Is 4 to 5 thou too much main bearing clearance

[slow car]

Hi has anyone ran 4 to 5 thou main bearing to crank clearance?
3.4 std oil pump,10.5 :1 com 7500 rpm?

[afterburn 549]

only in drag machines might you get away with it for a little bit. JMO
Why so loose?

[otto_kretschmer]

my Haynes manual calls out .0003 to .0028 for bearings 1-7

[afterburn 549]

AFAIK typical is 1.5 all the way around ( oil clearance, most engines)

[3literpwr]

Mine just came in around 0.0348mm or .00137in... Stock SC crank with 100k on her.

[afterburn 549]

some people make a mistake using plasti guage
When crank is laying dwn against main bearings on one side so you get double the gap on the measured side.

[slow car]

Hi I have been using plasti gauge and are getting around 0.004 "
Crank is in spec, case is in spec have tried 3 different bearings still over 0.003"
Where to from here, Grind crank, line bore,?
Does any one know how much thicker genuine Porsche bearing shells are over Glyco?

[afterburn 549]

Mag case ?

[slow car]

3.2 alloy case

[slow car]

If i grind the crank , do i really need to re case harden it?

[afterburn 549]

Leave it to shop that does P.cranks.

I do not think so as far as re nitride or something
Check on bearings B4 you go too far...some have become real expensive and not available .

[mytoy]

How does grinding the crank reduce bearing clearance?

[RedCoupe]

You reduce clearance by running oversize bearings. Just to be clear, the bearings fit a standard size bore in the case, but have more material so they fit a crank journal that is smaller than standard.

[mytoy]

Understood. But no where was he stating using oversized bearings.

The statement was that he had 4 to 5 K clearance at present and wanted to know if line boring and crank grinding was necessary. Yes, it would be if going to oversize bearings but not if you were trying to reduce clearances.

I do see there is another thread where he asks about hardening a crank after grinding and using oversized bearings. So in that context take my earlier statement as non meaningful.

[afterburn 549]

From things i think I know.
Is your crank journals "egg shapped"??
If not your measuring must be wrong.
I would have a experienced person do it .
Its hard to remove that much material in normal wear .
If it is under sized you will to have the crank machined dwn to the next size and install bearing to match.

[tom1394racing]

Have you measured the bearings after they have been torqued in the case without the crank? Did you use a bore gauge that was zeroed with the same micrometer that you made the crank measurements with? Both of these are critical to get accurate bearing clearances.

Here is how it's done:

https://www.youtube.com/watch?v=7VayaKK2mJI

[mikedsilva]

Originally Posted by afterburn 549

some people make a mistake using plasti guage
When crank is laying dwn against main bearings on one side so you get double the gap on the measured side.

I've been wondering about this...
So if you use plastigauge, and get 4 thou as the clearance measured, does that mean your bearing clearance is actual 2 thou?

[Trackrash]

I'm not sure what Afterburn was thinking. Not true.

Did you properly torque the case through bolts? What motor are you working on?

Here is my crank's clearance.

[mikedsilva]

Hi Gordon... working on a 3.0SC...
I'm not an expert and have just started learning to use micrometers and bore gauges..
So far I have measured my crank to be perfectly in spec. Using the bore gauge, I am getting a clearance of 0.08mm which equates to 3 thou... 0.003 inches.

Using plastigauge, I am getting 4 thou.. 0.004 inches..

The measurements I am getting on the bore gauge are repeatable.. but I am a novice. So not sure if I am doing it wrong every time!

[Tippy]

Don't think you'd have any oil pressure at those clearances!