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

[Richy]

Harry Pellows [ The Maestro ] wrote about this problem in his book . There are some dimensions that simply will not work together .

[edmayo]

Measure with a dial bore gauge, then you know.

[NTW]

Once you are 100% confident in your measurements, grind the crank to work.

[BartdeBruijn]

Measure with a dial bore gauge, then you know.

Hi Ed,

I did measure with dial bore gauge for sure!

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…..

[edmayo]

I didn't see where you mentioned using a bore gauge. Sounds like the crank will need to be ground for clearance.

[tom1394racing]

You might try a quick check with Plastigauge to confirm your clearances are near zero.

[denniso]

Out of curiosity, what is the recommended required min clearance?

[edmayo]

Generally .010 to .072 mm. It would be good idea for you to find and buy the "spec book" You'll need it often.

[BartdeBruijn]

I didn't see where you mentioned using a bore gauge. Sounds like the crank will need to be ground for clearance.

Hi Ed and others,

I will obviously have to regrind my crank to create the needed clearance.

However my point/question is that despite what I measure, if you look at the max and min in specification you do not have any clearance left, how is that possible?
That is the point I do not understand. I am within spec machining wise 62.00 case bore and 56.740 crank journal, this will leave you 2.63 mm clearance which is the thickness of the original -0,25 under size original Porsche bearing shells.
This is what the spec book said
Case bore 62.00-62.019)
Crank journal (56,721-56.740)
I fyou machine min on bore (62.000) and max on journal (56.740) both within spec your clearnace will be 2.63 mm what is the bearing shell thickness leaving 0.0 for the oil film.
That is what I do not understand…….