This issue relates to the bulb holders for dash illumination lamps and indicator lights. They are 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.

One original factory issue bulb holder (socket) is shown at right. Inside the shell is a round disc with a central hole with a small internal notch (11:00 o'clock position). The internal edge of this disc is stamped into a helical shape to create a one-turn female thread to accept the screw base bulb. There are six spring fingers around the perimeter on the open end to allow the assembly to snap into a round hole for mounting. The center disc and spring fingers make the ground return contact for the threaded shell of the bulb base.

Deeper in the socket is a round fiber disk carrying a single center wire contact, backed up by a preload spring to push the disc forward in the socket making the power input connection with the bulb center contact. The single wire is in most cases part of the wiring harness, so after factory assembly these sockets are also an integral part of the harness. These original sockets work well but may be corroded after decades in service, and the wire insulation may be badly aged, so these things will be replaced along with installation of a new wiring harness.


Fast forward 25 to 30 years. Pictures above show a bulb holder on a replacement harness purchased from Moss Motors USA in 1986. These sockets are obviously a cost reduced design with only two metal pieces, the shell housing and the spring finger ring. The female thread is effectively formed by indenting the socket wall to form an internal helical ridge on two sides that will engage the male threads on the bulb base. This one also works well, and these have been in service in my MGA for more than 20 years and 200,000 miles, still in good serviceable condition. So far so good.


Move forward again, and the pictures above show a similar socket on a harness procured in 2004. This time the ridges forming the female thread are much less pronounced leaving a larger inside diameter, and this "thread" will not hold the screw base bulb in place. You might screw it in lightly with your fingers just to have the bulb pop out in your hand. These sockets have been part of wiring harnesses for multiple vehicle models, may have been supplied by multiple harness manufacturers, and come to market through various retailers.

I also have in my possession a Moss part number 142-400 lamp assembly, MGA turn signal dash lamp, recently procured. This has a few problems of its own, see Faulty Parts report FT-012. 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 assembly). 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.

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.

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

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 inventory, 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 Edison E10 Miniature Screw Base, nnominal 3/8" diameter screw base bulbs and sockets. Thread major diameter = 0.375" max. and 0.365" min. 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 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. The new bulb sockets are defective, which will probably affect quite a number of lamp assemblies and various wiring harnesses.

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.

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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.

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 enhance the indent. This helped the fit and certainly helped him work off his frustration! 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 science.

Best Regards
Lesley
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Addendum: March 4, 2011:
At 04:43 PM 3/4/2011 -0800, Ronald Perring in Salt Spring Island, BC, Canada wrote:
"I am fortunate to have a lathe and came up with a jig. I had a one inch bar stock and cut off a piece one inch in length. Drilled a hole with a letter Z drill (.413") - then cut out a slot of approx. 1/4 of the barrel. It looks like a flare wrench. It makes a snug fit over the bulb socket. I then screwed a 3/8-16-UNC bolt inside. I was then able to tap the indent with a flat screw driver causing no distortion to the bulb socket".

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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 bayonet 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. These are plastic sockets with an additional black wire for grounding. See Electrical Modifications for details.
Lesley
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Addendum July 6, 2010:
Now we have a new problem exactly the opposite of the old problem. Picture here shows a new metal bulb holder with deeper indents in the socket wall to form the "thread". Unfortunately the indent is now too deep and the bulb will not fit into the socket.

At 04:12 PM 7/6/2010 +1200, Dave Addison in New Zeland wrote:
"This is replacement wiring loom for my '58 MGA 1500. The loom in question is Moss UK part # ML594 - fabric covered loom for 1500cc, but I suspect the issue affects all current Moss looms. I have been in touch with Moss UK, and they acknowledge the issue.

"The replacement holders have a small 'chop' mark on opposite sides, supposedly to allow the bulb to thread onto them. Unfortunately, the 'chops' in the holders are too severe, and the bulb thread won't start on them. I removed the map light holder (which is on a separate wire direct from the switch and not joined to the rest of the loom) and removed the wire, the black plastic insert and spring from inside it. I was then able to put a metal rod inside it (actually a large screwdriver) and with it carefully positioned on my vise, tapped the screwdriver on the side a few times. After several gentle attempts, that eased out the 'chops' enough until I was able to get the bulb thread started.

"I contacted Moss UK technical support and got a reply from a guy called Jeff, who said:
'It seems that the bulb holder is a little oversize. I squeezed one up with a side cutters until the bulb gripped when screwed in, I have been unable to speak to the loom manufacturer to ask them on their view, but I imagine they have made tens of thousands of these as they were until fairly recently supplying plastic holders which required capless bulbs which no one liked using, so it seems we can't win at the moment but will work on it'.

"From that, I take it they are seeing the opposite issue to me - and a much more easily solvable one. I'd be quite happy if I just had to squeeze them closed a bit - much easier than removing them and opening them out. I have no idea if this issue affects looms purchased through Moss USA". -- Dave Addison

On 11/2/2010, Grant, Michael Grant at Moss Motors Ltd, USA wrote:
"We have been after the manufacturer in the UK to go back to the original type sockets as soon as possible. The confusion caused by the plastic sockets is significant. And the available plastic sockets are not suitable for all applications. They don't fit some sockets on the instruments in some cars. As new OE type metal sockets became available, the manufacturer began switching their harness back to OE type metal sockets. .... The sockets being used now are of new manufacture.

We have original harnesses and bulbs and we have checked it out very carefully.
Original bulbs fit the new sockets just fine.
New Wagner bulbs from the local auto parts store fit fine.
There are bulbs out there that are undersized and we believe that is the issue.
....
This is complicated by the fact that these undersized bulbs appear to fit the aftermarket sockets which also do not seem to meet OE specifications. We are not completely done with our investigation but we plan to include the necessary bulbs of the proper size with the harness until we get the aftermarket bulb-separate socket mess sorted out.
....
Best Regards,
Michael Grant
Product Manager