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Join Date: Apr 2007
Location: wisconsin
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Bearing clearance... whats optimal?

Porsche gives a huge range for main bearing clearance... .0004 to .0028.

Rods is even greater, ok to .0035.

Rule of thumb mentioned in various threads is .001 per inch, then add .0005 for racing, or about .0025

Then there is threads that .0025 is too much, and almost completely out of spec...

Then there is threads that 200k SC's are specing out at .0015...

The there are posts from engine builders that "in spec is in spec."


Is there a clearance folks are shooting for (not like we have 5 different bearings shells to play with like the chevy guys do)?


Hypothetically if you can achieve either a rod spec of .0022 or .0032 (both in spec), which is better. The latter will give lower oil pressure...

Same with the crank. If one can get .002 or less, or .0028, is one "better," or as Porsche specified, they are both "in spec."

Porsche's normal crank spec is as tight as .0004... which is hard to believe!!!
Old 10-19-2017, 09:36 AM
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I would not assemble an engine with 4 tenths mains that runs 20-50. 2 is better than 2.8 on the mains for the street. 2.2 is better than 3.2 on the rods. Race engine running high revs and straight 50 the larger is good. Rod big ends don't stay round at high rpms so the add'l clearance accomodates that. Tight clearances require perfect geometry on the parts involved along with thin oil. The old standby of 1 thou per inch of journal for the street has stood the test of time.
Old 10-20-2017, 05:37 PM
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That's what I figured... Car will be street/hpde...

That's why I was surprised that Porsche specs .001 to .0028 on the mains (much less .0004).

Even .001 sounds really tight, though I do see folks post on here that they are running .0015 on the street.

Plan to stick with .0022 for the rods...

Shooting for the same on the mains, need to double check.

thanks for the info
Old 10-20-2017, 05:52 PM
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Quote:
Originally Posted by boosted79 View Post
I would not assemble an engine with 4 tenths mains that runs 20-50. 2 is better than 2.8 on the mains for the street. 2.2 is better than 3.2 on the rods. Race engine running high revs and straight 50 the larger is good. Rod big ends don't stay round at high rpms so the add'l clearance accomodates that. Tight clearances require perfect geometry on the parts involved along with thin oil. The old standby of 1 thou per inch of journal for the street has stood the test of time.
Follow up question...

With Porsche we don't have the luxury of multiple over and undersized bearings...

So what is an acceptable bearing clearance spread? .0025 on all bearings would be great, but unlikely...

So, what do you do if some are .0022 and some .0025 and some .0027?

Seems most folks on here use plastigauge, which in no way would show those differences. Others use lower priced bore gauges, which only measure to .0005. So, most folks simply don't realize a difference exists.

My indicator goes to .0001... And it's incredibly repeatable.

Am I over thinking this??? With a Chevy you can mix and match 1/2 shells and buy special thicknesses. Not so much with Porsche...

Live with it I assume?
Old 10-31-2017, 06:11 PM
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Unless you want to grind and fit oversize you have to live with it. Maybe try moving the shell sets around to different journals to try to get them closer.
Old 11-01-2017, 05:48 AM
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