Somerford Mini Ltd. Ordering Parts Using This Catalogue

Cylinder Head - 970, 1071 and 1275cc (Cooper S)
No. Part No. Description Qty. Remarks No. Part No. Description Qty. Remarks
970, 1071 and 1275cc Cooper S
Cylinder Head Identification
Cylinder head details in this section are appropriate for all variants of Cooper S engine, built between
1963 and 1971. Three cylinder head designs were used, identifiable by casting numbers 12A185, AEG163
and 12G940. All three will fit any capacity Cooper S engine and all three are interchangeable as
complete assemblies. This does not, ho wever, mean that all components can be easily swapped between
them.
All Cooper S cylinder heads are referred to as ‘eleven stud’ heads because of the attachment method to
the cylinder block (actually inaccurate since ten studs and a bolt are used); all other Mini cylinder heads
are attached with nine studs. The two extra fixings are clearly visible at either end of the rocker cover,
the gasket flange of which is notched to accept them. The only other BMC/Leyland engine to be fitted
with an eleven stud cylinder head was that to be found in the high performance derivatives of the
Austin/Morris 1300 (1300 GT, MG 1300 etc .) and the Italian-built Innocenti Mini Cooper 1300,and
indeed the cylinder head fitted was the 1969-71 Cooper S item. This does not mean however that the
1300 GT engine is a Cooper S engine, since there are many other differences between them, including
the engine block itself, but it still makes a very desirable power unit to transplant into a Mini.
The first Cooper S cylinder head, introduced on the 1071cc model in 1963, was marked with the casting
number 12A185 and additionally may be identified by a yellow colour coding. The valves fitted are
distinctly larger than those in the cylinder heads of non-Cooper S Minis - the inlet valve is 1 13/32"
(35.6mm) in diameter and the exhaust valve is 1 7/32" (31mm) in diameter. Additionally, the valves
are uniquely manufactured in Nimonic 80 (EN214N) steel and use double valve springs.
Towards the end of 1963 a new cylinder head casting was introduced, marked with the part number
AEG163 and colour coded blue or brown. The only difference between the 12A185 and AEG163 castings
is a production modification in the internal oil ways in the later casting; it affects neither performance
nor reliability of the head. The valves , valve springs and rocker gear all continued unchanged from the
12A185 head. The AEG163 cylinder head was fitted from late 1963 through to 1969 and thus saw use
on the last 1071cc engines, all 970cc engines and all but the last 15 months or so of 1275cc engine
production.
The 12G940 cylinder head casting was officially introduced on the 1275cc Cooper S engine at the
beginning of Mini Mk III production, for the 1970 model year. Alongside the casting number in the
centre of the top face of the head, the other identifying mark is the part number 12G1805 stamped by
the thermostat housing. It may well also have been fitted to the last few Cooper S Mk IIs; this is
difficult to prove, not least because the factory parts and service department soon offered the 12G940
head as the only replacement for all Cooper S heads, meaning that a significant number of 1960s cars
are now fitted with them. Using the eleven stud 12G940 head on the Cooper S saved manufacturing
costs through component sharing with other models (the aforementioned Austin/Morris 1300 and
Innocenti).
It also improved Cooper S cylinder head reliability, for a small cost in performance , since smaller exhaust
valves in the 12G940 head allowed for more material to be left between the inlet and exhaust valves;
this area was prone to cracking on the 12A185 and AEG163 heads . The inlet valve size on the 12G940
is, as previously, 1 13/32" (35.6mm) diameter, but the exhaust valve size is reduced by 1/16" to 1 5/
32"(29.4mm) diameter. The valves are made from standard, not Nimonic , material and ha ve longer
stems to compensate for the higher valve spring seat on the 12G940 casting. Thus even the inlet valves
are not directly interchangeable with earlier Cooper S items, though their diameter is the same.
The nine stud fixing 12G940 cylinder heads used on other 1275cc BMC/Leyland/Austin Rover engines are
not, of course, capable of being fitted to a Cooper S engine without very costly modification - they
usually have smaller valves than the eleven stud 12G940 (MG Metro etc . excepted) an yway.
At various times during A Series engine production, new and reconditioned cylinder heads from the
factory were either in ‘assembly’ or ‘bare’ condition. Assemblies usually included studs, valves, valve
springs and guides (but not rockers, rocker shaft and pedestals) whereas bare heads included valve guides
and studs or valve guides only. These states of build were however not hard and fast, so if you are
offered a new or reconditioned factory cylinder head on an exchange unit scheme you should establish
before ordering exactly what you will receive, to guard against the possibility of giving a way components
on your old head that you will still need in future!
