2.0L S crank measurement
 

Hello all,

I recently received my crankshaft back from a well-known, reputable Porsche-only machine shop. The crank was STD/STD and I originally sent it in for magnafluxing, cross-drilling, cleaning and polishing but the machinist felt that some scratches on one of the rod journals were deep enough to warrant re-grinding. After getting it back I measure all the rod journals at 2.242" +/- .0002" which is .001" under the low end of STD dimension specified in the Porsche manual. The mains are fine.

It's my understanding that a re-grind to first undersize would be .25mm or .0098"

Is there such a thing as .001" undersize rod bearings? With STD rod bearings my rod clearance will be too large, no?

I plan to call the machine shop Monday to clarify, but in the meantime I'm puzzled and wondering if anyone knows if .001" under rod journals are OK.

Thanks.

-Paul

Paul,

Just to confirm this is a 2,0 S crank, 66mm stroke, counterweighted.

OK. You measured your rod journals and they came in at 2.242" +/- 0.0002"

Converting from decimal inch, your measurement is 56.947 mm +/- 0.005mm.

And as you correctly quote from the little white book, the wear limit is 56.960mm for the "Shaft" of the "crankpin to connecting rod bearing."

So, you're below the wear limit all right. And in that situation, the correct thing is to regrind the crank to the next size rod bearing, in this case, 0.25mm undersize.

I faced a similar situation with my original 2,0 crank. You can see how the measurements stacked up. I thank Tom Butler (tom1394racing) for his development of the graphs we use to visualize this. Here is mine:

http://forums.pelicanparts.com/uploa...1176156531.jpg
Here's a link to the whole thread. I have learned a lot since then I like to think.
http://forums.pelicanparts.com/911-engine-rebuilding-forum/328636-measurement-instruments-metrology-2.html

As you can see visually, the wear limit is a long way below the max clearance.

The bore measurement is 57.020-57.059.
At 56.947mm without your tolerance range, your clearances are between .073 and .112mm vs. the bore of the rod. The standard clearances are .030-.088 at the max, and the wear limit clearance is 0.099mm.

If we give you the tight side of your tolerance of .005, the crank is still between .068 and .107mm. On the loose side, between .078 and .117mm. One tenth of a millimeter is a pretty big gap, you can hear that clearance on the valve adjusters for sure!

It's unlikely to me that this gap could be made up with the rod bearing. If the bearings are in spec on correctly resized rods, then you should measure between 57.020 and 57.059 bore diameter on the big end.

Anyway, long winded way of saying that an extra $200 is a lot for undersize rod bearings, but if it were my engine I'd have the crank ground to the tight end of the factory spec, micropolished to a nice Ra, make sure they got nice fillets on the edges and call it cool.

Others may have different opinions or tell you to just go for it, but the numbers are the numbers. Of course there is a rounding error in going from decimal inch to the factory specs, you might borrow a metric mic and repeat the measurements for more of a picture of what is going on.

Thanks for the detailed reply and for the link.

Yes, that's correct -2.0L 911S 66mm counterweighted crank.
Your conclusions are the same as mine. I just wanted to be sure that:

a.) .001" undersize rod bearings didn't recently become available and

b.) that current conventional wisdom doesn't recommend that a short-stroke motor for a track car, with oil system updates, should run looser clearance on the rod bearings.

I'm going to recalibrate my Mitutoyo .00005" mic and re-check those measurements then have a chat with the machine shop Monday. I can't imagine why they would grind or polish the crank to some random undersize dimension for which no bearings are available. That certainly was never my instruction. I authorized them to do everything they recommended to check and update the crank.


Thanks again.

-Paul

Paul,

Nothing wrong with recalibrating your mic against a standard every once in a while. When I measure something I'm usually out of practice so I practice on the standard to make sure I'm not using too much gaging pressure. With a ratcheting barrel it's harder to make wrong measurements but it's certainly possible.

'065's original crank had been ground .5 under on the rods. I had it micropolished and then let it sit in a humid garage covered with WD-40, which is a pretty lousy corrosion protection fluid. It rusted. I had it polished again, but the pits didn't come out.

So. . . this crank will become the basis for a regrind to NASCAR bearing size-- turns out that the 2.125" journal size (53.975mm) not only has less friction, but it allows you to use Mahle/Clevite bearings, which are of much higher quality than the Glyco bearings we get these days. A set for a V8 is like $33. Unfortunately the Pauter rods are $1700 for a set of six, but look on the bright side, you save $500 by not having to have your rods resized and balanced. So between saving money on undersize bearings. . . it's only about $1,000 more, or about 5% the cost of a race engine build, to go with the reground journals. That estimate doesn't include the crank mods. . . :)

Thats precisely what I'd do with this one. :)

Thanks. Not sure I can handle (psychologically) Nascar parts in my 911, but you guys know what you're doing.

Meanwhile, I spoke with the machine shop. I'm going to go ahead and mention the name because these guys seem to have an excellent reputation as well. It's Ollie's in Havasu AZ. I'm not criticizing or disputing anything about their work - I just want to understand why they did what they did. According to George, at the shop, they've been grinding 911 rod journals .001" undersize, to run with STD bearings, in high performance engines for the last 25 years.
They believe the life of the crank is improved due to better oil flow from the greater clearance, with less chance of spun bearings. If this is a proven fix for a slightly worn crank - and indeed an improvement, as Ollies claims, I'm puzzled by why it isn't well known and mentioned in the excellent books on Porsche engine building.

-Paul

Paul, I have had 2 different 2.0 cranks rebuilt by Ollie's for HP engines. Both cranks had there rod journals turned down .001 for better oiling at high RPM's. Both cranks see 8000 rpm's at times and have been running great so far. Over 15K miles on one.