Help! Main bearing clearance mystery

[BartdeBruijn]

Hi!

I am building a 2.45 l MFI high compression engine and have a question on main bearing clearances.
After line boring the case back to STD and having the CW crank been grinded to 1th undersize (-0.25 mm.) and ordered the correct factory bearing shells through Mittelmotor in Germany I build the case and crank together to check clearances.

After torquing all down in two stages to 35 Nm. The crank turned smooth. However when stop turning a kind-of stick-slip occurred.

Because of this I dismantled all and start measuring precisely the bore and crank.

The interesting thing is that the case is back to Porsche specs, 62.0 mm and the crank is first undersize back to 56,74 mm. So case is back to minimum and crank is grinded to maximum. This will result in a clearance between the two of ((62.00 - 56.74)/2)=2.63 mm. When measuring the Porsche bearing shells, I measure 2.63 mm. This means in my opinion that if the bore is minimum and the crank is maximum Porsche spec wise there is no clearance left.......

So how is it possible that if both Case and crank are in spec that with correct bearing shells the clearance is zero or close to zero, see picture of excel sheet.
I also added a picture how to convert a micrometer to micrometer that can measure shells. Use a ball bearing and a shrink fit tubing!

Any ideas, advise would be more then welcome.

I also checked an old set of std bearing shells and the difference between them is precisely 0.25 mm.

regards,
Bart

[r lane]

Torque the case together with the mains installed. With a dial bore gauge, make your measurements. If still a problem, redo that procedure with bearings removed. I know a lot of work, but with all of the machining, things happen.

[r lane]

Have you checked crank for straightness. I am assuming you have measured the crank, but not the case bore. If so, I am thinking the case assuming the crank is true. Bob

[dannobee]

Check the clearances using plastigage and see if they measure the same.

[BartdeBruijn]

Thanks for your replies!

What I tried to say is that my case has been line bored back to STD. This means 62.00 mm. Also when I measure with a dial bore gauge all the mains measure 62.00 upto 62.02 (see table)
The crank has his first undersize grind making it 56.740 mm. Also measured and crank is on all journals precisely 56.740.

This means that case is on the lower end of the spec range (62.00-62.019) and crank is on the maximum (56.721-56.740)
Calculating the theoretical clearance between case and crank journal, (62.00-56.740)/2=2.63 mm.
This 2.63 mm is exactly what my bearing shell thickness (1th undersize -0.25 mm.) is. Measuring these Porsche original ordered bearing shells measure 2.63 mm.
(Measuring standard bearing shells give 2.5 mm. So the 2.63 for 0.25 mm. Undersize makes totally sense.)
Conclusion is that there is no clearance…….
For me this is extremely strange that if you have spec ranges for bore and crank and you take min and max that all clearance is gone…..

Hope it is clear where my question came from!?

By the way the crank has been measured for straightness.

I did plastigage and they flatten extremely, I also get shiny spots on my new (expensive) bearing shells, only by torqueing down. I do this in two steps, 17 Nm and then 35 Nm.


Regards,
Bart

[Billthebuilder]

Are you sure you got your crank back maybe they gave you back a standard crank belonging to someone else? Never mind I’m a dumb ass 56.740

[BartdeBruijn]

It is for sure my own crank.

My point is that I stay within Porsche spec and end up with close to no clearance…….
The bores are 62.00 mm. Which is minimum in spec dimension (62.00 - 62.019]
Crank journals are 56.740 mm. Which is maximum in spec dimension (56.721 - 56.740]
This gives me more or less exactly the 2.63 mm. Clearance the same as the original Porsche ordered bearing shell thickness.
These bearing shell thicknes is also correct as if I measure old StD ones they measure 2.5 mm. So difference is close to first undersize (-0.25 mm.) of my crank.


So staying within Porsche spec seems not to give me any oil film clearance….

[Magyar Kiwi]

Hi
Please add pics of your new bearing shell markings and packaging ?
Many thanks.

[BartdeBruijn]

Hi Magyar,

Please see pics attached.
The question still stays that doing the math there is no clearance left……
The STD shells and the -0.25 mm. ones differ precisely this o.25 mm. Thickness difference, so correct in my humble understanding

Regards,
Bart

[Rosco_NZ]

Perhaps you have -0.50 bearings??

[stownsen914]

From the info presented, it doesn’t sound like clearance is off by .25 mm. That’s .010”. For clearance you’re looking for ~.002”.

Someone here may know better why a clearance that should be in the neighborhood of .002”, is instead zero. I’ve heard of people ‘“massaging the bearings” but I’m not sure if you can get .002” that way.

You might consider dannobee’s advice to verify your measurements using plastigage.