Rebuilt or Repaired Cylinder Heads
Somerford Mini Ltd. offers a bespoke cylinder head rebuilding and repair service. In our experience, a
large number of customers require some degree of modification to their cylinder heads and as a result,
whether you want a standard head or one that is uprated to any degree, we ask you to contact us to
discuss your requirements. It would be helpful if you could also supply us with the details of your
engine: full engine number (if known) and type (capacity, high or low compression, gearbox, breathing
system, etc.), plus any existing modifications. The more you can provide us with, the better we can help
you. All of our cylinder heads are built to run on lead-free fuel, unless you specify otherwise.
1 - CYLINDER HEAD ASS’Y, with valves 1 970, 1071 and 1275cc 1963-69.
(12A185 or AEG163 casting)
- CYLINDER HEAD ASS’Y, with valves 1 1275cc 1969-71.
(12G940 casting)
2 - CYLINDER HEAD, bare, with guides and 1 970, 1071 and 1275cc 1963-69.
studs only (12A185 or AEG163 casting)
- CYLINDER HEAD, bare, with guides and 1 1275cc 1969-71.
studs only (12G940 casting)
Gasket Set
3 GEG103SMC GASKET SET, cylinder head 1
The cylinder head gasket set, also known as a ‘decoke’ or ‘decarbonising’ set, includes the cylinder head
gasket, manifold gasket, two thermostat housing gaskets (one extra to cater for other 1275cc Mini engines
with sandwich plates), HS2,HS4 and HIF carburetter gaskets, rocker cover gasket, plus heater take-off
gasket. The rocker cover gasket will require two small notches to be cut in it to allow for the extra
Cooper S cylinder head fixings. Valve stem oil seals are not included in the set, leaving customers to
choose the type and quantity they need and order them separately.
Cylinder Head Components
Cylinder Head Fixings
4 GEG300 GASKET, cylinder head, copper 1
5 CAM150 STUD, cylinder head, long 4
6 CAM151 STUD, cylinder head, short 6
7 51K1193 NUT, cylinder head to block 10
8 PWZ106 WASHER, plain 10
GB126B WASHER, plain 9 Alternative heavy duty washer.
Withstands up to 80lbf ft torque.
9 AEG248 BOLT, cylinder head 1
10 PWZ106 WASHER, plain 1
Plugs and Adaptors
11 2K1345 PLUG, oil hole, cylinder head 1 Alternative to 6K808.
6K808 PLUG, oil hole, cylinder head 1 Alternative to 2K1345.
12 12G2092 PLUG, water hole 3
13 AEA771 PLUG, water and dowel hole 2
14 12G3503 PLUG, cleaning hole 3/2 Qty. decreases for 12G940 head.
15 2A243 ADAPTOR, by-pass hose, shor t 1
Thermostat Housing and Fixings
16 - HOUSING, thermostat 1
For details of thermostat housings, please see the ‘Cooling - Thermostats and Water Pumps’ section in the
Cooling and Heating area of the Somerford Mini catalogue.
17 GTG101 GASKET, thermostat housing, cork 1
GUG705558 GASKET, thermostat housing, paper 1
In theory the two gasket materials are interchangeable, but the cork gasket seems to be preferred by
Somerford Mini customers for a simple thermostat housing fitment, whereas the later paper gasket is
preferred for a thermostat housing with sandwich plate (not applicable for Cooper S engines).
18 CHS2515 STUD, thermostat housing 3
19 FNZ105 NUT 3
20 PWZ105 WASHER, plain 3
Heater Valve and Fixings
21 - VALVE, heater 1 See note below.
For details of the heater valves, please see the ‘Heating - Heater Hoses, Water Control Valves and Cables’
section in the Cooling area of the Somerford Mini catalogue.
22 2A180 PLATE, blanking heater take-off in head 1 For models without heater.
23 88G221B GASKET, heater valve 1
24 53K402 STUD, heater valve 2
25 FNZ104 NUT 2
26 LWZ304 WASHER, locking 2
Manifold Fixings
27 GUG704053MG GASKET, manifold, standard 1
28 53K487 STUD, manifolds 6
29 12A1211 WASHER, plain, large, clamping manifolds 4
30 PWZ105 WASHER, plain 2
31 GHF261 NUT,brass, manifolds to head 6
51K1177 NUT,brass , manifolds to head, long 6 Alternative; easier to access.
The standard brass nut GHF261 is the usual fixing for manifolds - being brass it doesn’t sieze on the
steel manifold studs too easily. The long brass nut 51K1177 first saw light on Mini manifolds when BMC’s
Competitions Depar tment and Special Tuning wanted a manifold fixing that wasn’t so fiddly to get at. It
does the job very well and is far easier to do up or undo than the standard nut, but for some reason -
probably cost - was never adopted on the production line.
Tel. 01249 721421 Fax. 01249 721316 www.somerford-mini.co.uk
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