aracer

NOT IN POSES OR ANYWHERE? ANY HELP APPRECIATED

Jake Raby

These are not published anywhere. As part of our engine program we have developed our own specifications.
NOTE: There are NO under or oversized bearings available.

Macster

What Jake R. said.

But there might be ways to get what you need:

Here's a formula from Bosch Automotive Handbook, 5th edition:

h○ = ( D - d ) / 2 - e = 0.5 D * ψ * ( 1 - ε )

with relative eccentricity

ε = 2e/(D - d)

where:

h○ = minimum lubricant depth
D = inside bearing diameter (nominal diameter)
d = shaft diameter (nominal diameter)
ψ = s/D
s = bearing clearance s = ( D - d )
e = eccentricty ( displacement between shaft and bearing centers)
ε = relative eccentricity 2 e / s

There's a good drawing of a journal bearing that illustrates the various features listed above in the Journal or Sleeve Bearings section of the Machinery's Handbook. I have the 20th edition but there are I'm sure newer editions. This reference gives several pages of formulas that one I believe could work out and arrive at some reasonable numbers.

That is, one could fill in the knowns above and solve for the unknowns.

Or you can just buy new bearings and install them and install the crank and make sure it is straight, and if it is then torque things down to their proper settings with PlastiGage (I hope I remember this correctly) positioned in between the crank journal and bearings and then loosen the crank and carefully pull things apart and determine the clearances from the PlastiGage. Unless the crank is terribly worn out on every journal you should find every journal's clearance nearly the same, so you know about what the clearances should be.

And if you find one that has a much larger clearance, say 150% the others, that's probably a worn out journal. (You can plug the clearances you find in the above formulas and possibly work out what's what.)

If you find one (or more) bearings with excessive clearances then it is time to replace the crank if there's no way to grind crank undersize and fit oversize bearings (and as Jake stated there are no over or undersized bearings available) or if there is no way to build up the worn journal then resize it to have the proper clearance using a standard bearing (and I believe Jake has stated before hard chroming or welding up these cranks does not work well.)

If you find one with a real tight clearance you might have installed a bearing incorrectly, had some dirt, a burr, etc., behind it.

I used this method to get some experience: I checked the bearings in a Datsun 510 4-cylinder SOHC engine I was rebuilding -- I had the bearing clearances/tolerances though and the micrometers (and bore gages) to measure the crank main/rod journal outside and inside bearing diameters) this way and found one journal worn a bit on a taper, but still not enough to worry about. And I learned later that it would have been ok for a street engine to have this worn journal ground undersized and fit an oversized bearing just to this one journal.)

Understand, though I've never done the above for a Porsche engine and it would be much better to have the clearances/tolerances in hand.

Sincerely,

Macster.

onefastviking

The only spec I see from Porsche is this ...... and it's not much help.

Crankshaft Endplay
New Installation 0.05-0.24 mm
Wear Limit 0.28 mm

DreamCarrera

I second what Macster said and would only add E=mc2.

J/K I really have no idea what Macster said but it does sound good.

Macster

There are many formulas related to the plain bearing journals. Using what is known, crankshaft journal diameter, oil viscosity (at temperature), and more, many more than I have the time to type in, one can with algebra calculate the unknown values.

(Another way to arrive at the numbers, or at least have some confidence as to whether the numbers are in the ball park or not is to refer to the clearance numbers published in various Porsche engine rebuild books. I have one -- the title escapes me -- that has a list of all the Porsche flat 6 (and flat 4) air cooled engine crank and rod bearing clearances. I don't have the book to refer too but I seem to recall the bearing clearances/etc for at least one MY water cooled engine was given as well. Now sure, these numbers are for air-cooled engines but we are talking ball park here. If anything I would suspect the water cooled engine bearing clearances would be tighter.)

Or use the existing crank to determine if the numbers are ok. I was taught that if the engine was running ok prior to disassembly for rebuilding upon rebuild check the clearances with micrometer and bore gages or even easier use some special plastic stuff/pieces which are crushed into the space between the crank bearing and shell and from the flattened out plastic strip one can deduce each bearing's clearance.

And the benefit is this is done with the crank in place and torqued down.

Of course I had the factory (and some 3rd party engine builders') bearing clearance numbers handy.

But even if one doesn't have the numbers the odds are high the numbers will all be nearly the same. If there is an outlier bearing showing too much or too little clearance, this suggests a problem and possibly the crank is not reusable.

Now I'm sure that a professional engine builder like Jake has through years of experience and trial and error arrived at optimum clearances. For instance, he may have found that some bearings require tighter clearance and some a bit more clearance. Perhaps using heat sensors located to measure the oil temperature thrown off by the crank bearings a hot running bearing can be id'd and its clearance increased. Or too cool oil suggests the bearing's clearance may be too much and it can be tightened up. Too much clearance affects oil pressure and a bearing with a large clearance represents a large fraction of the oil pump's oil flow to the main bearings, to the engine in fact.

There are some pretty sophisticated techniques and the shops/engine builders guard these techniques and the numbers they have arrived at are held as trade secrets, as they rightly are. I was invited to visit a professional race engine builder's shop some time ago and I asked about pics. He said in some areas, some fixtures, dyno setups, etc., no pics allowed. Unfortunately my schedule has not allowed me the time to visit this shop but I plan on it sometime this spring.

Anyhow, I would be surprised if Jake didn't come by his numbers at considerable cost in time and probably a few engines...

Sincerely,

Macster.

Jake Raby

..Have that crank magnafluxed... At least 50% of the cranks that are checked, are cracked.

EVERYTHING related to specifications in this engine has been hard earned.. Thats why we just had 7 technicians attend our M96 101 Engine Rebuild School from all over the country.

aracer

so much for simple help!
anyway, I don't consider the main/rod specs for 996 engines such a "hard earned" subject.
HERE'S THE INFO FROM THE ENGINE BUILDER ASSOCIATION

ROD JOURNAL DIAMETER-2.1642-2 01650" OR 59.971-54.991 MM
MAIN JOURNAL DIAMETER-2.3611-2.3618" OT 59.972-59.99 MM
STROKE-3.012 OR 76.505
BORE- 3.937 OR 100 MM
with the amount of engines we do.......info can be shared, as we're in NYC and busy.

aracer

OOP'S THAT'S FOR THE 964 ENGINE....WILL GIVE EVERYONE THE SPECS FOR 996 LATER

Macster

I asked today and was told I might be able to have the 987/991 crank specs. The 996 (and 986) crank specs were issued some time back when the engine was in release. Many techs that attended the M96 engine intros and such received such literature but after this many years that literature gets buried and forgotten or tossed.

So there is more that just asking for the info. The tech has to agree to spend the time to search through all his stacks of literature and find it.

No promises but I'll see what I can gather up.

Sincerely,

Macster.