[Magyar Kiwi]

Hi Bart
Thank you for the photos, unfortunately I wasn't able to learn anything from them.
Some ideas:
1/
From Technical Specifications, WKD422 022, page 23:
For Bearings 1 to 7 STD Crank and Shells clearance should be 0.010 to 0.072mm
From page 24:
For Bearings 1 to 7, -0.25 Crank size should be 56.740 to 56.721.
2/
If your Crank had been ground to middle of tolerance: 56.730 your clearance would be 0.010, better.
If your Crank had been ground to lowest tolerance 56.721 your clearance would be 0.019, better.
3/
I have heard that some crank grinding engineers aim for the smaller end of any diameter tolerance to prevent issues with the assembled engine "nipping up" (not having enough clearance).
4/
Wondering why the crankcase bores are out of round by up to 0.10 mm i.e. at Journals #1 &# 4.
5/
When you assembled and torqued up the crankcase halves did you have the number 8 bearing in place ? This bearing acts as the "dowel pins" to align the case halves at the pulley end.
6/
Recommend you install the all crank shells and number 8 bearing in the crankcase, torque up all the thru bolts and measure the bore diameters.

Best regards.

[Dpmulvan]

What kind of bore gauge/ micrometers are you using? Could your measurements be off??

[BartdeBruijn]

Hi Gents,

Thanks for replying!

There are for me two ‘levels’ of consideration if I can call it like that!?

1) is based on the specs given by Porsche.
Bore STD: 62.000 - 62.019 mm.
Crank 1th undersize (-0.25): 56.721 - 56.740 mm.

If you will take minimum bore and maximum journal/2 you end up with 2.63 mm. This 2.63 mm. Is the thickness of the Porsche ordered bearing shells.
Conclusion only following Porsche given specs you can end up with no oil clearance……

2) is based on measurements.

My bore iI measured with an quality bore dial gauge. Bean calibrated on q quality micrometer. ( even ordered an expensive 3 digits (0.001 mm.) digital one.
So my measurements are on the bore (see picture of excel-sheet) 62.000 - 62.01. My crank journals are all 56.740 mm.
As explained in 1) leaves me with close to no oil clearance.
I also torqued my case in two steps without number8 bearing as my case is shuffle pinned. I was tolded that shuffled pinned case does not need number8 for measurements. It will also be difficult/impossible to get the dial bore gage in on that side. It is also the rare small spigot 7R case, so the stiffest as you can call a mag case stiff ;-)
I will do the Plastcage measurement again.

Will report back on case measurements, with and without bearings and the plasticage measurements too!

Thanks again and will be continued!

Cheers,
Bart

[djpateman]

As discussed on Early 911S Registry:
https://www.early911sregistry.org/forums/showthread.php?174048-Help!-Main-bearing-clearance-mystery
There much more to the clearances than just the case bore and the the crank journal . Your combination of max range journal and min range case are not an acceptable combination because it fails to provide the oil film clearance within it's tolerance range. You have to balance all three ranges at once. But first you need to decide what is your target clearance value. Your case is done, and you have your bearings. So assemble and plastiguage your clearances. Grind crank to get your target clearance.

BTW, this reminds me of a Honda Accord engine that got hydro-locked and we had to build a new engine. The procedure was to obtain block, crank and rods. Then lay out the rods on the crank the way you will assemble. Observe the markings on the block: each piston bore is marked for the size of piston to order, and each bearing location is marked with a size. Also observe the markings on the crank: each bearing location is marked with a size. Consult parts book to determine the individual bearing to order for each location; that is block mark x and crank mark y= bearing z, and similarly for the crank and rod markings. NO crank bearing sets, and NO rod bearing sets; all bearings are custom selected.

[icarp]

I believe you must have the number 8 bearing in for any proper measurement.
Even with thpins the 8 bearing must be installed
I hope this helps
Just my opinion
Good luck

[BartdeBruijn]

I will perform measurements again with #8 bearing in! Will be difficult to measure bearing 5, 6, 7 as my dial bore gauge dus not have the length to reach from bearing 1 to those bearings. However measuring 1-4 will give indication if #8 helps.

Interesting is also that after torqueing down the case with bearing shells without #8 and no crank that measurements alter/change over time/days. The measurement inhorizontally direction reduces over days with almost 0.005-0.01 over 1 day and again over some more days…..
Almost if there is creap.
Yes Ik calibrate my dial gauge with a micrometer before every measurement!

[Neil Harvey]

If I can add and give how we do this. For us it when we are asked to repair (re bore) the engine case main housing.

We have sized sleeves that we install in place of the #8 bearing and another that replaces the countershaft. These are sized for the housing bores not with bearing shells. Assemble the case, then install a hollow dowel pin sleeve in place of the front bottom 8.00mm stud. Once the new dowel pin is installed we can remove the inserts and re assemble the case without the front sleeve to measure and repair the housing bore.

In your "case", fit the countershaft and its bearing shells reassemble the case and re measure. Or do as we do (above) using your #8 bearing as the alignment tool.

[stownsen914]

You can use the #8, or a trick I've seen mentioned a few times is use a VW type 1 bearing in place of the #8. The Beetle bearing has a larger ID so a bore gauge can fit through.

[Rosco_NZ]

I had similar situation with Std Case & Crank ... crank was only just Std & too much clearance .. I ended up having to buy oversized mains and grind the crank to suit. Interested to know whether the journals should be rehardened / tenifer treated