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Bits & Pieces IV

Sept/Oct 2002

Oil Consumption & Low Oil Pressure

Regarding Jerry Windle's question at the end of May/June 2001 "Engine Talk" -- he was experiencing both excessive oil consumption and low oil pressure in his 48 coupe -- I doubt that the two problems are related.

For the oil pressure problem, I will take three guesses.

    3.)  There may not be a restrictor fitting in the bypass oil filter feed line.
    2.)  There may be cam bearings at standard on a cam with -.010 journals.
    1.)  The wrong 1/8" NPT plug (with no restrictor) may be installed at the front of the main oil gallery, behind the cam gear.

As a rule, if the oil doesn't leak but it disappears, it is going out the tailpipe, smoke or not.

Possible causes could be incorrectly bored cylinder walls, rings installed incorrectly, or something I'm not thinking of at the moment.

Is there excess blowby?  The evidence of this is oil fumes coming out the oil fill cap, and out the road draft tube. If not, the rings are sealing the compression properly.  Many times when there is no visible smoke with high oil consumption the problem is with cylinder wall preparation. The walls may not have been honed enough to remove all of the metal craters torn by the boring bar.  Each time the piston goes down, the oil is partially purged from these craters and some of it is carried out the exhaust with the spent gases.

The other common cause of high oil consumption is cylinder walls that have been finished too rough, with oil filling and then coming out of the honing scratches.

If there is excess blowby, that indicates that the rings are not seated properly.  At 5000 or more miles, they probably will never seat properly. 

If the high volume oil pump had any effect on oil consumption, I would expect that it would increase it.

Maximum Safe Bores

The figures I use for maximum safe bores follow. This assumes normally aspirated engines using gasoline.

All 21 stud blocks:  3 1/16 +.080. The limiting factor on these is usually head gasket availability and sealing between the bore and the stud at the bottom of each bore.

24 stud 3 1/16 blocks are usally OK at 3 3/16. I have seen some that were good at 3 3/16 +.040.

24 stud 3 3/16 blocks: 3 3/8, although I think that 3 5/16 is more prudent.

There will, of course, be blocks that will not bore safely to the above sizes. This could be due to core shift, sand pockets and/or holes in the casting caused by trapped air.

Core shift and corrosion from the water jacket side are often problems.  Sonic testing is available to measure wall thicknesses before you bore. If you suspect corrosion, pay extra attention to the areas near the bottom of the bores.

Sand inclusions in the cast iron can cause problems with any bore size.  Sonic testing will not usually find sand inclusions.

Of course, there will be engines that run cool and long at bore sizes larger than those noted. They may be the exceptions that prove the rule.

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