1939-40 Mercury
1939 saw several changes to V8 engines. The biggest changes were due to the introduction of the Mercury, those motors are covered in the next section. The Mercury had an engine based on the Ford V8, but with a noticeable increase in torque and a nice bump in horsepower. The additional power came as the result of additional displacement, 239 cubic inches instead of Ford's 221 cubic inches. The additional cubic inches came from a 1/8 inch larger bore, to 3 3/16 inches. Now we have the reason for 24 studs per head. The lower line of studs on the 21 stud motors is too close to the bore for safe production of 239 cubic inch motors. With the addition of more power came larger main and rod bearings, this time 2.500 inch diameter mains and 2.140 inch diameter rods. Another difference started with 1939 motors is a longer crankshaft. Ford and Mercury mounted the engine fan on the end of the crankshaft in 1939, 1940, and 1941. To allow the fan to be held onto the snout of the crankshaft, the snout was made longer. Starting in 1939 some V8 blocks were relieved between the valves and the bore at the factory. This was done to reduce compression and detonation problems on large trucks. Both 3 1/16 bore and 3 3/16 bore blocks may be factory relieved. All blocks still have four freeze plugs in the oil pan rails. Mercury blocks have round center water holes in the decks that can be used to identify a 239 motor from the smaller Ford motors of the same years.
The 239 cubic inch Mercury engine was also optional in trucks. The water holes in the deck feature a circular center instead of the upside down keystone. Some 239 blocks had 99 stamped into the passenger side intake rail near the front, but not all. As with the LB, an unscrupulous person could have added this marking to a less desirable engine also. The determination on one of these blocks should be made using the center water holes in the deck. Mercury motors will have an upside down keystone at the bottom and a circular hole in the middle. These can be seen in the The 239 cubic inch Mercury engine was also optional in trucks. The water holes in the deck feature a circular center instead of the upside down keystone. Some 239 blocks had 99 stamped into the passenger side intake rail near the front, but not all. As with the LB, an unscrupulous person could have added this marking to a less desirable engine also. The determination on one of these blocks should be made using the center water holes in the deck. Mercury motors will have an upside down keystone at the bottom and a circular hole in the middle.
The series engines got the 99 identification since they started in 193
9 and displaced 23
9 cubic inches.
Features of the 1939-40 Mercury
- 24 studs retaining the cylinder heads
- Water pumps in the block
- The water drains are angled
- Passes the pencil test
- Core plugs in the oil pan rails
- Crankcase ventilation
- Insert main bearings 2.499” diameter retained by studs
- Round center water hole in the deck
- Upside down keystone water hole in the lower center of the deck
The circular center water hole and upside down keystone can be seen in this picture.
The crankcase area with the core plugs in the oil pan rail can be seen in this picture.
