Con rod query

[Stuart Giles]

Unless finished in place all shells are made with an exaggerated clearance at the parting line although it may not be obvious.

Bob, surely you mean interference? (or 'crush' as it's known in the trade). The shells stand a few thou proud of their housing so they are forced firmly into it on assembly. It is this radial force (not the tabs) which prevents the bearing shells turning in service.

I think what Bob is referring to here is the fact that big end bearing shells aren't usually a uniform thickness, they are thinner near the split line, so will present an oval hole giving greater side clearance to the crankpin even though they will (hopefully) be sitting with an element of "crush" in a dead round hole in the big end. You can see this quite clearly  if you use Plastiguage near the split line of a shelled big end and then repeat in the centre of the big end cap

It's certainly true that soft white metal will absorb particles, but the load a white metal bearing will take is a lot less than a thinwall shell bearing of similar area. Also, with a white metalled bearing you are dependent on the quality of the metalling process as well as well as the material used. It's not too clear in this photo of a Ford Model "A" rod from my workshop, but this "run" big end has large areas (the grey bits) where there is no tinning, so, when the white metal got hot, it just melted rather than conducting the heat away through the rod. It probably looked just fine when it came back from the (US based)  metallers. Aftermarket shell bearings are available for Ford Model "A" rods, so problem solved in this case.



I do think that shell Vs. white metal bearings  in vintage motors is to some extent a religious debate; if so, I suppose I have declared my allegiance above .