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Ford 9N, 2N & 8N Discussion Forum
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Re: Need advice re ring/crank bearing replacement

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Author  [Modern View]

04-09-2013 09:56:53

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Always understood that most of the journal wear occurs at start up rather than at normal operating conditions.

That is normally the case since that is the transition from boundary to hydrostatic regime. Luging an engine alsl contributes to accelerated wear since increased load at lower RPM equates to a reduced oil film thickness and mixed mode lubrication.

Ok, so why does the flatheat N have the 0-50 psig oil pressure gauge and the OHV NAA have the 0-80 psig oil pressure gauge?

WAG - the engineers wanted a higher pumping pressure and increased flow rate to accomodate the greater demands of the OHV design. A flathead doesn't need top end lubrication but an OHV does and it needs it nearly immediately on startup. The oil pressure gauge on my 1963 Triumph equipped with a similar size and vintage Standard engine is 100 PSI and the Leyland specification is a minimum idle hot oil pressure of 30 PSI hot. With an OHV you have to move more oil, move it further, and get it there faster. An OHC is even more demanding in that regard.

So what is the concern about oil back pressure, since the oil pressure is not in the equation for oil film thickness?

It is a easily collected performance metric. The issues I just mentioned are part of that performance but pump oil pressure is also a very good indirect measure of bearing clearances. Bearing clearances ARE a factor in oil film thickness and a low pumping pressure indicates increased clearances and a reduction/loss of the protective oil film.

In my opinion the N series engine needs to have minimum of 15 psig oil pressure @ 1500 rpm.

A fresh engine with good oil clearances will easily make 40 PSI at 1500 RPM. I would say that an engine with 15 PSI @ 1500 RPM is telling you it has wide clearances and/or a worn oil pump and is nearing the end of it's service life. It's those widened clearances that will cause accelerated engine wear - not the low oil pressure.


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