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MG Midget and Sprite Technical - Fastener size & spec from BMC part number

Hi

After many years of ignorance I have just realised that British Motor Corporation (BMC) had a standardised part numbering system for fasteners (bolts, set screws, washers and nuts) that gives their type, size and specification within the part number. Can't comment on what BLMC or BL did for later cars.

As usual wiser souls got there many years before me including I bet some excellent folk on here who have answered my screwy questions! Thanks to our MGA cousins for the following:

The following gives tables to look up:
https://mgaguru.com/mgtech/restore/pdf/fasteners_bmc_std.pdf

The following 'Notes on nomenclature' on page 2 shows you how to decipher the fastener type, size and spec from a part number: http://web.archive.org/web/20170918163404if_/http://www.mgaroadster.co.uk:80/Technical_Information/MGA%20Nuts%20and%20Bolts.pdf

This is handy for me who has been reading my beloved factory parts handbook ('Austin Service Parts List AKD1763' for a MkII Sprite) - my rebuilder's equivalent of Nigel's 'Good Book' (the factory 'Driver's Handbook') to replace missing/worn/damaged/incorrect fasteners, or planning repair jobs from the comfort of the sofa and needing to find/order up fasteners.

My standard mixed bag of UNF fasteners from Namrick helps me a lot, but sometimes all I have to go on is a part number (and not always the car parts suppliers stock them, or list their dimensions). I now can determine when something isn't UNF just from the part number.

So I now know what a PMZ305 is! And can go buy them from fastener suppliers, not just the car people.

Cheers
Mike

PS this explains why my otherwise superb 'Service Parts List' is silent on fastener size and specification - it's in the part number already! Doh (and faith restored in my 'Good Book')
M Wood

Not sure what happened to the link on nomenclature, here is some of the relevant info (from a helpful MGA person, un-named):

Notes on nomenclature used by BMC.

Bolts and set screws
First letter.

H = hexagonal head
C = countersunk head
P = pan head

Second and third letters
B = bolt
S = set screw
M = ?
N = non coated
Z = zinc coated
P as second letter = pointed
P as third letter = chrome plated

Examples:
HBZ HZS HPZ CMZ CPZ PMZ HNS CMP PMP


First and second digit.
Diameter of bolt or screw in sixteenths of an inch.

NB. If second digit is a "2" it is not UNF but UNC No.6.

Third and fourth digit.
Length of bolt or screw in eighths of an inch, excluding the head or the length point, only if it has a hexagonal head, otherwise it is in sixteenths of an inch.

Eg. HBZ0509 = Hex headed bolt, zinc coated, 5/16" dia. 9/8" (1" long
CPZ0412 = Countersunk screw, pointed, zinc plated,4/16"(1/4") dia. 12/16", (3/4") long.


Washers

First letter.
L = locking
P = plain

Second letter
W = washer

Third letter
N = non coated
Z = zinc coated

First digit

Plain washers
1 = small old
2 = large old

First digit
Spring washers
1 = thickness is 15% of i/d
2 = thickness is 20% of i/d
3 = thickness is 25% of i/d

Second and third digits
Diameter of bolt or screw in sixteenths of an inch.

Nuts
Letters used for the three common types:-
FNZ = full nut zinc (or probably fine nut zinc, UNF).
LNZ = locking nut zinc (nyloc).
CNZ = coarse nut zinc, UNC.
134

First digit
1 = thick
2 = thin
3 = slotted thick
4 = slotted thin
5 = ?

Second and third digits.
Diameter of bolt or screw in sixteenths of an inch.

