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Oil Drain back .......What Drain Back

Pete Lewis

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there is lots of conflict . idea's  solutions and myths about oil Drain Back

myself it doesnt worry me at all but to many its a paranoid problem

well this week for some daft reason i have replaced the oil filters on two Vauxhalls one diesel on 1.4 petrol both have an upright filter removable canister 

with element clipped inside,   on removal  neither had any oil inside no mess no spillage....empty ......so drain back is nothing old its also very  NEW

they dont knock presumably tighter tolerances ,  they do knock after a oil change  till its refilled dont knock on normal start up  but the filter is  very empty

when switched off.


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My little old Ami has the Citroen GS flat-four 1015cc engine in it, and those engines use an inverted filter with an anti-drain valve in it.  I'm advised this is essential with those engines to prevent their O/H camshafts from running dry.  

From what I understand modern engines have moved on to free spinning roller, needle and ball bearing races in preference to plain bearings and bushes, so the design of oil system is very different and very much more cleverly calibrated.  I presume those components with plain bearings are focused in the system's design to accommodate any short lapse in oil pressure. This can be done by focusing high pressure first to those places &/or by dedicated material faces and spiral oil groves ..and of course in closer production tolerances. In contrast to white metal and bronze bushes they are pretty resilient, especial where lightweight synthetic oils with additives and detergents (..that eat bronze) are used.  Disclaimer : The last modern engine I rebuilt was a KTM motorcycle engine and the big end & mains were needle rollers, but I have no hands-on knowledge of modern car engines. .

Regarding churning the motor over on the starter..  I have to wonder what revs-per-minute that achieves ?  Surely, an engine which fires instantly and pumps its oil pressure up and splashes it around very much sooner (the engine not under load, and 'splash' is crucial to lower cylinder lubrication and roller bearings) is less damaging than (relatively) slow and prolonged churning with low oil pressure and hardly any splash.?  During which time some petrol / ethanol is sucked in ..perhaps not enough to fire the engine but some to start washing oil from the bores.!


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I don't think modern car engines use roller bearings. They're a motorbike thing. They're actually not as good for shock loads (which is what the crank bearings are all about) and they increase cost and reduce service intervals, both totally alien concepts to car manufacturers.

The benefit of the "churning ... on the starter" is that you don't have the shock loads when doing that. And the RPM is irrelevant - the pump works at cranking speed and you're only trying to fill the oil ways. An engine that fires instantly may fill the oil system faster in time but it's working the bearings much harder during that time, and for just as many rotations.

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 all the modern cranks I have seen are shell bearings. The grinding wheels that grind the cranks are hydrostatically lubed. No replaceable shells as there is never metal to metal contact.

It's not the pressure that is important for minimal wear; It's the flow. A reason why pre-oilers will decrease wear.




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