GregBBRD

Yes. I don't use a true "ball" end micrometer, but use one that has a ball on one side and a flat on the other. And all bearings have eccentricity (sp?), as they move from the center of the bearing towards the "split". Racing bearings have more eccentricity than street bearings. This is designed this way, since the rods "pinch" at the parting lines as the loads go up. When comparing bearings, you need to measure at the very middle of the bearing, in the same place on each bearing.

Plastigage is sold to rookies that don't have measuring tools...

Imo000

How accurate are they compared to a mirometer?

BC

Not very from what I had seen.

davek9

I would like to thank everyone for the quick responses this morning, including AO and Dennis for lending a technical eye.

I’d like to through out a question to the group.
Given that the bearings are designed to wear and not the crank, if the crank is undamaged and a “standard” size bearing is removed, would it not be logical that the replacement bearing would be another “standard” size?

I noticed that these bearings are marked “Standard”.

Thank you,

Dave K.

soupcan

https://rennlist.com/forums/928-foru...lem-found.html

I had bought a couple sets of gyco bearings years ago, thankfully I had them for this engine. Found 5 out of the 16 sets were to tight, the original bearings were in great shape and right in the middle of spec.

Imo000

If standard was in it, and the crank is not being re ground, then the replacement will have to be standard size too. Standard out, standard in.

GregBBRD

Yes. No.

Certainly your crank will look like new. And the inside of the connecting rod will look the same.

So, you will definately be using "standard" rod bearings.

"Standard" is the problem. There is a pretty big variance in "standard" rod bearings. Porsche puts three different sizes of "standard" in their boxes of bearings. Glyco replacement bearings can very all over the map. You can get "standard" rod bearings that are so tight on the crank that it will cause damage/engine failure....many have been "down this path". You will need to measure your original bearings and replace them with bearings that are the same size. Beg, borrow, buy, steal a micrometer that is made to measure bearings. Hire someone who knows how to use it. Measure things very carefully. Put the bearings with the biggest clearance (they will measure the thinest) into the #2 and #6 position.

hacker-pschorr

This reminds me, years ago when local guys Jim and Todd were building a 928 motor for track use. I cannot remember how many sets of Porsche bearings they ordered, but it was a lot. All measured, catagorized and the best combination picked for the motor. The rest are probalby on a shelf somewhere.

GregBBRD

Yes. I've done this, too. It gets down to the point where you start wishing for a slightly "worn" crankshaft, so you can use the "tighter" bearings.

The problem with 928 crankshafts is that they are seldom worn. Pretty good piece of steel.

blown 87

What do you consider a good clearance for all the rod bearings in inches for us folks that do not know that .1mm = .004 thousandth?

hacker-pschorr

Come to think of it, eight of those "on the shelf" are probalby in my motor, I wasn't picky at that point

blown 87

Your plastigauge had gotten hot or damaged, done right, with good plastigauge it is dead on most times with really good mikes.

blown 87

with used rod bearings there is a lot of "feel" to it that is much more than just clearance IMHO.

dr bob

I like Plastigage as another confirmation that there's no contamination, no crud behind a bearing, and that I can sometimes do the math correctly to get the right clearances. Divide by two, carry the six, multily by the square root of three, then by .667, add in the time of day and air temperature, should be about right. Oh yeah, ALWAYS subtract the crank pin diameter. You can probably see why I need the backup.

Had a customer bring me an engine way back when, one he was building himself to save money. Still in the stand, it was locked up tight. When did it lock up? What was the last thing you did before you couldn't turn it? He didn't know. He'd just started bolting things together until he was out of pieces to bolt on. Then it wouldn't turn. Clueless... I'll speculate that Greg assembles each engine no less than three times if you count all the measure/fit/test/rotate/measure/test/assemble/test steps for each moving part. Ever wonder why some seem to be 'better' than the one you buy from Bernie's Wholesale Discount Barely Used or Reconditioned Shortblock Mart?

davek9

Thank you Greg and all, now I understand that "standard" are not all the same size and the need to measure and fit.

Edit: so now the real question is: How many sets of "standard" bearings do I need to order ?



Dave K