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As it is possible, even common, to swap one head for another on 1275 engines, it is quite easy to become confused, or worse yet, confuse the parts-guy when it is time to order new valves! Be aware that if the head has been worked on previously, it may have oversize valves; it may have undersize it seats have been installed. Unless you are 100% sure what is in your cylinder head, it is a good idea to remove a valve for accurate measurement. There are two lengths used (nominally 3.44" & 3.56") three different keepers, (wide, narrow, and three-groove) and many different head diameters! These valves are in stock at Mini Mania! Intake valves 12G1316 (Smog-Type), all variations of the 12G940, and the latest CAM head castings all use the following 3.56" long valves.
MSE6 - POST 1992 unleaded stage 2 (Road Rocket) large-bore head
Modified to give maximum performance gain for cost. Combustion chambers, inlet and exhaust ports extensively re-worked. Stone-ground finish in ports promotes ultimate fuel atomisation. Three-angle valve seats in head. Super-quality MG Metro valves modified to increase airflow. Combustion chamber volume used to give slight static compression ratio increase over standard. Head mods employed greatly increase volumetric efficiency, boosting actual running (dynamic) compression ratio. Where much sportier cam profiles are used, a higher compression ratio is advised to achieve maximum power potential. Seek advice on what is required - a modified ECU will be required.
Before fitting to engine, ensure block and head mating surfaces are thoroughly cleaned. If fitting to a new/re-machined block, clean surfaces with a grease-free cleaning agent (cellulose thinners highly recommended). Lay gasket on block (always use a good quality gasket), then fit studs - minimising gasket damage during fitment. If fitting to a previously run engine, remove all head studs so block face can be thoroughly cleaned. Remove all traces of old gasket and carbon deposits. Finish clean with cleaning agent, then continue as previously outlined. Position pistons 1& 4 at top dead centre to minimise loading on valve gear, allowing even torque-down of head. Thoroughly clean head face with cleaning agent, then lower lightly down onto block/gasket - DO NOT DROP/FORCE IT DOWN, the gasket may get damaged. ABSOLUTELY NO GASKET CEMENT OF ANY TYPE IS NECESSARY. The proper A-series head gaskets have a coating of lacquer that adheres to head and block surfaces when hot.
Fit push-rods and any relevant ancillary studs (rocker posts, thermostat housing, etc.). Undo rocker adjuster screws all the way out. Fit rocker gear to head ensuring push-rod cups line up under adjuster screw ball ends. Fit head nuts (and washers where applicable) finger tight then nip down with socket in sequence described in workshop manual. Torque down in sequence to 35lb ft first, then 50lb ft. Torque forward rocker studs to 25lb ft. Set valve clearances to manufacturers recommended settings. Turn engine over by hand to ensure no spring crush is occurring.
If no by-pass hose is being used, it is essential to drill 4 to 6 1/8"-diameter holes in the thermostat outer ring to allow coolant circulation prior to thermostat opening. If no thermostat is being used, a blanking sleeve MUST be fitted. Failure to use either method will result in localized over-heating, causing excessive heat leading to pre-ignition, detonation, piston failure and cracked head. Water pump port must be blanked off (can use blanking plug CAM4126 and clip 3H2963.