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QUESTION - How can I tell if my 2-1/4L petrol engine has 7:1 or 8:1 compression? If I have a 7:1 head can I mill it to make an 8:1 head? ANSWER - Look at the center head bolt between the carburetor and the valve cover. The surface of the head under that bolt may look like the other head bolt mounting surfaces or it may be a raised flat surface about 2.5 cm (1 inch) wide that goes between the valve cover and the edge of the head. If the mounting surface under this head bolt looks like the surface under the other head bolts, you have an earlyish 7:1 head. If you have the raised rectangular surface look carefully to see if there is a number stamped on that surface. If there is no number or a 7 stamped there then you have a late 7:1 head. If there is an 8 stamped there (Mine looked like two zeros that were stamped to make an 8) then you have an 8:1 head. Only heads with the raised flat surface that have an 8 stamped on it are factory 8:1 heads. The early heads are all 7:1 and do not have much metal between the water passages and the bottom of the cylinder head. They can not be safely milled to 8:1. You can expect these heads to crack in a short time if milled that much The later head castings are thicker between the bottom head surface and the water jacket. This makes the head less prone to cracking and provides enough metal to mill the head to 8:1 compression. A late 7:1 head can be milled to provide 8:1 compression by milling off 100 thousandths from the head surface. A 7:1 head that has not been shaved is 3.690 inches thick from the bottom surface to the valve cover mating surface (An unshaved 8:1 head is 3.590 inches thick). Before milling a head check to see how much may have been removed in the past and subtract that from the 100 thousandths needed to increase a head by a full point over stock. A 7:1 head can not reliably be milled to 9:1 Raising compression is the easiest way to provide an increase in reliable horsepower across the entire RPM range. Many people have raised the compression of the late 2-1/4L 8:1 head to 9:1 with good results. This should not be done with a 7:1 head. Milling the head changes the rocker arm geometry. The push rods that came with a 7:1 head are 7.300 inches long. The push rods that came with 8:1 heads are 7.175 inches long. Last time I looked, 7:1 push rods were no longer available new. When a head is milled from 7:1 compression to 8:1 you should use the 8:1 push rods to correct the rocker arm geometry. If you mill a 8:1 head to 9:1 go to an automotive machine shop and have a shorter set of push rods made up for the engine. There is very little clearance between the underside of the lower thermostat housing and the water pump housing. After resurfacing, ALWAYS check to see that the block's water pump housing is not interfering with the head's lower thermostat housing by laying the head on the block without the head gasket in place. You should be able to freely slide a sheet of paper between the lower thermostat housing and the water pump housing when there is no head gasket in place. If the surfaces touch, grind metal off both surfaces. NOTE that the latest heads have metric threads & studs. If you are installing a unknown head into your engine and the bolts do not seem to want to go in easily check to see if they are metric or SAE. Do not force the wrong thread bolt into the threaded holes. Do not retap the holes with different pitch stock dia taps. There will not be enough thread left to hold the bolts into the head and you will ruin the head. If you find that you have a newer metric head and you have SAE bolts and studs there is a head metric bolt kit available from Rover to let you properly mount the head. Also note that latest heads do not have a hole for mounting the cold running sender unit. Head casting numbers car not visible on a fully assembled engine. But if you have an unmounted head that you want to identify, here are the casting numbers.
Note: I do not know when Rover stopped using a cold running lamp Likely the sender mount would have been removed from the head at that time. |
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