Background

When new con-rods were fitted (in 2017) the existing big end bearings were re-used. In February 2023 the opportunity was taken to inspect them during the winter overhaul.

Whilst the actual bearing surfaces seemed good, it was clear that the shells had up-and-down clearance in the rods. We decided that new bearings were required.

We believe the original Panhard-manufactured bearings would have been solid bronze. The current bearings were bronze shells lined with white metal. These had been fitted in a major engine overhaul prior to the 1996 London to Brighton run.

If we were to revert to solid bronze it was felt we had the skills within the volunteer team to make them ourselves. If we decided that white-metal lined ones were required then the manufacture would have to be done by a specialist.

We took advice and were told that white metal linings are more forgiving than solid bronze. Lack of clearance or lubrication will lead to potential seizure. Under that circumstance a white metal bearing would melt and give a very noticeable rattle. A solid bronze bearing would lock solid with potentially catastrophic results. If we were to use solid bronze then a greater clearance – suggested at 0.003” (3 thousandths of an inch) on the diameter would be required (for white metal 0.002” clearance would be the aim).

Measurement of the current clearances suggested that we already had 0.004” in the bearings plus whatever clearance there was between the bearings and the rod housing, so noise from the extra clearance was unlikely to be an issue. On grounds of cost and originality the decision was taken to use solid bronze and aim for a clearance of at least 0.004”.

Description of work

All work on car done in the conservation laboratory at the museum

•    Crankshaft supported on purpose-made fixtures (made during con-rod replacement work) and sump removed

•    Manual assessment of bearing clearance by checking for movement of con-rod relative to crank. Excess clearance

     apparent

•    Big end caps and bearings removed

•    Front cylinder head removed to allow con-rods from cylinders 1 and 2 to be removed (making and checking the  

      new bearings would be much easier with a con-rod available on the bench, rather than in the engine)

•    Each big end crank pin measured with a micrometer in 6 places (on 2 diameters at right angles, and at front, 

      middle and back of each pin). The condition and roundness of the pins was good, but the pin for No. 2 cylinder 

      was significantly smaller than the rest – presumably a consequence of the repair work required when this rod had 

      broken and been replaced sometime in the early history of the car

•    The con-rod big end bores were checked and found to be in good condition

•    Discussion with Smiths Metals resulted in us specifying SigmaBronze 7 as a suitable material, combining excellent 

     machinability with good wear resistance and excellent anti-friction properties. This is a material that is available in 

      a range of cored bars which minimises material waste, and hence cost.

A Team Panhard member with access to the appropriate machinery agreed to make the new bearings, using the 

following procedure.