DO MAIN BEARING CLEARANCE TIGHTEN OR LOOSEN with heat



DO MAIN BEARING CLEARANCE TIGHTEN OR LOOSEN with heat

Postby grumpyvette » July 17th, 2014, 10:52 am

I got asked this and while I thought it was rather obvious after giving it a bit of thought I decided to answer this .
oil pumps provide oil flow almost directly related to the RPM they spin at UP TO THE POINT where pressure allows the bye-PASS circuit to allow some volume to RE-CIRCULATE back thru the oil pump, limiting oil pressure.
oil pumps provide FLOW VOLUME! its RESISTANCE TO OIL FLOW thru bearing clearances that REGISTERs ON THE GAUGE AS PRESSURE!
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keep in mind the sbc oil pump has 7 tooth gears and the big block pumps have 12 teeth making the oil flow smoother and less pulsed, plus having larger gears they tend to supply more oil at lower rpms
look closely and youll see the big block oil pump has a 5 bolt lower cover and the oil pump pick-up with its 3/4" feed seats into the main pump casting while the small block oil pump has a 4 bolt cover and the sbc oil pump pick-up with its 5/8" feed seats into the pumps cover plate
READ THIS THREAD
viewtopic.php?f=54&t=3536

the contention of the person asking this was that since metal expands as its heated, both the block and crank obviously will slowly expand as they heat up so the clearances should remain rather consistent as both will remain at similar,even if elevated temps. but because cast iron expands at a different rate than forged steel, and the bearing shells are frequently aluminum., the bearing clearances may change
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the first thing I'd point out is that oil being pumped thru the bearing clearances tends to loose viscosity or thin as its heated, and the thinner or lower the viscosity the lower the resistance it has to being forced thru bearing clearances so as a result hotter oil having a lower viscosity and a lower resistance to flow reduces the oil pressure reading on the oil pressure gauge.
as a general rule as your engine oil viscosity is reduced the effort required to pump the oil thru clearances is lower and the pressure reading on the gauge drops, thats not necessarily an indication of lower bearing protection, as thats generally a function of oil quality and its formula, and basic components used, in its design, and generally its increased flow rate increases bearing cooling, a good quality 10w30 should ideally provide 20-22 psi at 800rpm idle (anything over 15-17 psi at 800rpm is fine) and 60-75psi by 6000rpm and UP,AS LONG AS ITS CONSISTENT, which is all you can use
blocks are generally made from cast iron or aluminum, cranks from forged steel or cast nodular iron,now aluminum expands faster than either cast iron or forged steel, but in the temp ranges were talking about the expansion won,t be critical to the bearing clearances.
IF you care to do the math heres a chart, but youll find the oil viscosity change is a more significant factor than the metal expansion over the expected temp ranges in question
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IF YOU CAN,T SMOKE THE TIRES AT WILL,FROM A 60 MPH ROLLING START YOUR ENGINE NEEDS MORE WORK!!"!
IF YOU CAN , YOU NEED BETTER TIRES AND YOUR SUSPENSION NEEDS MORE WORK!!
grumpyvette

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Re: DO MAIN BEARING CLEARANCE TIGHTEN OR LOOSEN with heat

Postby grumpyvette » July 23rd, 2014, 1:14 pm

theres three potential GATES or valves in the oil flow path thru the blocks oil passages, the pump provides oil flow, and a bye-pass valve internal to the pump limiting max oil pressure to the spring resistance selected (usually 60-75 psi)
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many oil filters have an ANTI-DRAIN BACK VALVE
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many oil filter adapters have a oil filter bye-pass valve, that opens if the resistance to oil flow exceeds 10 psi
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viewtopic.php?f=54&t=3536
IF YOU CAN,T SMOKE THE TIRES AT WILL,FROM A 60 MPH ROLLING START YOUR ENGINE NEEDS MORE WORK!!"!
IF YOU CAN , YOU NEED BETTER TIRES AND YOUR SUSPENSION NEEDS MORE WORK!!
grumpyvette

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Site Admin
Site Admin
 
Posts: 14105
Joined: September 14th, 2008, 1:40 pm
Location: florida


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