Overhead Valve Model A

Using a homemade adapter plate, we mounted a 1927 Chevrolet ohv head on a Ford Model A block. Never mind that the head was offset relative to the cylinder bores: the thing ran!

Homemade parts consisted of block-to-head adapter plate, water-pump mount, engine side covers, intake manifold, and rocker-arm cover.

Herman Garret and his buddy Charlie Boyd cobbled together a Model A speedster in 90 days, a feat made possible by extreme pragmatism. When Charlie complained that the car wouldn’t turn left, (a shock mount had fouled the steering), Herman reminded him that a race car only needs to turn right. Ditto for the barely functional brakes.

Eventually the car was sorted out well enough to enter a dirt track race in Florida, where it won seven out of eight heats. Charlie was ecstatic, but wanted to go faster.

The two stalwarts came back from Kansas City with a collection of homemade parts for adapting a 1928 Chevrolet overhead valve cylinder head to a Model A block. I volunteered to assemble the parts and get the engine running.

Mating a Chevrolet or Oldsmobile four-cylinder head with a Model A is an old racer’s trick dating back to the time when these cars were new. But it’s not easy to pull off, since the bore centers, head-bolt patterns and water-jacket ports bear only the most casual relationship with each other. An adapter plate with staggered or angled bores must be used. The sequence of intake and exhaust valves for both engines are identical and with a little judicious bending of the rocker arms, it is possible to minimize pushrod angularity. The stock 1928 head gasket works for the Chevrolet, and Charlie came up with a copper gasket for the Ford, which blanks off the valve areas on the block and has holes for the pushrods. We would have to fabricate something to hang the Model B water pump on.

The block-to-head adapter plate appeared to be made of alloy steel. Width varied from .835" to .845," and we found several low spots, one deep as .006," together with some deep scratches. The plate should have been sent out for resurfacing, but this was a trial assembly and Charlie expected to tear the engine down again after startup. To make that task easier, we assembled the plate and its copper gasket dry, without gasket sealant.

The adapter plate and spark plug recesses in the head were cc-ed, which indicated a compression ratio of 5.48:1. Pretty low, although a point higher than the stock Model A.

Work was interrupted by a trip to California where I was fortunate enough to be able to visit Neal Jern at his shop in Soquel, just outside of Santa Cruz. Neal is probably the foremost builder of Chevrolet-based ohv kits for Model T's and A’s. He can supply all the necessary parts for the conversion, including cast aluminum adapter plates, valve covers and side plates. Customers specify the compression ratio, which is a function of the thickness of the adapter plate.

Neal does exquisite work, as befits a man who spent 40 years as an instrument maker at Lick Observatory. His ohv Model T speedster is the finest example of this type of car that I have ever encountered -- absolutely museum quality.

Our engine used pushrods of unknown origin that seated in Allen head bolts screwed into the adjustable lifters. The bolt-head recesses had been dimpled to match the spherical ends of the pushrods. Oversized valves 1.5:1 rocker arms and rocker-arm shafts appeared almost new.Head modifications included oversized valves. But there was no provision for oiling the valve mechanism. Charlie said that this feature was historically accurate: Chevrolet owners were expected to be handy with an oil can.

Once past the restrictions imposed by the mounting flanges, the intake and exhaust ports were caniverous. The beautiful foundry work aside, you had to wonder why the designers had such little concern for flow velocity. Even so, the 1928 four-port casting must have been a vast improvement over the two-port heads used previously.

The copper block-to-adapter gasket, intended for a Winfield ohv head, required modification. We drilled additional water passages and increased the bore diameters to four inches to match the block. That left something like 3/16" of gasket material between adjacent cylinders.

We grafted a 6" section of ˝" steel rod to the distributor shaft in order to raise the distributor clear of the valve cover. My good friend and Model A expert George Sprotte cautioned us on the importance of mating the extension with the same tongue-and-groove arrangement Ford used. A single-piece drive shaft or a solid joint would result in rapid distributor-bushing wear.

The Model A cooling system works on the thermo-siphon principle, with the pump functioning as a helper. Hot water, rising through the head, passes freely between the pump impeller and the sides of the port. Two vestigial vanes, cast into the port, function at high speeds to direct some flow to the pump.

Because the Chevrolet water outlet is only 1 ˝" wide, we had to rely entirely upon the pump for circulation. An adapter was fabricated from 1" aluminum plate, with the bore tapered 30 deg to match the profile of the impeller. Water enters the pump cavity at the hub area and through a slot cut into the lower part of the plate. In theory, the pump should function like any other centrifugal pump, accelerating the water and throwing it outward for collection at the discharge port.

Using the adjustable shaft collars provided, we set the pump impeller within a few thousandths of the housing for maximum pump efficiency. With a little machine work to one of the collars and a plumb bob, we were able to obtain a fairly close alignment between the pump drive pulley and the engine pulley. The fan was not considered necessary for for quarter-mile dirt track racing.

If the engine overheats --the almost bald impeller can’t be much of a water mover – we may go to a 216 or 235 Chevrolet pump, as suggested by George. These pumps are self-contained and can be fairly easily mounted to deliver water to the block at the lower hose connection. In other words, rather than pull water out at the head, the pump pushes water through the engine, Corvette fashion.

Details absorbed more time than anticipated – the better part of one Sunday was spent fabricating a throttle linkage, which still might be improved. Finally, after about two weeks and several sleepless nights, we got the engine together and started. The assembled engine looked good, but there were problems with the adapter plate. It ran, but leaked water like a sieve at the block/adapter-plate interface. The copper gasket could not adapt to the warped and gouged plate. Nor did the inherent stiffness of the steel plate help matters. The plate blanchard ground to ensure accuracy and generate a crosshatch pattern for gasket bite. I suggested that he find someone to make a proper composition gasket for the application, using the copper part as a model.

This view shows
    raised distributor. Herman has another project underway, this time mating a Ford ohv tractor head with a 1932 Ford B block. Seems that the bore centers are almost identical. Ultimately we would both like to adapt a double-overhead-cam, multi-valve head, driving it with a belt. A high-pressure fuel pump would provide lubrication. Now that would be something!