Thread: Questions about a Plymouth 217ci Flathead

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If you pull the engine for a rebuild and/or to bore the cylinders here is the last word in souping up a flathead (side valve) block. The easy modern way to do this would be to find a milling machine large enough to tilt the block slightly about 2 degrees (depending on the width of the block) with the valve side down and mill out a wide channel from the valve seats to the edge of the bore. If you plane the head too much you will actually choke off the flow from the valves to the cylinder. Yes, a "relief" will lower the compression ratio but increase the flow. This used to be done by hand grinding in a very tedious artful way. The problem is that there is no easy way to make the reliefs the same volume using hand grinding since you can't cc the results easily. Thus I am giving you this old picture from a 1951 paperback to show how a modern milling machine might do it. Some of the later flathead Ford V8 engines had factory machined reliefs in the block but having shown you this I am not aware of anyone who has used a milling machine to do this accurately. Note the depth at the edge of the cylinder bore should not cut into the water jacket or go below the top ring on the piston at TDC, but maybe 1/16" to 1/8" is possible. Actually most of this art was applied to flathead Ford V8s but it should work for any side valve engine. I guess if I were doing it I would worry about cutting into the water jacket and limit the depth to 1/16" and/or look around for some diagram of a cut through Plymouth block to make sure there is enough metal on the top of the block to allow such a cut. OK so you say this is not necessary but if you plane the head too much you will actually reduce the flow over the "hill" between the valves and the cylinder. On the other hand if you know where there is a large milling machine with a bed to mount the whole block and you can work out the extra cost, all that is needed is a straight cut from the valves to the bore without all the tedious hand grinding. If you do this send us some pictures and tell us the approximate extra cost beyond a usual rebore. Hey I spent my whole career in "Theory" so I would like to know if anyone out there has done this using a milling machine rather than a hand grinder.

Don Shillady
Retired Scientist/teen rodder