Identifying 12G940 heads

[Smilin' Ric]

I am researching the large valve 1275 head, casting number 12G940. I have found there are 3, 4, or 5 variations of this head, with one (two?) having a larger intake valve, and maybe some improvement in the ports. My main question is how to ID the desired large valve head without removing it to measure the valves. Is there some mark or other telltale sign?

Cheers,

Ric
'60 Bugeye

 

[SD Bugeye]

My 12G295 head has a casting on the top side of the head this ids the layout of the part.

are you saying that your 12g940 could have different sized valves?
that is the way I thought you woukd ID it
that seems odd

 

[Smilin' Ric]

My 12G295 head has a casting on the top side of the head this ids the layout of the part.

are you saying that your 12g940 could have different sized valves?
that is the way I thought you woukd ID it
that seems odd

Yep. Confusing, eh?

12G940	1 5/16"	1 5/32"	21,4cc	1275GT,A/M1300	Can be modified to the specifications below
9 Bolts	33,3mm	29,53mm		Sprite MK IV, Marina 1,3
12G940	1 13/32"	1 5/32"	21,4cc	Austin 1300 GT	Fits on 1275 Gt when bigger valves are needed
11 Bolts	35,6mm			MG 1300 MK II, late Cooper S
AEG165	1 13/32"	1 7/32"	21,4cc	Cooper S	Can be used instead of the 11 bolted 12G940
12A185					12A185 was early casting coded yellow AEG165 is later colour coded brown or blue
12G940	1 5/16"	1 5/32"	21,4cc	Turbo MG	Sodium cooled exh. Vlaves & improved cooling but thinner deck

I am looking for some very smart person to tell me some magical way to tell which one is which, without pulling the head off to put eyes on the valve heads.


Cheers,

Ric

 

[dklawson]

Sorry, I'm not smart enough to give you definitive information. However, I did search for a while.

From the Mk1 Mini Forum:
https://mk1-forum.net/viewtopic.php?f=3&t=4902&p=34140&hilit=12g940+heads#p34140

The numbers after the casting mark are in inches/mm for the intake... then exhaust.

Only one of the later 12G940s in the list below mentions special marks/stamps. I think for most you need to measure the valve diameters. Threads I found while searching suggest there were at least 7 or 8 different raw castings for the 12G940.

My Mini was built with the AEG163 head but like many of those castings the valves had sunk into the head. Though my car's head was not cracked, in general they are prone to cracking between the inlets and exhausts. I replaced my car's head with a large valve 12G940 and have been happy. However, I didn't know about the physical appearance differences. The head I bought was from an A+ engine so the head is not perfectly flat around the thermostat area (see the text below the list below).


"[FONT=&quot]AEG163 1.401"/35.6 1.22"/31.0 21.4 Later Mk1 & Mk2 Cooper S[/FONT]
[FONT=&quot]12G940[/FONT][FONT=&quot]* 1.312"/33.33 1.15"/29.2 21.4 1275GT, Austin 1300, all late A+ models inc. Turbo but not MG Metro[/FONT]
[FONT=&quot]12G940[/FONT][FONT=&quot] 1.401"/35.6 1.15"/29.2 21.4 [/FONT]
[FONT=&quot]Stamped '12G1805' on flat area by thermostat - MG1300, 1300GT & Mk3 Cooper S[/FONT]
[FONT=&quot]12G940[/FONT][FONT=&quot]* 1.401"/35.6 1.15"/29.2 21.4 As above plus MG Metro[/FONT]

[FONT=&quot]*Note:[/FONT]

[FONT=&quot]A+ head castings - despite having the same [/FONT][FONT=&quot]12G940[/FONT][FONT=&quot] casting number - are considerably different in appearance, loosing the flat area behind the thermostat housing and around the rest of the rocker/head stud areas to a sculptured look - where these areas are replaced by a 'sunken' cast finish. [/FONT]
[FONT=&quot]The A+ castings were introduced with the appearance of the Metro in 1980, although some late 1275GTs (1979-on) had them fitted. Generally they are easily recognised by their colour - a putrid yellow on the small-bore castings and bright red on the large-bore ones, a paint finish that is VERY difficult to remove, even in a chemical tank.[/FONT]

[FONT=&quot]The MG Metro Turbo shares the same valve sizes with the other non-'big valve' [/FONT][FONT=&quot]12G940[/FONT][FONT=&quot] variants but has 8mm, sodium-filled exhaust valves.[/FONT]

[FONT=&quot]A+ [/FONT][FONT=&quot]heads[/FONT][FONT=&quot] generally have three collet grooves in the valves, collets are 14-degree cone taper instead of the 10-degree on single groove collets (so parts are not interchangeable between single and triple groove types) and the top-caps have a raised ridge around the collet hole."


[/FONT]

 

[Smilin' Ric]

Sorry, I'm not smart enough to give you definitive information. However, I did search for a while.

From the Mk1 Mini Forum:
https://mk1-forum.net/viewtopic.php?f=3&t=4902&p=34140&hilit=12g940+heads#p34140

The numbers after the casting mark are in inches/mm for the intake... then exhaust.

Only one of the later 12G940s in the list below mentions special marks/stamps. I think for most you need to measure the valve diameters. Threads I found while searching suggest there were at least 7 or 8 different raw castings for the 12G940.

My Mini was built with the AEG163 head but like many of those castings the valves had sunk into the head. Though my car's head was not cracked, in general they are prone to cracking between the inlets and exhausts. I replaced my car's head with a large valve 12G940 and have been happy. However, I didn't know about the physical appearance differences. The head I bought was from an A+ engine so the head is not perfectly flat around the thermostat area (see the text below the list below).


"AEG163 1.401"/35.6 1.22"/31.0 21.4 Later Mk1 & Mk2 Cooper S
12G940* 1.312"/33.33 1.15"/29.2 21.4 1275GT, Austin 1300, all late A+ models inc. Turbo but not MG Metro
12G940 1.401"/35.6 1.15"/29.2 21.4
Stamped '12G1805' on flat area by thermostat - MG1300, 1300GT & Mk3 Cooper S
12G940* 1.401"/35.6 1.15"/29.2 21.4 As above plus MG Metro

*Note:

A+ head castings - despite having the same 12G940 casting number - are considerably different in appearance, loosing the flat area behind the thermostat housing and around the rest of the rocker/head stud areas to a sculptured look - where these areas are replaced by a 'sunken' cast finish.
The A+ castings were introduced with the appearance of the Metro in 1980, although some late 1275GTs (1979-on) had them fitted. Generally they are easily recognised by their colour - a putrid yellow on the small-bore castings and bright red on the large-bore ones, a paint finish that is VERY difficult to remove, even in a chemical tank.

The MG Metro Turbo shares the same valve sizes with the other non-'big valve' 12G940 variants but has 8mm, sodium-filled exhaust valves.

A+ heads generally have three collet grooves in the valves, collets are 14-degree cone taper instead of the 10-degree on single groove collets (so parts are not interchangeable between single and triple groove types) and the top-caps have a raised ridge around the collet hole."

Thanks Doug.