3 replies to this topic

[DeadSquare]

Years ago, I had a rear main bearing crack.  I obtained another, but it wasn't concentric with the other half in the block, so I had it carefully hot-dip galvanised, fettled it, fitted it and line bored it with a cylinder boring bar.

 

I had assumed that being an English vehicle with an inch and 3/4 diameter crank journal, if I measured the internal dimension of the centre main bearing housing, that it would be an inch and 7/8 or 15/16, but it didn't even come out as 1/10s of an inch.

 

I have just measured a block and it is 1.894"

 

Does anyone know why simple fractions were abandoned or why the shells, which were made under licence from America by Vandervell, weren't made to fractions of an inch ?

[Spider]

The nominal tunnel dia for the Mains in a Small Bore Block is 1.8960 to 1,8965".

I'm pretty sure they ended up at these sizes as there's limits as to just how thick the wall can be in slipper type babbitt bearing shells, if they are made too thick, they can't transfer heat as well, so they would have had to start from the thinnest they could be made keeping in mind the thickest that they can be made too for when the crank is ground. Since the original design, babbitt materials have improved considerably and they can be made thicker now days, so I doubt it was a case of working to a 'round number' for the shell tunnel size, but practical limits when working from a 'round number' for the crank journals.

[DeadSquare]

  Thank you for the correct dimension of 1.896".  It still seems to me that .073" is a very odd dimension for either Austin to have specified of for Thinwall Bearings to have chosen. 

 

.075" would have been logical as America worked to thousandths of an inch while England worked in fractions, a problem that meant converting all the drawings when Rolls Royce licenced Packard to make Merlin engines during the war.

 

Heat dissipation is a reasonable theory, but against that is the introduction of pressurised oil flow which carries away a lot of the heat.

 

Tony Vandervell called his company "Thinwall Bearings", and I remember having to re Babbit the conrods of a prewar 1172cc Ford Ten because the shells fitted to the postwar engines were loose in the prewar rods, the Babbit being thicker than the shells, so perhaps he just made them as thin as he could get away with.                                        

Edited by DeadSquare, Yesterday, 01:52 PM.

[DeadSquare]

I have just measured the shell from a BMW K1100 motorcycle, and they are .0785", = a very logical 2mm.