEMS at the same time.

While I was trying to install the loose bulb in the loose socket, the bulb popped out on the table and broke. I took another bulb from my invetory, procured from Moss Motors a few years ago. This bulb is identical and also pops out of the socket.

For additional check I went to my MGA and pulled the bulb holder for the map light out of the dash. This is part of the wiring harness procured from Moss Motors in 1986. The bulbs currently in my car were all replaced again as a batch in spring of 1998, also procured from Moss Motors. Now it gets stranger. The 1998 vintage bulb will stay adequately in the new bulb holder. The new bulb will stay adequately in the 1986 vintage bulb holder. Apparently the older bulb socket has smaller thread diameter, and the older bulb has larger thread diameter. So I measured all parts with a dial caliper.

Old bulb thread OD = 0.367-0.370 dia (not quite round)
Old holder thread ID = 0.340 dia (at smallest space between ridges)

New bulb thread OD = 0.358-0.361 (not quite round)
New holder thread ID = 0.359 dia (at smallest space between ridges)

Old bulb in old holder gives 0.027-0.030" grip.
New bulb in old holder gives 0.018-0.021" grip.
Old bulb in new holder gives 0.008-0.013" grip
New bulb in new holder gives 0.001" interference to 0.002" clearance, depending on orientation the of bulb, so no grip at all.

These are supposed to be nominal 3/8" diameter screw base bulbs and sockets. The old bulb appears to be perfect size, and the old socket is a nice clearance fit for easy assembly.

The new bulb thread is about 0.010" smaller, and appears to be slightly smoother across the apex of the threads when the two bulbs are held side by side. I think this by itself is not of much consequence, and should not be a problem.

The new socket has thread ID nearly 0.020" larger which is most likely the serious problem here. The outside grooves and inside ridges in the new socket thread, are visibly very small compared to the more generously formed threads in the older socket. It appears that the new bulb sockets are defective, which will probably affect quite a number of lamp assemblies and various wiring harnesses.

Maybe new socket forming tooling is different. Maybe old socket forming tooling is just worn out from years of use. I don't know. As an engineer, if I had to fix the new sockets I think I would make a small crimping die with sharp edge "V" jaws that would pinch the sides of the socket at the grooves to downsize the thread slightly for more proper fit with the OLD bulb (with proper OD). I have no idea how many bulb holders or how many wiring harnesses might be in inventory of various distributors or the harness manufacturer.

As the MGA wiring harness with the bad bulb sockets is "suspected" of being a Moss Motors supplied part, Moss Motors has been notified. I am awaiting word from them on the status of their inventory parts. I am also awaiting word from fellow-1 and fellow-2 on verification of the supplier of the MGA harness with the problem bulb holders. Any additional information will be posted here when available.

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Addendum: July 18, 2004
From: Lesley at British Wiring Inc <BritishWiring@Ameritech.net> www.britishwiring.com

This is a problem with all bulb holders of this style currently coming out of the UK. There is only one manufacturer for this item and the problem stems from old machinery and a very old dye. The company is not willing to retool. The last of this style of bulb holders is very close. After that the only remaining option will be a plastic substitute.

That slight indentation in the side is all that holds the bulb in place. Some bulbs fit better than others (I have a supply coming in that our manufacturer says fits better) but the situation still will exist.

A nice "fix" that one of our customers found was to cut a dowel rod of the right internal size, put it inside the holder, positioned a screw driver on the indentation and tapped it lightly with a hammer to inhance the indent. This helped the fit and certainly helped him work off his frustration!

If the bulb seems to be popping out from some sort of pressure either filing a little off the bottom of the bulb or the contact terminal will sometimes help.

Putting the bulb in with a sort of push/turn, push/turn motion may also help - this seems to help get the bulb in straight and increases the likelihood of a first time fit.

I'm sorry I can't offer a cut and dried solution for you but as you well know the restoration of these old cars is not an exact art.

Best Regards
Lesley

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Addendum: November 26, 2006
The news is out of the bag. Some new replacement wiring harnesses now have plastic lamp holders for dash lights. The strange part is, these are for bayonett base bulbs, so now you will have to stock another part number of light bulb for spares and keep another note for another modification to your car. And of course I hope you are not a concours enthusiast. See Electrical Modifications for details.