Crankshaft bearings and journals

[Tuesday]

Hi. Well this is my first post and you are probably going to hate me because it's about a BMW I6. But it is a general crankshaft question and you guys are smart so you can probably help anyway.

Crank main and rod bearings are sold (on this site anyways) in sets with all the same size bearings. Do they machine the journals on the bearings all the same, or is it possible that one journal may need one size, and one needs another? Also why do they even sell different sizes (ie oversize) bearings, why don't they just make all cranks of the same engine the same?

If you have any more info that you think I'd like to know about crankshafts, bearings and/or rods I'd love to know.

Thanks!

[ChrisBennet]

You infidel!

Just kidding.

We're car guys first, not Porsche snobs.

To answer your question, the main journals will be machined all the same size "standard" (STD) and the rod journals will all be machined the same size "standard" (STD). A crank will come from the factory with standard size mains and rod journals (STD/STD).

If a crank journal is damaged or worn out of spec, all the journals will be machined to the next size. If say a rod journal was damaged, all the rod journals would have some material removed and those journals would be undersized or "1st undersized" or "1st under". If the damage was so deep that even more material had to be removed, or if the rod journals were being machined a 2nd time, the next size would be "2nd under".

If the case has to machined making the holes (half holes?) for the crank main bearings bigger. You would use an oversize bearing.

Does that make sense?

-Chris

[Tuesday]

Originally posted by ChrisBennet
You infidel!

Just kidding.

We're car guys first, not Porsche snobs.

To answer your question, the main journals will be machined all the same size "standard" (STD) and the rod journals will all be machined the same size "standard" (STD). A crank will come from the factory with standard size mains and rod journals (STD/STD).

If a crank journal is damaged or worn out of spec, all the journals will be machined to the next size. If say a rod journal was damaged, all the rod journals would have some material removed and those journals would be undersized or "1st undersized" or "1st under". If the damage was so deep that even more material had to be removed, or if the rod journals were being machined a 2nd time, the next size would be "2nd under".

If the case has to machined making the holes (half holes?) for the crank main bearings bigger. You would use an oversize bearing.

Does that make sense?

-Chris

Yes that makes sense. So oversize bearings are for cranks that have been machined due to damage, correct? My crank is in perfect condition so I guess standard would be the way to go. I guess that I could plasti-guage it, but for some reason I threw away the main bearing cap bolts, so I'd have to buy 2 new sets to do that which is $$$! Thanks

[NevenM]

Hi

For the purposes of measuring you don't need stretch bolts any bolt will do. Also I'm surprised at the useage of plastigauge by you guys, the general opinion here (in NZ) is thats its awful stuff and we generally measure with a bore micrometer

HTH
Neven

[Jim Sims]

Hard to get a bore micrometer in between the journal and the bearing when assembled. "Free state" measurements are not the same as "all up" measurements especially with a magnesium or aluminum case. Plasti-gauge is a way to bolt up the system and get an indication of the actual clearance. Jim

[NevenM]

Jim

If anyone can explain to me why "Free State" measurements and "all up" measurements differ I'd be interested, As as I'm concerned a - b = c, and though there are 2 error factors in (a & c)
versus 1 in c, I'd bet that the error in plastigauge is greater than the cumulative error in 2 micrometer measurements

Neven

Neven

[john walker's workshop]

plastigage goes back to the age of poured babbit rods, and shimmed mains. if the case or rod bore is right on, the bearings are new, and the crank is in spec, everything is going to be correct, clearance-wise. this is assuming you are using the proper calibrated measuring tools, and you know how to use them. it's pretty much predetermined.

[ChrisBennet]

Originally posted by Tuesday
Yes that makes sense. So oversize bearings are for cranks that have been machined due to damage, correct?

Not to nit pick but undersized bearings are for cranks that have been machined. The "size" refers to the hole that the bearing goes into or the journal it goes around.
A crank bearing can actually be two sizes at once!
- If the hole in the case where the bearing sits was machined, the bearing would be oversized.
- If the main journal on the cranks were machined (reduced in size) the bearings would need to be under sized.
- If both the case and the crank were machined (case=over, crank=under) they have bearing for that too!
-Chris

[Jim Sims]

"Free state" measurements only equal "assembled measurements" if the size, shape and position of features don't change as a result of assembly loads. If you really believe there are no loads between the crank journals and the bearing inserts and hence no deformations of these features then you are correct. Being able to turn the crank on the assembly lube doesn't mean there aren't forces from a imperfect crank loading the bearings as it "whirls"; it just means friction is low enough to easily overcome them. Under running loads things get much different (worse) in terms of clearances with a case as limber as the 911 unit. Cheers, Jim

[NevenM]

Jim

I should have qualified my statement and said that I measure everything under assembly loads and yes we are talking a very
dynamic design.

Cheers