Model A & B

Ford Garage

Babbitt Bearings and Shims

Shown below are NOS original Ford A-6340 engine crankshaft rear main bearing shims. These are thin individual steel shims, not thick laminated brass shim packs. The front and center crankshaft main bearings also used similar steel shims (different shape than the rears).

These shims are precision milled or ground to 0.002 - 0.0025 inch thickness each. The finish milling/grinding marks can be seen on the shim surfaces. They are not simply stamped or die-cut from rolled shim stock.

The original precision steel shims were released two per side. Not laminated, not thick, and not 'soft'.

The thin steel shims allowed the oil gap between the block and cap at the bearing to be minimized, and more importantly, allowed the bearing cap to be solidly torqued to the block and maintain that condition. This allowed minimizing the thickness of the soft babbitt bearing metal , as Ford intended in the original design.

Why have shims at all?

Shims allow for some future tightening of the bearing (by shim removal) to reduce the increased crankshaft-to-babbitt clearances caused by compression of the babbitt metal in use. Bearing wear is not really the factor, nor should sloppy engine rebuilding work be a factor requiring the use of shims.

After Ford assembled the engine, it was run-in using an electric motor to drive it. In this process the points of highest contact (high spots) between the babbitt and the crankshaft were heated by local friction heat. (likewise piston rings and cylinders were seated).

The babbitt bearing high points of contact were selectively heated, softened, and reformed dynamically in this run-in process. As they were reformed by the crankshaft, the high spot 'interference' areas were decreased, the bearing 'contact' area was increased, the oil film clearances were equalized, the babbitt hot spots were cooled, and the overall frictional forces were reduced to the 'acceptable' resistance level. The acceptable level was determined by the amperage drawn by the electric motors.

Thus the bearings were fitted with good contact and minimum consistent clearances, and there was no need for excessive shims for later adjustment. By removing a single 0.002 inch shim from one side of a cap, the total up/down bearing clearance could be reduced 0.001 inch. Remove a shim from the other side of the cap to get another 0.001 inch of clearance reduction between the crankshaft and bearing.

Do it one more time on each side for a total of 0.004-0.005 inch of bearing clearance take-up, which should be more than sufficient for the life of the engine if the bearings were made properly in the first place.

Aftermarket thick brass shim packs: A not-so-good idea!

The 1/16 and 1/32 inch thick laminated brass shim packs for engine rebuilding may have made millions of dollars in profit for Federal Mogul for the last 70 years, but they are basically a bad idea. The thick brass is quite 'soft' compared to the original thin steel shim design.

Take apart any old rebuilt engine and what do you find? The thick brass shim packs are unnecessary, counterproductive, and allow the cap fasteners to lose torque and clamp load as the soft brass and tin is further compressed over time.

As the clamp loosens and the cap moves slightly, the shim gets fretted and hammered to foil pulp. Every old rebuilt Model A engine I have taken apart is like this! How many good used brass shims have you ever found? Not many I bet.

It is much the same phenomenon as that of a soft head gasket which compresses and then continues to compress and lose clamp/torque in early use and must be re-tightened. The brass is relatively soft and is more easily compressed than steel, and allows the joint to loosen over time.

As the brass shim compresses in use, the bolt torque and clamp is lost and the cap can move around and fret the shim and loosen further. The clearances between the crankshaft and bearing are thus increased and allow the crank to whip more and hammer the babbitt and loosen the clamp joint even further. It is a self-fulfilling prophecy! (Gee Boss, we better use thicker shim packs on the next rebuild to allow for all this bearing slop!) Not!

NOS Cap Showing Original Babbitt Thicknesses

Aggravating factors: Who needs them?

Babbitt compression is also further aggravated by the fact that in a rebuilt engine the crankshaft is often ground undersize, which increases the nominal thickness of the soft babbitt in the block and cap greater than necessary and greater than originally intended.

Likewise, the thick shim pack moves the steel cap away from the block and also further increases the thickness of the soft babbitt in the cap. Even if you were using an NOS crankshaft which had never been ground undersize, you would be faced with excessively thick babbitt bearings in the caps due to using thick shim packs.

Compared to thinner babbitt, the thicker babbitt is more prone to excessive compression and resulting increased bearing clearances. The resulting clearance leads to hammering, cracking, and ultimate failure of the babbitt. This failure would happen at some point sooner rather than later compared to an engine with fewer or no shims. Thicker is not better when it comes to babbitt bearings!

The soft thick brass shim pack cannot hold the bolt torque and clamp load in service, and the cap loosens slightly allowing the shims to be fretted. The hammering crank and lost torque allows the caps to be deformed over time as well. Steel caps do not wear or deform in normal circumstances and if properly fitted. The solution is not thicker shims!

A similar failure mode mechanism is at play on the engine's connecting rods, caps, bearings, and shims.

At the end of the day...

All in all, the final result of thick brass shims is extra-thick babbitt which can be more easily hammered and compressed by the crankshaft, resulting in subsequent loss of torque and clamp load of the bearing caps, and increased bearing clearances.

The increased bearing clearances lead to further increased hammering and compression of the babbitt, ever-larger bearing clearances, and finally babbitt failure.

It is a vicious circle and we know how the story often ends.

I have seen 1/16 and 1/32 inch shim packs in brass. If I had to use them I would delaminate them and use two 0.002 inch shims per side as Ford originally did.

What do you think? Original thin steel shims on the left, versus thick laminated brass shim packs on the right. I know what I think.

I would peen the babbitt (mechanically pre-compress it before machining) and minimize the thickness of the shims used for boring and final assembly. This is to keep the babbitt (and shims) as thin as possible, and keep the bolted joint as strong and solid as possible, and to maintain the torque and clamp on the fastener joint.

Unnecessarily thick brass shim packs and excessively thick babbitt is the enemy of modern babbitt engine rebuilds. That is my story and I am sticking with it.


More related information on Ford Garage:

  1. Model A & B Babbitt Bearing Material Specifications
  2. Model A Bronze Rear Main Bearing Thrust
  3. Model A & B AER Insert Main Bearing Data & Instructions

March 2001