So what is it about that damn cylender 3 anyway?

[Ferret]

Is there an official M50B25 cylender 3 conspiracy thread somewhere?

I've had a hard hunt to find an un-cracked replacement 2.5 M50 cylender head - every scrap yard I've gone to has tried to sell me a head with a crack in cylender 3...

I've finally located one without a crack but it's a strange one with casting marks I've never seen before... which reminds me I need to post a photo of the one I dont consider to be strange.

Now I need to decide whether to pull piston 3 and replace its rings - I have no idea of the condition of them. The 2.0 head that's on there currently is so buggered it's clanking at tickover but not under power.

While pulling away the cars not smoking much, odd bit of blue smoke, but I think the valve guides have gone on cyl 3 as it's oiling up spark plugs and misfiring at idle.

Trouble is I gots to lift the front of the car to do the pistons with the block still in and I've no garage. Thinking about it I'd have to put it on ramps before I took the head off and that's gonna make getting the head out a a right biatch of a job...

Hmm... to risk it or not to risk (changing the rings), that is the question.

[DaveVoorhis]

Same problem finding M20 heads -- they tend to be cracked through the cam journals. I suspect it's because good engines get sold intact; bad engines get parted out. BMWs tend not to wind up at the breakers unless the engine is sacked, and the usual failure mode is an overheat bad enough to crack the head.

[E34-520iSE]

That's why I'm stockpiling good M20 heads, to keep my beastie going and going!

[E34-520iSE]

Now I need to decide whether to pull piston 3 and replace its rings - I have no idea of the condition of them. The 2.0 head that's on there currently is so buggered it's clanking at tickover but not under power.


Clanking? My M20B20 did that clanking thing 4 yrs ago. It was excessive wear in the big end journal of piston 6. One complete crank regrind & oversize bearings later and the engine sounded sweet, and still does!

HTH,

Shaun M

[Ferret]

Clanking? My M20B20 did that clanking thing 4 yrs ago. It was excessive wear in the big end journal of piston 6. One complete crank regrind & oversize bearings later and the engine sounded sweet, and still does!

HTH,

Shaun M

It's a strange thing though, it comes and it goes - if you put a can of oil rejuvinator in there it shuts up pretty quickly... I've done the hose to ear trick and the noise is definitely coming from the head...

I knew when I put the 2.0 head on the 2.5 engine it wasnt gonna last very long - but I think the head was shot from the word go.

[Hallmark]

They're called Wells Racing (they've got a website) in Duncanville, Texas. I took the head from my 525 in and he showed me cracks in cyl 3 and 4. They grind out the crack, weld it up and then recut the valve seats. They also straighten and pressure test the head. The guy who does the machining is David and he says that a number of independent BMW shops send heads to him for valve jobs and somewhere on the order of 35-50% of them are cracked at cyl 3.
David tells me that after they weld it up they never crack again.

[E34-520iSE]

Some cracks can't really be repaired.If you imagine the cracked area being like an eggshell, you start cleaning out loose metal but the crack just runs away from you, and it never ends!

HTH,

Shaun M

[rob101]

Some cracks can't really be repaired.If you imagine the cracked area being like an eggshell, you start cleaning out loose metal but the crack just runs away from you, and it never ends!

HTH,

Shaun M
So long as the crack is ground out to its root, then it will not be "running away from you". that said, the welded bit won't have the same strength as the homogenous metal that it replaced though. depends on the quality of the weld, if the edges of the weld don't fuse to the metal properly, you can expect it to crack in the same place again.

[Ferret]

I'm also wondering whether it's worth breaking down the two step head bolt torque-up into four steps, IE 4 x 45 or maybe 3 x 60 degrees rather than 2 x 90... It's just strange that they always seem to crack in the same place...

Looking at the components, I'm wondering if bmw buggered up their fluid dynamics where the out or inflow is halfway along the left side of the head...

[DaveVoorhis]

I doubt the cracks are caused by the torque staging. I suspect they have one and only one cause -- overheating.

[Ferret]

I doubt the cracks are caused by the torque staging. I suspect they have one and only one cause -- overheating.

Hmm, you're probably right, but what if it is the torque staging, interesting that the cracks happen on cyl 3/4 when the two bolts between are the first two to get torqued up...

Maybe the area is overstressed because of it?

http://i11.photobucket.com/albums/a190/Ferret101/IMG_6159-1.jpg

[Hallmark]

Admittedly, the welder has got to know his business, but the welded area should be stronger than what it replaced. The alloy in the welding rod is of a higher quality than the aluminum in the head. One of the steps is to pressure test after the weld is done to assure the integrity of the weld.

[E34-520iSE]

Hmm, you're probably right, but what if it is the torque staging, interesting that the cracks happen on cyl 3/4 when the two bolts between are the first two to get torqued up...

Maybe the area is overstressed because of it?


Quite a good point you've made there Ferret, maybe splitting the tightening into 2x 45 deg. steps could be worth a try. Likewise, some cases of cracking may be due to not having the head skimmed and/or bits of dirt, old gasket etc, not being cleaned away.

Cheers,

Shaun M

[DaveVoorhis]

Hmm, you're probably right, but what if it is the torque staging, interesting that the cracks happen on cyl 3/4 when the two bolts between are the first two to get torqued up...

Maybe the area is overstressed because of it?
Could be, but once the head is torqued down the stress should be spread evenly across all the bolts. However, I'd suspect the area between 3 and 4 is both the hottest part of the head and the point where an overheated head is trying the most to bend away from the block. If it is overly stressed, there should be cases where a known-good, properly skimmed, properly installed head cracks without being overheated.

[tangles01]

So long as the crack is ground out to its root, then it will not be "running away from you". that said, the welded bit won't have the same strength as the homogenous metal that it replaced though. depends on the quality of the weld, if the edges of the weld don't fuse to the metal properly, you can expect it to crack in the same place again.

I'm a boilermaker and well weld metal is if done correctly and with the right elecrode selctoin up to 50% stronger then any parent metal, also the cracking at 3 probably has something to do with it's position around halfway down the head, whings always crack at the point of greatest strees so the must me the greatest strees in the middle area of the head, that means 4 should go readily as well but may be a common machining fauls like less than a though but et's enought with large forces over a long period