Info please

[A7novice]

It is usually stated that when testing cylinder compressions, it is important that any one cylinder value should not be more than 10% different from the other three cylinder values.
Just idle curiosity,  but what is the logic for this. 
Cheers.

[Tiger]

Would indicate problem with the low cylinder/s
Rings Valves General wear

[Chris KC]

Compression testing is more useful as a comparative tool than as an absolute. Even among Austin Sevens you would measure quite a range of compression levels depending on the head and block fitted, oversize, etc. etc. not to mention their condition (e.g. brand new, just run in, high mileage...) However if you measure 90 on three cylinders and 55 on the fourth something is clearly amiss and you can then conduct further investigations to figure out what.

[A7novice]

Many thanks for replies. I realise that it suggests that low cylinder has problems , but I wondered if the suggestion was it would somehow shorten the life of the crankshaft.

[Andy Bennett]

I think there are a few problems which could be seen through a reduction in a compression test that could be stressing the crankshaft, like a failing head gasket allowing water into a cylinder or a loose gudgeon pin scouring a bore and then breaking/getting jammed.

I think the point is simply that if you have done a compression test and found a wide range of compression test figures then you are best off investigating the cause.
A stitch in time saves nine comes to mind.

[bob46320]

its a bit like a tandem bike. If the bloke at the back decides not to pedal, then the bloke in the front does all the hard work. It would induce uneven running , more noticeable at tick over.
I guess that in the past, a poor compression in one cylinder wasn't noticed until it packed up completely, and noises came from the engine that became intolerable when the piston or valve (or whatever the cause) caused the engine to stop.