Cylinder head - 848 and 997cc

848 and 997cc Cylinder Head Identification
Cylinder head details in this section are appropriate for the 848cc engine used in Minis between 1959 and 1984 and the 997cc engine used in the earliest Mini Coopers built between 1961 and 1963. The 848cc transverse engine was also to be seen in earlier Mokes and a small number of Austin Metro vans, plus in in-line' form, in an even smaller number of Austin A35 vans. The earliest variant of the Mini Cooper was the only BMC/Leyland product ever to receive the 997cc engine.

The cylinder head design for the 848cc Mini engine is shared with the standard tune Mini 998cc engine, but not the 998cc Cooper unit. It was also shared with other BMC model engines, including 803 and 948cc units to be found in Morris Minors, Austin A30, A35 and A40 models and the 'Frogeye' Austin Healey Sprite Mk I. Only the castings marked 2A628 (usually earlier models) or 12A1456 (later models) are appropriate for Mini - the outwardly similar 2A629 is not. The other obvious distinguishing feature of this cylinder head is the inlet valve diameter, which at just 1 1/16" (27mm) is the smallest of any A Series head design.

The 997cc Cooper cylinder head was also fitted to various standard tune 1098cc engines, including the single carburetter Austin/Morris 1100 and the 1098cc Minis, plus the high performance 948cc Austin Healey Sprite Mk II and MG Midget Mk I. For identification purposes the casting number is 12G202 and the inlet valves are 1 5/32" (29.4mm) diameter.

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 usually included valve guides and studs 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 away components on your old head that you will still need in future!

Rebuilt or Repaired 848 and 997cc 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.

Ill. No.	Part No.	Description	Qty. Rqd.	Remarks
1	-	CYLINDER HEAD ASS'Y, with valves	1	848cc; please contact us for details.
		(2A628 or 12A1456 casting number)
	-	CYLINDER HEAD ASS'Y, with valves	1	997cc; please contact us for details.
		(12G202 casting number)
2	-	CYLINDER HEAD, bare, with guides and studs only	1	848cc; please contact us for details.
		(2A628 or 12A1456 casting number)
	-	CYLINDER HEAD, bare, with guides and studs only	1	997cc; please contact us for details.
		(12G202 casting number)

Gasket Set
Ill. No.	Part No.	Description	Qty. Rqd.	Remarks
3	GEG101SM	GASKET SET, cylinder head	1	See note below.
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 later engines with sandwich plates), six HS2 carburetter gaskets, three HS4 carburetter gaskets, rocker cover gasket and heater valve gasket. Valve stem oil seals are not included, leaving customers to choose the type and quantity they need and order them separately.

Cylinder Head Components
Cylinder Head Fixings
Ill. No.	Part No.	Description	Qty. Rqd.	Remarks
4	GUG2506C	GASKET, cylinder head, copper	1
5	CAM150	STUD, cylinder head, long	4
6	CAM151	STUD, cylinder head, short	5
7	51K1193	NUT, cylinder head to block	9	1959-78 standard fitment.
8	PWZ106	WASHER, plain	9	1959-78 standard fitment.
	GB126B	WASHER, plain	9	Alternative heavy duty washer. Withstands up to 80lbf ft torque.
9	CAM4545	NUT, flanged, cylinder head to block	9	1978 on standard fitment.
		(does not require plain washer)		1959-78 heavy duty alternative.

Plugs and Adaptors
Ill. No.	Part No.	Description	Qty. Rqd.	Remarks
10	2K1345	PLUG, oil hole, cylinder head	1	Alternative to 6K808.
	6K808	PLUG, oil hole, cylinder head	1	Alternative to 2K1345.
11A	2A243	ADAPTOR, by-pass hose, short	1	1959-69; use with long by-pass hose.
	12A2075	ADAPTOR, by-pass hose, long	1	1969 on; use with short by-pass hose.
10B	CAM4126	PLUG, bypass adaptor blanking	A/R	Use if thermostat blanking sleeve is fitted.
10C	3H2963	CLIP, bypass blanking plug	A/R
12	ADP210	PLUG, temperature sender unit boss	1	Models without temperature gauge.
13	1B3664	WASHER, sealing	1	Models without temperature gauge.

