Thread: Questions about a Plymouth 217ci Flathead

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www.stoveboltengineco.com sells headders for your flat 6 and intakes also I think, also some nice Holley/ Webber 5200 carbs for $89 each

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

The factory reliefs on Ford flatheads was done to reduce the thickness of the metal in this area and reduce the tendency to crack the block, due to uneven expansion. Any extra flow was more than offset by the reduction in C.R.
Rounding the corner at the top of the bore will gain some flow, but the extra hassle involved when re-ringing the engine makes it questionable.
Most flatheads work better with aftermarket high comp. heads, but a Mopar 6 can be planed without any problems.

Our family had several of the Plymouth six flatheads but before the age where I could mess with them; although I helped my Dad more than once do a "driveway rebuild" where I held the light for him while he tried to rering, scrape (clean carbon) and replace bearings over a weekend and invariably ran into a time constraint to be able to drive the car on Monday morning. I am sure that we included several moths that hovered around the light and got trapped in oil. As I recall the Plymouths we had ('35, '36, '40 and '51) had awesome torque in low gear and then it was over, and that may have been due to the rear ratio but I don't know what those rear gears were. Anyway, it is true that the Ford flathead FACTORY reliefs were shallow but I guess I was brainwashed because every Hot Rod feature car I studied from '53 to '56 or so always had a Ford flathead "ported and relieved". Maybe R Pope knows the limit to mill a Plymouth head? I recall that a Model-A head could be milled up to 0.125" (1/8" !) and would run for a short time before something failed but then the combustion chambers in the Model-A heads are huge and the T heads are like soup bowls! So the question is by the time Mopar got to the last gasp flathead in '54 how much room was left in the "bowl" for a safe cut on the head, .060"? As far as the rering after relieving the block, I would "guess" one should be able to stagger the ring gaps away from the relief? Of course the other advice given above is sensible in going to a shaved head, dual carbs and some sort of headers, but I guess it is a pet idea that I would like to see someone do in using a straight mill cut on a shallow angle to relieve a side-valve engine. I do know that as a kid working in a gas station and moving all types of '50s cars, there was a very noticeable power increase in going from Plymouth, Dodge, DeSoto and Chrysler straight six engines based solely on increasing cubic inches, so maybe someone out there can advise our friend about the maximum safe bore for a 217? By the way the '53/'54 Belvidere body is an unappreciated classic in my mind so whatever you do to the engine, it should still result in a very cool ride!

Don Shillady
Retired Scientist/teen rodder