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The Story of Bearings...conclusion

Sept/Oct 2001


Rod Bearings

The rod bearings that came in our flathead V8 engines were full floating from 1932-48, and more conventional inserts from 1949-53.

The full floating bearings have only two bearing shells per crankshaft rod journal.  The steel "backer" is actually in the middle of each shell, with bearing material on all sides except for the parting lines.

These bearings are free to spin in their rods, as well as on the rod journal.  Any one of the three surfaces may sieze without causing rod failure.  Other engines have used this design feature, but none are as popular or populous as our V8's.  Some of the early fuel burning dragster Chrysler Hemis were converted to full floating rod bearings to lower friction and enhance reliability.

The 1932-38 rod bearings had thrust flanges on their sides.  These were used with rods that did not usually have a consistent forging number.  The Ford part number prefix for these bearing is usually 48. the standard journal size is 1.998 - 1.999"

Perhaps because end thrust on rods should not be very large force, this was eliminated on the 1938 and newer rod bearings.

The 1938 - 1946 (?), 221 cu in engine, rods used bearings with the prefix 81A. They had bearing alloy on their end thrust faces but no flange. The rods that they were used with had forging numbers 21A and 91A. These may be used interchangeably; the weight ranges are similar. The standard journal size is also 1.998" - 1.999".  These are the rods that most flathead stroker crankshafts use.

The 1939-48 239 cu in engine engine rods had forging numbers of 29A and 99A. These forgings may also be used interchangeably. These rods and the 1949-53 rods can not be used without modification in 3-1/16" bore blocks.  The bearings that they use have the Ford prefix 99A.  The standard journal size is 2.138-2.139".

In 1949, the rods changed again to the more common insert type.  This required new forgings, the 8BA. These were made with no squirt holes, or with squirt holes in either of two locations.  The rod journals stayed the same dimensions, but they had to have two oil holes, one for each rod bearing.

The rod bearings changed to match, they were much thinner, with locating tangs as an assembly aid. As Ford added and changed squirt hole locations, the aftermarket added holes to the bearings.

On the full floating bearing rods, it is important to remember that the ID of the rod big end is a bearing surface and should be polished.  On 8BA rods, a conventional cross hatch is desired to help retain the insert.

Ford used various rod bearing materials. Babbit, sintered bronze, lead silver, and cadmium silver are the ones that I have seen.  Ford also made full floating rod bearings in standard, +.004" and +.008" OD. I have never seen the oversize OD bearings made by aftermarket suppliers.

Cam Bearings

Cam bearings for our flatheads are micro-babbit on a steel back. The front and rear are same.

They are a medium press fit in their bores in the block. This means that their installed ID is different than their free ID and varies somewhat depending on the exact size of their bores in the block.

They each need to be installed with their oil holes lined up with those in the block.  In addition, the rear cam bearing must be installed with its larger hole lined up with the fuel pump pushrod bore.  If you neglect this, you will always be forced to run an electric fuel pump.

Cam bearing are available standard (for a 1.796-1.797") journal, and .010", .020", and .030" undersize. If you have poor oil pressure hot in a fresh engine, you might have an undersize cam on standard bearings.

Ford made some blocks with oversize cam bearing bores, usually +.080". Ford also made some 1934 blocks with no provision for cam bearings; the cam ran in the block directly, much like the Model A.

Some of the flathead racers have run cams on stock or larger cam bearing bores to get a larger base circle on high lift cam lobes. Perhaps because of higher springs forces and rpm, this has not worked well.

Pin Bushings

Pin bushings may be bronze or bronze on a steel backer.  They are usually replaced during overhaul. 

The old bushings are pressed out and new ones pressed in.  The steel backed bushings are a much heavier press fit than the solid bronze.  The solid bronze bushings are swedged (or formed) into the eye of the rod.

Either type is then fit to the piston pin, usually by honing, but possibly by boring.  In the old days this was done with a reamer.  The recommended clearance is .0003" to .0005".  Pistons pins vary at least much. Give your machinist a couple of pins to fit your bushings to if you are having this done.

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