Rebuilt bottom end - oil pressure ?

[nigeldtr]

This has turned my understanding of oil completely on its head. I thought that oil viscosity (thickness) was measured by the time it took a fixed amount to pour through a standard sized hole - the longer it took the higher the viscosity - the lower figure on the multirade being the cold measurement and the higher being the hot measurement. 10W (winter) /50W being a good oil for winter use to stop the oil turning to treacle when cold.
I thought that the idea of a multigrade was that when the oil was cold it was thinner to allow quick circulation on start up and as the engine temperature increased and bearing tolerances widened, the oil thickened up to the higher range to fill the space in the bearings and maintain oil pressure.
It seems from the previous answers that the opposite happens.
Now I'm totally confused. 

Jim

Jim,


The more I read, the more confused I get. I was of the same opinion as you and to my mind, a 20W/50W should be ideal but it seems the oil does not get thicker - help can anyone explain the magic of multigrades

[nigeldtr]

Jim,

I think Triton Thraser just cleared the fog in "Filter in Oil Feed Line" thread take a look!

Nigel

[BSA_54A10]

1) yes you are correct, I should use the preview more often .
Kick till it just slips.
Then engage 4th and try the stall test.

2)

The more I read, the more confused I get. I was of the same opinion as you and to my mind, a 20W/50W should be ideal but it seems the oil does not get thicker - help can anyone explain the magic of multigrades

The 2 numbers on multigrade oils are the viscosities of the oil at stp and at 100 deg C
STP is 25 deg C & sea level ( or some where near that )
So a 20 /W50 has the same viscosity as 20 when cold
And the same viscosity as dose 50 @ 100 deg when it is at 100 deg. So, no it dose not get thicker as it get hotter, it just thins a lot slower

[alanp]

There's a nice diagram on this oily subject in 'Filter in Oil Feed Line' dated 20 Jan from alan@Ncl.