Most internal parts that you would use for a major overhaul are interchangeable between early and late model engines. However, some of the parts used in fuel, ignition, and cooling systems are unique to early or late models.
It’s generally true that engines with an oil fill tube over the flywheel housing will use late model parts, and engines with no oil fill tube over the flywheel housing will require early model parts. However, during the transition from early to late models, some engines ended up with a mixture of early and late model features, so when first ordering parts for your engine, it’s best to have the following information in hand before placing an order.
1. To order ignition parts (including the “Ignitor”), you will need to know if your distributor was manufactured by Delco (used on late model engines), or by Prestolite (used on early model engines). Prestolite distributors are distinctively smaller in size, and their condensers are mounted on the outside of the distributor housing.
2. To order cooling system parts, you will need to know if your engine has a thermostat housing (a small dome-shaped casting) located on the front corner of the head, nearest the starter. This thermostat housing would indicate that your engine has a late model cooling system.
3. To order carburetor parts, you will need to know if your carburetor is made of aluminum or cast iron. Aluminum carburetors were used on late model engines, and cast iron carburetors were used on early model engines. All carburetors were manufactured by Zenith or Bendix, so the manufacturer does not help in determining your carburetor vintage.
4. To order gaskets, you will need to know (1) and (2) above, plus the shape of your transmission cover. Late model engines used a square cover, and early model engines used a rectangular cover with a curved top (“tombstone-shaped”).
5. Late model (Delco) starter ring gears have slightly smaller teeth than early model Prestolite ring gears. For this reason, Prestolite and Delco starters cannot be exchanged between early and late model engines unless the ring gear (or entire flywheel) is changed with them. In addition, late model ring gears have a slightly larger diameter than early models, so when moving a late model flywheel to an early model engine, either a late model housing will also have to be moved along with the flywheel or the six (6) small boss areas on the inside of an early model flywheel housing will have to be ground down slightly. These boss areas are for the ¼ – 20 bolts that hold the flywheel cover on, and they will have to be ground down almost to the bolt holes themselves.