Thermostat Housing and Fixings
Ill. No.	Part No.	Description	Qty. Rqd.	Remarks
14	-	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.

Ill. No.	Part No.	Description	Qty. Rqd.	Remarks
15	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 848 and 997cc engines).

Ill. No.	Part No.	Description	Qty. Rqd.	Remarks
16	CHS2515	STUD, thermostat housing	3
17	FNZ105	NUT	3
18	PWZ105	WASHER, plain	3

Heater Valve or Tap and Fixings
Ill. No.	Part No.	Description	Qty. Rqd.	Remarks
19	-	VALVE, heater	1	See note below.
20	-	TAP, heater	1	See note below.
For details of the heater valves and taps, please see the 'Heating - Heater Hoses, Water Control Valves and Cables' section in the Cooling area of the Somerford Mini catalogue.

Ill. No.	Part No.	Description	Qty. Rqd.	Remarks
21	2A180	PLATE, blanking heater take-off in cylinder head	1	For models without heater.
22	88G221B	GASKET, heater take-off	1
23	53K402	STUD, heater take-off	2
24	FNZ104	NUT	2
25	LWZ304	WASHER, locking	2

Manifold Fixings
Ill. No.	Part No.	Description	Qty. Rqd.	Remarks
26	GUG704053MG	GASKET, manifold	1
27	53K487	STUD, manifold	6
Very early 848cc cylinder heads used two each of long, medium and short manifold studs; these were soon replaced in production by six of the medium length studs, which can also be fitted to the earliest engines without any necessary modification.

Ill. No.	Part No.	Description	Qty. Rqd.	Remarks
28	12A1211	WASHER, plain, large, clamping manifold	4
29	PWZ105	WASHER, plain	2
30	GHF261	NUT, brass, manifold to cylinder head	6
	51K1177	NUT, brass, manifold to cylinder head, long	6	Alternative; easier to access.
	FNZ105	NUT, steel	6	Alternative.
The steel nut alternative was fitted off and on over the years (most likely as a panic measure when the production line ran out of brass nuts) but does tend to sieze on the manifold studs unless plenty of copper grease is applied.

Rocker Cover and Fixings
Ill. No.	Part No.	Description	Qty. Rqd.	Remarks
31	12A475	ROCKER COVER, steel, with breather pipe	1	1959-69 open circuit breathing models.
32	12A1530	ROCKER COVER, steel, without breather pipe	1	1964 on closed circuit breathing models.
33	CAM6822	ROCKER COVER, alloy, without breather pipe	1	Accessory (also MG Metro) item for closed circuit breathing models.
A range of polished or painted alloy accessory rocker covers is available from Somerford Mini; please contact us for details. Note that some accessory rocker covers require different fixings from the standard items.

Ill. No.	Part No.	Description	Qty. Rqd.	Remarks
34	-	PLATE, engine name	1	See note below.
	-	DECAL, engine name	1	See note below.
	-	PLATE, engine patent	1	See note below.
	-	DECAL, engine patent	1	See note below.
For details of the large range of plates and decals fixed to rocker covers, please see the 'Chassis Plates and Decals' section in the Interior Body Fittings area of the Somerford Mini catalogue.

