Main Bearing Questions

[Earendil]

I'm having trouble locating any information on how one might diagnose main bearings on their last leg. Is there any symptom besides noise? If the oil pan were off, would there be anything visible that would show without disassembling things?

On a related node, say I decided to replace the bearings myself. What else would be prudent to replace at the same time? I can't seem to find a DIY that would provide me with keywords to search with.

Sorry about the ignorant questions. I've rebuild the M20 head 3 times now, but the bottom end I've never touched. As far as I'm concerned it's just magic encased in metal

[95BMWIC]

Wow...

I think you and I are on the same last leg..

I recently discovered copper metal shavings in my oil - I have 220K on the engine, with a newly refurbished head..

I posted some questions in the forum also.

http://www.r3vlimited.com/board/showthread.php?t=362186

For fun (and education) I tore apart an old E30 engine and here is a couple of pics of what I found.. (175K miles)


Note - What you see is the metal wearing down and the copper showing (not a reflection) - This was the main bearings and pic #2 is the thrust bearing-

Funny though I suspected that the rod bearings would be worse - but the main bearings were.

If you let it get to the death rattle - then catastrophic failure may happen.. Why do you think the main bearings are going - the miles?

[jlevie]

The only way to check the crank main journals is to pull the engine and tear it down. Have a shop check the journal dimensions. The journals must be measured to a tenth and the bearing color is picked from that. Since the necessary micrometers are a tad pricey, for a one time deal have shop do the testing.

[dnguyen1963]

The only way to check the crank main journals is to pull the engine and tear it down. Have a shop check the journal dimensions. The journals must be measured to a tenth and the bearing color is picked from that. Since the necessary micrometers are a tad pricey, for a one time deal have shop do the testing.

Haven't seen you on this forum in awhile. Good to see you are back.

[Earendil]

Wow...
If you let it get to the death rattle - then catastrophic failure may happen.. Why do you think the main bearings are going - the miles?

Two things, 1) At over 300K I'm concerned 2) I had a guy ask me at a BMW dyno day "Is that sound your bearings or your valves?". It was another E30 owner, and I had been thinking for a while that my valves were noisier then they should be. I don't suppose anyone around here could diagnose the sound via a bad audio recording? :)

The only way to check the crank main journals is to pull the engine and tear it down. Have a shop check the journal dimensions. The journals must be measured to a tenth and the bearing color is picked from that. Since the necessary micrometers are a tad pricey, for a one time deal have shop do the testing.

jlevie, I never want to question your wisdom, but I'm wondering if you could expound on "The Only Way". What if I just decided to replace them, regardless of their condition? Could I do that with the car on jack stands with the subframe and oil pan pulled? Or do I need the same tools to properly install them, not just verify they are bad?

I have at least 120K that I've put on the engine, and it came with 180k. Multiple odometers have been used, but it's safe to say over 300K miles with 10 or so track days. It's a track car, and I'm planning my winter maintenance. I'd rather not blow an engine on the track, but paying multiple grand to have the bottom end of an M20 refreshed doesn't sound appealing either.

[TobyB]

The rod bearings are usually stressed harder than the mains. If the engine's had
good oil all it's life, the mains seem to be pretty damned indestructable.

I agree mostly with Jim- there really isn't an easy way. You can add (or subtract) evidence
with oil pressure (more is better, but less may be just the pump) and you can try
rocking the crank around tdc for each piston (minimal dwell is good)
but at the end of the day, lots of things can mask bad shells...

It's a big job, but if you don't rush it and clean it and clean it and clean it and
clean it some more, and work carefully, it's not hard- it's just a precise puzzle.
And once you do one, you'll know how. It's not hard.

t

[95BMWIC]

jlevie, I never want to question your wisdom, but I'm wondering if you could expound on "The Only Way". What if I just decided to replace them, regardless of their condition? Could I do that with the car on jack stands with the subframe and oil pan pulled? Or do I need the same tools to properly install them, not just verify they are bad?

I have at least 120K that I've put on the engine, and it came with 180k. Multiple odometers have been used, but it's safe to say over 300K miles with 10 or so track days. It's a track car, and I'm planning my winter maintenance. I'd rather not blow an engine on the track, but paying multiple grand to have the bottom end of an M20 refreshed doesn't sound appealing either.

Jim's answer was prolly directed a little towards my previous posting (and yours) - since he is talking about the main crank journals.

I know you can replace the rod bearings this way - (dropping the subframe) BUT Like Jim was stating - you need to check and measure the main rod journals to know what rod bearings to use.

HOWEVER - You cannot replace the main bearings without removal of the crankshaft (both front and rear seals need to come off to do this). And if you are replacing the rod bearings why not do both? When I pulled the 325e crank - the main bearings were worn more so than the rod bearings..

As TobyB stated I too always heard the rod bearings were the first to go - but since the bottom end has never been done - it might be time for main bearings, seals, pistons and piston rings as well...

http://www.pelicanparts.com/BMW/techarticles/101-Projects-24-Rod-Bearings/101-Projects-24-Rod-Bearings.htm

I'm in the same boat..

[Earendil]

Thanks for the tips, Toby. For this part in particular:

It's a big job, but if you don't rush it and clean it and clean it and clean it and
clean it some more, and work carefully, it's not hard- it's just a precise puzzle.
And once you do one, you'll know how. It's not hard.

t

Could you compare it to pulling a head and completely rebuilding it? I assume it's more, but maybe a technical and time approximation? Yes, I know it'll vary, I'm just looking for a ballpark.

I suppose the downside to the block vs the head is that, worst case, you take the head to someone who knows wtf they are doing. You can't do that so easily with the block...

[TobyB]

I'd say if you didn't find the valve work to be too daunting, the block will be no harder.

Different, and take your time, especially in torquing the rod bolts (I use a sharpie to mark
them as I do the angle thing) and especially with the cleaning.
Timewise, he bottom end doesn't take too long- after cleaning, you can be leisurely and
careful with it and only spend a couple of hours. The cleaning will take a couple of hours
and some long, small brushes.

And yes, you certainly can walk into a machinist with a block and crank and rods and stuff.
But you won't need to.

t

[jlevie]

I'd tear down the block and take it and the crank to a good machine shop. Let them clean it, but not with a hot tank as that will destroy the intermediate shaft bearings. Then have them measure the crank journals and cylinders for wear.

At 300k, the cylinders are almost certainly badly worn and need an overbore and new pistons. The alone will run close to $1000, in which case a lower mileage used engine will be more more economical.