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Oil clearance VS viscosity/oil W

General Engine Building Discussion

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Would like to hear some input on what oil weights are suitable for main and Rod bearing clearances and if their is a sort of rule of thumb ..... I’m building a 2.4L iron block inline 4 banger with oil clearances right about 2 thou ....30w?40w?50w? Discuss

There's a few more aspects to consider than just the bearing clearances. one of the biggest deciders of oil viscosity in modern engines is variable cam control as a heavy weight oil will make the cam control difficult or impossible. If you are running conventional fixed timing then this is less of an issue and your oil choice may need to be adjusted based on your oil pressure. You've listed the clearance at 2 thou however the clearance needs to be considered along with the journal diameter as for example 2 thou clearance on a 1.5" journal would be considered loose while 2 thou on a 2.5" journal is considered a little tight. To give you some insight on our 4G63 drag engine program we were running 2 thou on the big ends and 2.5 thou on the mains with a 15 w 50 Motul 300V and had good control over hot oil pressure and excellent bearing life. I would generally suggest you start by stepping one stage heavier in oil weight from the stock recommendations and then see what your oil pressure is to help guide you to the best oil for your engine and application.

Thanks for the reply Andre! I love your content and think it’s very informative ! This reply makes a good bit of sense to me ..... I have an SR platform that I run Rotella T6 5W40 and usually see between 20-25psi at idle warm and 60 to 80psi up top ..... the way this sound is like I’m more after a good or set oil pressure I’m after .... do you think that on a 2 inch journal 2.5-3.0 thou will make that drastic of a oil pressure difference ? And if so as an example , In the event I’m at the extreme and went 3tho on a 2in journal let’s say the standard oil pressure is around 20psi idle would it be likely to drop to 10psi? And if so I would be increasing the weight of oil to bring it back to 20psi ?