Classic cars forum & vehicle restoration.

[Ray White]

Something which I have always been confused about is how to describe the bearings needed for a reground crankshaft.

If a crankshaft journal has been ground down it will require new bearing shells. These are advertised as + sizes but I would have thought they should be - sizes and described as "undersize" not "oversize"?

[peter scott]

I suppose it's + as far as shell thickness is concerned.

Peter
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http://www.nostalgiatech.co.uk
1939 SS Jaguar 2 1/2 litre saloon

[Ray White]

[Ray White]

[emmerson]

But the crankshaft would have had metal removed, so would be undersize, ie u/s.
A bored-out cylinder, however, would be bigger, therefore oversize, ie o/s.

[Ray White]

So, even though the crank journal is undersize with regard to it's thickness the bearing shell surface despite being correspondingly smaller is provided in oversizes.

It's all done to confuse the less clever students amongst us.

[emmerson]

In my days on the spanner (admittedly a long time ago) shell bearings for a reground crank were always marked (eg) 020 u/s, and pistons for rebores would be marked 020 o/s.
I blame it all on metrication!

[Ray White]

I think I have found the answer to my confusion. When a bearing has "spun", The machining required to clean up the housing results in the requirement for an "oversize" bearing without necessarily changing the pinion size.

[emmerson]

Ah, now that is a totally different thing, Ray. You're talking about increasing the thickness of the back of the shell, not the wear face.
This might be the case if the block had been line-bored to rectify damage caused by "spinning" of the main bearings. Undersize shells would be used in the con-rods.
This is getting just too complicated after a couple of pints and a huge lamb and mint pie down the pub!
I'm going to bed.

[Ray White]

I think what has happened is that over the years there has been a drift towards describing the thickness of the shells in oversize terms where as in the dim and distant past we always referred to undersize measurements in relation to a crankshaft re grind.

[MikeEdwards]

I came across similar issues when looking for parts for my recent rebuild, and I think it's really just a laziness thing, because there can't really be any confusion in the vast majority of cases. While I might think that a main bearing should be "oversize" because it's bigger to meet up with an undersized crankshaft, that's the only direction that would normally be encountered. Wear tends to only happen in one direction, whatever the part, so it doesn't really matter that the terminology may, strictly speaking, be incorrect.

This is probably as irritating to any proper engineers as the modern interchangeability of the terms "GPS" and "Sat-Nav" was to me at one point.

[Ray White]

[roverdriver]

I think that the confusion arises from our frequent activity of dropping out words from a description. I seem to remember the word 'for' being part of the description of bearing shells designed to fit ground crankshafts. In that case it makes perfect sense, even though the shells need to be oversize in order to 'fill the gap' as it were.
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Dane- roverdriver but not a Viking.

[emmerson]

I blame Boris!

[Ray White]