Nuts
Letters used for the three common types:-
FNZ = Full nut zinc or probably fine nut zinc, UNF
LNZ = Locking nut zinc (nyloc)
CNZ = Coarse nut zinc, UNC

First digit.
1 = Thick
2 = Thin

Second and third digits
Diameter of bolt or screw in sixteenths of an inch.
M Wood

Moss use a similar method though not the same as BMC. They have an identifier for coating and the length and diameter are in increments of 1/8" and 1/16" respectively.

e.g. BH604081
BH = Bolt with Hex head
First number 6 = UNF. 5 = UNC
Second and third numbers = width in 1/16" increments
Fourth and fifth numbers = length in 1/8" increments
Last number is finish 1 = Zinc

Example above is Hex head Bolt, UNF, 1/4" thick, 1" long in Zinc finish.

It should be possible to do a sreadsheet to convert from BMC to Moss. Any volunteers?

Rob
MG Moneypit

Mike, Rob,
you might also like the following 'Translate Fastener Part Numbers' from Paul Hunt's excellent mgb-stuff site (although available elsewhere too).
http://www.mgb-stuff.org.uk/partnos.htm

And p-clip sizes I picked up from somewhere -

PCR number = cable diameter in multiples of 1/16" and fixing hole diameter in multiples of 1/32"

e.g. PCR311 is (3) 3/16" cable diameter and (11) 11/32" fixing hole.

PCR707 = 7/16" cable, 7/32" fixing hole.
Nigel Atkins

http://web.archive.org/web/20170918163404if_/http://www.mgaroadster.co.uk:80/Technical_Information/MGA%20Nuts%20and%20Bolts.pdf

Works with cut and paste to address bar.
Nigel Atkins

This is almost fascinating! I hadn't attempted to decipher the Moss system but it now makes more sense. I tend to buy nuts and bolts from them because, despite being expensive, I'm never sure about length, whether a "shoulder" bolt is needed, what coating, what strength etc. Also they are packed with their ID code so are easy to sort when buying multiple items.
Bill Bretherton

Interesting stuff.

Thanks for the cut n' paste link tip Nigel; I have found a better link to this second doc I linked to: https://mgaguru.com/mgtech/restore/pdf/MGANutsAndBolts.pdf

The above document has compared MGA old and new fastener numbers (as Rob suggests would be a fun challenge for Sprites and midgets). I guess with this source and the MGB one that Nigel mentions would be useful places to start for Rob's proposal for a quest to produce a comparison table for Spridget fastener old and new part numbers.

The MGA notes on fasteners (linked above in this e-mail) has a potentially helpful comment about the new numbers; which goes along with Rob's great info:

'New system
In the current catalogues there seem to be a blend of the new and old nomenclature. However UNF bolts begin with BH6 and screws begin with SH6 . Both always end with the number one and otherwise follow a similar pattern to the old system. (UNC threads use digit S instead of 6).'


Cheers
Mike
M Wood

Nigel's MGB stuff lists grease nipples as 1/4 BSP. Wonder what that looks like - 1/4 BSP has a 1/2 inch diameter thread.
L Langley

Lawrence, AFAIK all databases have errors and omissions, and those errors and omissions get repeated, particularly from copy & paste. Not a good start to the list though, wonder if the previous source was the same or if they're an added-in typo on this list.

And, no source of information is infallible, particularly anything from moi.

Remember that, in Fawlty Towers, "Pretentious, moi?"
Nigel Atkins

No Nigel and I thought I knew the scripts off by heart, must be getting old. I had wondered what the grease nipple threads were, I was thinking about putting remote greasing points for the front suspension in an accessible place. I used to install these on machinery. They had a system on trucks, there would be a periodic pneumatic loud hiss when grease was pumped into the grease points.
L Langley

Lawrence, Sybil likes the monkey man and Basil asks him to produce his dear old mother from his room.

I used to think Fawlty Towers was one of the very best comedies but times change and I think it was perhaps Connie Booth's writing and contribution that perhaps lifted, or lifted more, the John Cleese elements.

I grease the front suspension every 3 months and rarely does it go without a small hitch or two and always messy clean and dirty grease to deal with, always a chore with very little satisfaction other than its over with for another 3 months.
Nigel Atkins

This thread was discussed between 25/07/2020 and 26/07/2020

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