Engine Bearings - Single vs. Double Classification?

[Rontgen]

I'm in the middle of rebuilding my first BMW engine, the M20. I've built a fair number of Ford V8s over the years and feel like I have a pretty good handle on engine building and machining in general. However, one concept that's not sinking in is: Single, Double, & Triple Classification of Rod & Main Bearings and the various associated colors. This appears to be BMW-specific and I haven't found much conclusive info after searching.

Could someone explain what they mean by Single vs. Double vs. Triple Classification? How would one know which Classification they should be building the engine to? I notice there are differing radial clearance specs, which seems strange to me?

Am I correct in assuming that this was a means to quickly assemble engines in a production environment?

Thank you!

[Rontgen]

OK, to answer my own question and hopefully help someone else in the future, I think I may have figured some of this out.

At the BMW factory, the engine blocks, cranks, and connecting rods would all come in marked with a color dot, depending on their exact measurements and variance from nominal. As the engine is assembled, the color dots are used to enable the production line worker to quickly choose a bearing which yields the optimal bearing clearance. Think production line where time is money - there's no time for bore gauges and math. In theory, you should be able to find these marks on your engine parts, spend a small fortune ordering the correct bearing shells from BMW, and your bearing clearance should be perfect (not accounting for 25 yrs of wear on the crankshaft).

However, if you are using aftermarket bearings, exactly none of this really matters. You have standard size bearings and two oversized sets if your crank has been machined. Simple. Choose the correct bearing based on your crank measurements, verify with bore gauges & outside micrometers during assembly, use Plastigauge as a sanity check - and you're good to go.

I still don't really understand the single/double/triple classification designation, unless perhaps it has to do with how many of your color dots match (i.e. a blue crank with blue block and blue rods vs. a blue block, red crank, and yellow rods?). Regardless, I'm not sure any of this matters to the casual M20 rebuilder. I did find that TIS specs a single radial clearance range, which is exactly what I'd expect and makes much more sense to me than what's listed in the Bentley.

[Streichholzschächtelchen]

That's whar I figured too. You're on the money with "spend a small fortune" btw. Super expensive for the oem ones.

[nando]

I would just like to point out - if there is "wear" on your crankshaft journals, then your crank is junk. I haven't ever seen a BMW engine that wasn't abused or neglected that had "worn" crankshaft.

[Rontgen]

I would just like to point out - if there is "wear" on your crankshaft journals, then your crank is junk. I haven't ever seen a BMW engine that wasn't abused or neglected that had "worn" crankshaft.

Agreed. And if the crank is junk, chances are the cylinder bores and pistons are beyond trashed so the economical choice is to track down a different M20 and start over. :)

I was just pointing out that there are variables at play (particularly after polishing the crank) that may not yield the exact same clearances as BMW had during the original assembly. Just didn't want to give the impression that blindly picking colored bearings is an acceptable substitute for mic measurements and/or Plastigauge

[TobyB]

I still don't really understand the single/double/triple classification designation

If you dig further, you'll find that the number of dots correspond to a clearance range.
I forget exactly how it runs, but it's in single tenths, so, for example:
2.1000- 2.1003 is one
2.1003-2.1006 is 2
2.1006- 2.1009 is 3
or something like that. It's a way to get the variances in production
crank grinding matched to the bearings more closely. (within a few ten- thou)
This system decreased in popularity as production tolerances got tighter.

Toyota's system for the Land Cruiser F engine (a dinosaur that lived forever) had 6 separate
bearing classifications....

And you're right- once you leave the world of the standard sized journal, it doesn't mean anything.

t

[Rontgen]

Interesting! I guess that makes sense - as machine tolerances have improved, there's probably less and less need for this. It wouldn't surprise me if this is something of a lost art in today's engine manufacturing world.

I'm sure we are way past the point of seeing revisions to the E30 Bentley, but it would be nice if they would give some insight into this rather than just regurgitating the information overload found in TIS. I think that's where the Bentley manual typically shines - it's able to translate the Genglish and over-complication from TIS into something any idiot like me can understand. Unfortunately I think they sort of fell short on this one.

Oh well, hopefully this thread will help someone out in the future. Thanks to all who responded!