Hungry m20...likes to eat crankshafts?

[ForcedFirebird]

Been wrenching for many years, never seen anything like this before.


...it was about 2013 or '14 and had a client bring in his e30 that had been sitting in the yard for some time. Had leaky valves, but the m20 appeared to be in decent condition, ran ok, good oil pressure, cylinder walls looked good etc, so we rebuilt the head and sent him on his way.


Less than 1k miles later, he brought it in for making a funny noise and the oil light was flickering (verified it had low pressure). Pulled the oil pan and found the rod bearings were in excellent shape, but the mains were worn right down to the copper and the crank had scoring.


We pulled it apart, freshened everything up, installed a new pump, oil relief etc, put it all back together.....and less than 1k miles later, the light was flickering....AGAIN?



He was pretty upset and decided to just sell off all the good stuff from the car and sold the rolling shell (even pulled/sold the head).


Fast forward to this past summer. Frank (helps me at the shop) came across the car and bought it from the new owner. He had a lower mileage m20 from an auto car, so we went ahead and swapped it in and tossed the old short block aside.


Today I was cleaning up and decided to break the short block to salvage what I could from it, oil pan, brackets and the like.



Well, tuned out the very same thing happened the second time, and snapped some pics.


As you can see, the rod bearings are great - and since the mains are fed first, I don't see how this is even possible!

[kronus]

caps out of order? did the crank get replaced, or did it get its journals cut down?

[ForcedFirebird]

caps out of order? did the crank get replaced, or did it get its journals cut down?

The first time it happened, the engine was never opened (short block). When we replaced the crank, the caps were put in the correct order. BMW was kind to us and cast the cap numbers on them, and have built many baby sixes before/after this and know the cap numbers always face the same side (plus the tangs always match and face passenger side on mains and rod caps).

[kronus]

yeah, I figured, but worth checking :)

[digger]

what do the shell wear patterns look like?

could be non round tunnel, tunnel not aligned, clearances off, unlikely both cranks bent

[ForcedFirebird]

what do the shell wear patterns look like?

could be non round tunnel, tunnel not aligned, clearances off, unlikely both cranks bent

I tossed the block in the scrap bin along with the crank.


The shells looked perfectly worn, copper showing all the way around. Not sure on the original crank, but the second set of bearings that went with the crank was checked for clearance. I chalked up the original mishap to a bad/failing oil pump, flushed the block and replaced those. I did not check them for round, though.



The block could be "bent" but that would be a first for me. Usually when we align bore them, they are straight, whereas upgraded fasteners to egg them, but maybe a couple of tenths of thousandth, no where near enough to eat away like this. We have seriously cooked some short blocks in racing-overheating situations, and they always survived - how do you suppose an engine with mileage would mysteriously warp?


The thing that gets me, is how much material was removed from the crank, and the bearings are just getting to the copper. I have only seen wear like this when a rod bearing set actually spins and lodges in place after an overlap - and usually on the rods, not mains, as the mains don't have the same reciprocating force on them like the rod ends do.

[digger]

if it was lubrication issue then rods would probably be affected to. so it would be some geometry issue. without parts to investigate will probably always be a mystery

https://www.mahle-aftermarket.com/media/local-media-north-america/product-files/ceb-2-1114-engine-bearing-failures-brochure.pdf

suggests maybe warped block 13 or bent crank 14 or surface finish on crank 21. its a bit of a generic document though

[e30davie]

I mean its obvious isn't it?

Its cursed. And only the blood of a 64 virgin m42s will break the curse.

[ForcedFirebird]

I saved the caps and bearing shells. Will try and get a photo and post tomorrow.

They are all fairly evenly worn. The car was sitting for some time and there was light rust on the back of the valve sealing surface during the head rebuild, I do remember that. The original short block came apart 4-5 years ago and have done many since then, some of it blends together. P.17 in your link shows oil starvation, over the years have experienced several of those scenarios, but when young, had a paperback book with pictures of bearings for reference lol. When taking the motor apart today, the oil pump is still new, so is the pressure relief.

Tempted to pull the block back out of the bin to check the main bore alignment, but in my mind, why would a block go 100-something miles, sit in a garage for a few years, then eat main shells?

[ForcedFirebird]

I mean its obvious isn't it?

Its cursed. And only the blood of a 64 virgin m42s will break the curse.

Don't get me started on m42's. They crack from the exhaust to water jacket, just the like the M/S 5x engines do. I have kept all the cracked heads over the years, the m20 885's take the cake, though. Probably have a dozen of them - figuring one day I may need to weld them for cores. :(

[digger]

I saved the caps and bearing shells. Will try and get a photo and post tomorrow.

They are all fairly evenly worn. The car was sitting for some time and there was light rust on the back of the valve sealing surface during the head rebuild, I do remember that. The original short block came apart 4-5 years ago and have done many since then, some of it blends together. P.17 in your link shows oil starvation, over the years have experienced several of those scenarios, but when young, had a paperback book with pictures of bearings for reference lol. When taking the motor apart today, the oil pump is still new, so is the pressure relief.

Tempted to pull the block back out of the bin to check the main bore alignment, but in my mind, why would a block go 100-something miles, sit in a garage for a few years, then eat main shells?

if it sat for a period of time corrosion/moisture perhaps. IMO its worth investigating

[ForcedFirebird]

To recap.

Car drove fine and was in good shape for ~90k miles. Sat in a garage for several years. Came to shop, got new head, ate main bearings in less than 1k miles.

At that time motor was broke down to replace bearings, clearances checked, washed down, reassembled, main bearings failed 1k miles later (less than 30 days). Car Sat for ~4-5 years, came back and got a "new" used motor.

It's not like the motor/car sat long between the first 2 events, but did sit for several years before this break down. The two bearing failures were months apart. Hence the haste in disposing of the block and second crank. Bad voodoo, was quick to throw it away in case it rubbed off on the other m20's.

[zaq123]

To recap.

Car drove fine and was in good shape for ~90k miles. Sat in a garage for several years. Came to shop, got new head, ate main bearings in less than 1k miles.

At that time motor was broke down to replace bearings, clearances checked, washed down, reassembled, main bearings failed 1k miles later (less than 30 days). Car Sat for ~4-5 years, came back and got a "new" used motor.

It's not like the motor/car sat long between the first 2 events, but did sit for several years before this break down. The two bearing failures were months apart. Hence the haste in disposing of the block and second crank. Bad voodoo, was quick to throw it away in case it rubbed off on the other m20's.

was the block ever line bored after its first failure? Who know why it failed the first time? But the bearing failure could very well warp the block enough where align boring would be in order before the second set went it

[AndrewBird]

was the block ever line bored after its first failure? Who know why it failed the first time? But the bearing failure could very well warp the block enough where align boring would be in order before the second set went it

This is what I was thinking. The first time might have been caused by a bad oil pump like you thought, but perhaps the block warped then too and caused the second failure.

[TobyB]

The thing that gets me, is how much material was removed from the crank

yeah, that's insane. I mean, worse than Ford V8 with crack- in- the- block insane.

What's even odder, if your pix are in order, is that it seems to alternate mains.

If the M20 wasn't normally so prone to taking out a rod before the main,
I'd say maybe it's an odd occasional loss of pressure, or crap in the galley, but
given that the rods can look worse than that and the mains look new, that just seems
so unlikely.

Too late now, but I would have gone over that block with the mag checker, and then
cut it into little chunks...
Typically, you can use a BMW block to support a small office building, but maybe something
was wrong out of the mould- I couldn't see overheating doing that, but at this point, most of
our assumptions are wrong, anyway, right?

Whack...

t