Ill. No.	Part No.	Description	Qty. Rqd.	Remarks
35	8G612	CAP, oil filler, metal	1	1959-62 open circuit breathing models.
36	12A402	CAP, oil filler, black plastic, without breather	1	1962-69 open circuit breathing models. Will fit 1959-62 models.
37	13H2296	CAP, oil filler, black plastic, with breather	1	1964 on closed circuit breathing models.
38	GFE6007	CAP, oil filler, red plastic, with breather	1	For CAM6822 alloy rocker cover.
39	12A403	SEAL, plastic oil filler cap	1	For 12A402 and 13H2296 caps.
40	GEG401	GASKET, rocker cover	1
41	CAM6822FK	CHROME FIXINGS KIT, rocker cover	1
Includes chrome nuts, chrome distance pieces, chrome cup washers and rubber bushes. For alloy rocker covers such as CAM6822.

Ill. No.	Part No.	Description	Qty. Rqd.	Remarks
42	2A150	NUT, rocker cover to engine	2	Original zinc finish.
	2A150C	NUT, rocker cover to engine	2	Chrome finish, for steel rocker covers.
	CAM6823	NUT, rocker cover to engine	2	Chrome finish, longer, for alloy rocker covers.
43	1B2925	DISTANCE PIECE	2	Original zinc finish.
	12G687	DISTANCE PIECE	2	Alternative chrome finish.
44	1A2156	WASHER, cup	2	Original zinc finish.
	12G679	WASHER, cup	2	Alternative chrome finish.
45	12A1358	BUSH, rubber	2
46	12A1968	LIFTING EYE, engine, temporary fitment	2	Fit to engine only while lifting work is taking place.

Rocker Shaft, Rockers and Fixings
Ill. No.	Part No.	Description	Qty. Rqd.	Remarks
47	12G1926	PEDESTAL, rocker shaft, with oil hole	3
48	12G1927	PEDESTAL, rocker shaft, with tapped hole	1
49	2A258	SCREW, locating rocker shaft	1
50	2A515	PLATE, securing screw	1	1959-74.
51	2A259	PLATE, securing screw	1	1974 on; will fit 1959-74 models.
52	51K885	STUD, long, rocker cover and rocker pedestal	2	Outer pedestals.
53	51K1473	STUD, short, rocker pedestal	2	Inner pedestals.
54	NH605040	NUT, pedestal to cylinder head	4
55	LWZ205	WASHER, locking	4	1959-74.
56	PWZ105	WASHER, plain	3	1974 on. Not used on tapped pedestal.
57	PWZ106	WASHER, plain	3	1974 on. Not used on tapped pedestal.
58	AEG399	ROCKER SHAFT ASS'Y	1	1959-74.
59	12A1950	ROCKER SHAFT	1	1974 on; will fit 1959-74 models.
60	6K878	END PLUG, plain	1	Included in AEG399 rocker shaft ass'y.
61	2K4608	END PLUG, screwed	1	Included in AEG399 rocker shaft ass'y.
62	2A964	ROCKER, pressed steel	8	First rocker type.
63	12G1221	ROCKER, forged	8	Second rocker type.
64	CAM289	ROCKER, scintered	8	Third rocker type, 1974 on.
The pressed steel rocker was the first to be fitted, back in 1959. Cooper engines saw the introduction of the stronger forged rocker as a production alternative in the early 1960s, the two types being used in standard and high performance engines as alternatives both in production and the parts department until 1974, when the scintered rocker was introduced. The scintered rocker was used in all engine production from that time onwards, although the pressed steel and forged types continued to be offered as alternative spare parts for earlier cars. For normal road use it does not matter which type of rocker is used, so long as (i) all eight on the rocker shaft are of the same type; (ii) all eight are either the same age from the same engine or are new; and (iii) the tappet adjusting screws and nuts are correct for the type of rocker. Note that the forged rockers have replaceable bushes, whereas the other two designs do not.

The pressed steel rocker listed here, 2A964, is the original 'small bore' A Series item; the 'big bore' (1275cc) equivalent listed elsewhere in the Somerford Mini catalogue is 12G1313. The difference in theory between the two is a marginal increase in lift with the 'big bore' rocker, although in practice this is often swallowed up by assembly tolerances. Concerning forged rockers, again originally there were small and big bore engine variants, but since the big bore item is the one available (and it works perfectly well on a small bore engine), that's the one we list here. There were no small or big bore variants of the scintered rocker - one type fitted all.

