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MG MGB Technical - Screw rear lucas lens

Hello,

Could anaybody please tell me the specification of the screw for the Lucas rear lens on an 974 MGB USA. Moss UK number is 518868?

Thanks Eric
Eric van den Berg

I have some of these bagged up ready for my rebuild so I've just had a look. The Moss part indicates these are used on the rear lens so I started there and found that on my '72 car the part used was PMP214, the same as used on the reversing lamp lenses. The difference between the two is that the PMP screw is a pan head whereas the 518868 is c'sunk. From the lens I can see that the panhead is what is required so it looks as if the Moss part is incorrect, for me anyway.
By comparing the two I've worked out that both are No6 threads with a threaded length of 7/8". Both are chromed. The length is way over what is required.
I think the part number for the 518868 could also be written as CMC214 which I think translates as:
C - c'sunk, M - metric, C - chrome, 2 - No 6, 14 - 14 x 1/16" (7/8") for the thread length. Might be wrong but it works for me.
Sorry if this is a bit long and hope it helps.
Richard Coombs

Eric, I'd be interested in knowing if the rear lens screws on your car were/are pan head or c'sunk.
Richard Coombs

Richard, thank you very much for the long reply. My 74 MGB USA spec has pan headed screws (3 are torx and 1 slotted head)and not countersunk screws used on the rear lenses. I have removed 1 screw and it is about 1/2 inch (13mm) long from bottem head to end. If I look at the lenses the pan headed must be correct.
Eric van den Berg

The correct screws are Posidrive pan head, chrome plated, 1/2" long and 4 BA thread. If anything else has been put in there, like #6 UNC machine screws or anything metric, then you have to use whatever will work in the damaged threads. Length is not critical, though way long could run into something.

FRM
Fletcher R Millmore

Agreed, the 518868 and the PMP214 are No6 UNC, so incorrect thread. The 518868 also has a c'sunk head instead of the required pan head. The diameter of both is 0.1mm less than 4BA but the thread pitch is quite a bit different at 1.514/mm versus 1.26/mm. Now it's a fact that the No6 will go into the 4BA threaded section of the lens support, probably more because the receiving thread material is not particularly hard and the OD is smaller. However, as Fletcher says this will damage the thread so returning to a 4BA may be more difficult.
So, the end results are use 4BA screws, and the Moss part supplied under 518868 is incorrect as it's c'sunk and I believe to be a No 6 UNC thread.
The MGOC supply part number PMP208 for this position which I expect will be a pan head, 1/2", and still a No6 UNC. Not sure of the finish. The PMP214s (7/8") I have appear to have a gloss finish but I'm not competent enough to decide whether its a 'bright zinc' or chrome.
So much noise about a single screw!!!! Where's my anorak?
Richard Coombs

According to my old parts book the correct part number is PMP208, which I am sure is Pozi, panhead, No6UNC x 1/2in.
FRM I think you are wrong as BA was not used on light units except maybe on the electrical size. I always use N06 UNC for these and other Lucas lenses, in fact I have some new genuine Lucas chrome screws still which are No 6 UNC.

CP
Colin Parkinson

RMP308 for the later lens, according to the Parts Catalogue, which translates as SCREW, CSK, RAISED HEAD, CHROME, UNF, NO.10 X 1/2". However they quote a quantity of two (which was the quantity for the earlier light unit) and it should be four.

Brown & Gammons state PMP208 which translates as SCREW, CROSS HEAD, PAN HEAD, CHROME, UNF, NO.6 X 1/2", quantity 4. They also have RMP308, but for the rim of the earlier light unit, quantity 2, so B&G are probably correct.

My later light units have pan-head and not countersunk screws, which again ties up with B&G.
PaulH Solihull

PMP208 it is then for '69 on cars i.e. later lens. That's a decision folks!!
Paul, my March 2009 Moss catalogue, and their website, gives 518868 for the later lens which, as discussed above, is a cross-head, chrome, c'sunk screw, size No.6, 7/8", and therefore entirely unsuitable.
Richard Coombs

Richard - Moss were probably confused as well, so decided to opt for a combination i.e. No.6 countersunk :o) But where the 7/8" came from is anyone's guess!
PaulH Solihull

I stand corrected on the thread - it is 6-32 measured off OE screws.

FRM
Fletcher R Millmore

This thread was discussed between 31/07/2010 and 02/08/2010

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