Several designs of high-lift and roller rockers are also available for tuning purposes, along with special valve springs to avoid coil binding, if necessary. Please contact Somerford Mini for details of these and other engine tuning parts.

Ill. No.	Part No.	Description	Qty. Rqd.	Remarks
65	2A21	BUSH, rocker	8	Included in forged rocker.
66	5C2436	RIVET, bush to rocker	8	Included in forged rocker.
67	12A1215	SCREW, tappet adjusting	8	Use with pressed steel rocker.
68	6K654	NUT, locking screw	8	Use with pressed steel rocker.
69	AEG167	SCREW, tappet adjusting	8	Use with forged rocker.
70	NT605061	NUT, locking screw	8	Use with forged rocker.
71	12H3376	SCREW, tappet adjusting	8	Use with scintered rocker.
72	51K1178	NUT, locking screw	8	Use with scintered rocker.
73	6K556	SPRING, rocker spacing	3
74	2A18	WASHER, double coil, outer ends of shaft	2
75	6K555	WASHER, plain, outer ends of shaft	2
76	ZPS314	SPLIT PIN, outer ends of shaft	2

Valve Guides and Valve Seat Inserts
Ill. No.	Part No.	Description	Qty. Rqd.	Remarks
77	12G1111	GUIDE, inlet and exhaust valve	8	848cc; standard size.
	12G1111010	GUIDE, inlet and exhaust valve	8	848cc; 0.010" oversize.
	12A186	GUIDE, inlet and exhaust valve	8	997cc; standard size.
	12A186010	GUIDE, inlet and exhaust valve	8	997cc; 0.010" oversize.
78	2A639	INSERT, inlet valve seat	4	Standard size.
	AEA762	INSERT, inlet valve seat	4	1/16" oversize.
79	2A640	INSERT, exhaust valve seat	4	Standard size.
	AEA763	INSERT, exhaust valve seat	4	1/16" oversize; for leaded fuel use.
	TAM2068	INSERT, exhaust valve seat	4	Lead free fuel compatible.
Note that standard size and oversize inserts should not be mixed together on the same cylinder head.

Valves and Valve Springs
There are two types of collet used to retain valves in place on these engines and three types of valve to take them. The earlier 848cc valve design up to the early 1970s and the 997cc valve design had a groove 1/4" deep into which the collets located, whereas later 848cc valves had a groove 1/8" deep, either 7/32" from the valve tip or 9/64" from the valve tip (the latter being the 1981 introduced A Plus type). Care must be taken to ensure that valves, valve caps, valve springs and collets are matched before attempting reassembly. Note also that while the earlier large groove collets were originally secured together with circlips, around 1969 these were judged to be adding nothing to the security of the collets and were discontinued.

A range of uprated valves, valve springs and caps to suit most uses are also available from Somerford Mini; please contact us for details.

848cc 1959-74 approx.
Valves With 1/4" Collet Groove
Ill. No.	Part No.	Description	Qty. Rqd.	Remarks
80	2A877	VALVE, inlet, 1 1/16" (27mm) dia.	4
81	AEA434	VALVE, exhaust, 1" (25.4mm) dia.	4
82	AEA311	SPRING, valve	8
83	2A11	SPLIT COLLET, pair, valve retaining	8
84	2A12	CIRCLIP, valve collet	8	1959-69; no longer fitted.
85	88G616	CAP, upper, valve spring	8
86	2A879	OIL SEAL, valve stem	8
87	2A545	SHROUD, valve guide	8	1959-69.

997cc 1961-63
Original Valves With 1/4" Collet Groove
Replacement Valves With 1/4" or 1/8" Collet Groove
Original 997cc Cooper valves are understandably thin on the ground these days; if difficulty is experienced in obtaining originals, AEA574 (1/4" groove) or 12G3036 (1/8" groove) inlet valves and AEA434 (1/4" groove) or 12G3035 (1/8" groove) exhaust valves may be used in their place. Take care to use the correct collets: 2A11 for 1/4" groove valves, 88G459 collets for 1/8" groove valves.

Ill. No.	Part No.	Description	Qty. Rqd.	Remarks
88	AEA399	VALVE, inlet, 1 5/32" (29.4mm) dia.	4	Original with 1/4" collet groove.
	AEA574	VALVE, inlet, 1 5/32" (29.4mm) dia.	4	Replacement with 1/4" collet groove.
	12G3036	VALVE, inlet, 1 5/32" (29.4mm) dia.	4	Replacement with 1/8" collet groove.
89	AEA400	VALVE, exhaust, 1" (25.4mm) dia.	4	Original with 1/4" collet groove.
	AEA434	VALVE, exhaust, 1" (25.4mm) dia.	4	Replacement with 1/4" collet groove.
	12G3035	VALVE, exhaust, 1" (25.4mm) dia.	4	Replacement with 1/8" collet groove.
90	AEA311	SPRING, valve	8
91	2A11	SPLIT COLLET, pair, valve retaining	8	Use with original 1/4" groove valves.
	88G459	SPLIT COLLET, pair, valve retaining	8	Use with replacement 1/8" groove valves.
92	2A12	CIRCLIP, valve collet	8	No longer fitted.
93	88G616	CAP, upper, valve spring	8
94	2A879	OIL SEAL, valve stem	8
95	2A545	SHROUD, valve guide	8

848cc 1974 approx. to 1981
Valves With 1/8" Collet Groove, 7/32" From Valve Tip
Ill. No.	Part No.	Description	Qty. Rqd.	Remarks
96	CAM6391	VALVE, inlet, 1 1/16" (27mm) dia.	4
97	12A1987	VALVE, exhaust, 1" (25.4mm) dia.	4
98	AEA311	SPRING, valve	8
99	88G459	SPLIT COLLET, pair, valve retaining	8
100	88G616	CAP, upper, valve spring	8
101	CAM6520	SHIM, valve spring	A/R

848cc 1981 On
Valves With 1/8" Collet Groove, 9/64" From Valve Tip (A Plus Type)
Ill. No.	Part No.	Description	Qty. Rqd.	Remarks
102	CAM6502	VALVE, inlet, 1 1/16" (27mm) dia.	4
103	CAM6503	VALVE, exhaust, 1" (25.4mm) dia.	4
	CAM6505	VALVE, exhaust, 1" (25.4mm) dia., chrome stem	4	Lead free fuel compatible.
104	AEA311	SPRING, valve	8
105	88G459	SPLIT COLLET, pair, valve retaining	8
106	88G616	CAP, upper, valve spring	8
107	ADU4905	OIL SEAL, inlet valve stem	4

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Cylinder head - 848 and 997cc

SPLIT PIN-3/32'' x 1 1/2'' LONG

INSERT-EXHAUST VALVE SEAT 848,997,998 & 1098cc UNLEADED FUEL

USE: 51K1178

NUT-5/16 UNF NATURAL FINISH

GASKET-THERMOSTAT HOUSING, PAPER

GASKET-MANIFOLD TO CYLINDER HEAD-CARB. MODELS

GASKET-HEAD-COPPER-850,997,998 & 1098cc

NUT-BRASS-5/16 UNF-MANIFOLD TO CYLINDER HEAD-STD LENGTH

CAP-OIL FILLER-MG METRO TYPE (red)

GASKET- ROCKER COVER-CORK

GASKET SET-CYLINDER HEAD (copper gasket) 848,997,998 &1098cc

WASHER-HEAD STUD-PRE A+ UPGRADED CAN STAND 80lb/ft

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