Oh POO! My head gasket just went pop! 56k GTFO, no chance!

[davebmw]

Sorry if forgot the setting was 1MB instead of the customary 1024 x 768.

As for the head and block, surprisingly there is no sign of erosion, it looks like the gasket has just been pushed aside and the hot combustion gases have sneaked past on the block side of the gasket.

The pistons, Block and head look to be un-touched by it all. the spark plugs had a nice normal tanned colour no salt and peppering, 3 and 5 looked a little wet as you would expect.
When I had these pistons made i specified extra wide ring lands as used on boosted motors from memory there is about 9mm in between each ring groove.
Compare that to the Stock BMW/Mahle pistons that have 3 or 4mm, they really are a robust bit of kit.
I have the old Stock and a spare set of JE forged pistons in the attic I'll post some snaps tomorrow of the differences, its like chalk and cheese!

The only way i know of to check and be 100% sure is to strip the bottom end which means engine out job!
I'm hoping that will not be required as the compression test gave 200PSI on all cylinders before the head removal.
when shes rebuilt and the gasket bedded in I'll do a proper hot leak-down test just to cover that base too.

Is there anything you guys can think of to test the integrity of the bottom end without a full strip-down??

[jharvey]

Hmmm, I don't think there is a feasible way to hold some pressure in the oil cavity is there? That way you could do a leak up test with the heads removed. Also Probably best to keep it under say 50 psi or equiv bar. Don't want to blow up the oil pan like a balloon.

[davebmw]

That sounds a little tricky, there are several oil squirters for the timing chains, 1 for each piston, the dipstick, and the large opening that the chains occupy,
the oil-ways dropping back down into the sump and the high pressure ones for the head, cams, hydraulic lifters and the vanos actuator, not to mention the breathers.
I'm not quite sure if its feasible on this engine, or whether its a good idea,
putting positive pressure in the crankcase could blow oil out of the seals that are designed for a slight vacuum.
this could result in an oil coated clutch and damaged crank shaft seals.

I think the only practical way to do this is get the new head gasket on, get the whole thing up to operating temp and do a leak down test with an airline.

After the last rebuild and after about 3000 miles i did a leak down at 140psi and after1 minute the pressure dropped buy 10 psi across them all give or take 3psi.
So i have some figures to go by this time.

I am a little apprehensive about it this time though as the last time, I rebuilt the engine in its entirety in my dads workshop under controlled and damn near surgically
clean conditions, this time its only the head gasket but its outdoors and a lot less controlled and a lot more awkward to locate the head onto the block
without scratching the mating face.

[Fred]

When I had these pistons made i specified extra wide ring lands as used on boosted motors from memory there is about 9mm in between each ring groove.

I'd say you would be ok :-)

I'm hoping that will not be required as the compression test gave 200PSI on all cylinders before the head removal.

What is that, 9.5:1 or so? 37 degrees with 200psi cranking pressure hmmmm :-p lol

I doubt you need to check it. No idea what Jared is on about with regards oil pressure. Given that the compression was good and it still ran fine, you are almost certainly in good shape right? :-)

Fred.

[davebmw]

I'd say you would be ok

I F****** hope so!

What is that, 9.5:1 or so? 37 degrees with 200psi cranking pressure hmmmm :-p lol

No its 10.5:1 standard, with 2 thou off the block, and 3 thou off the head, a 0.3mm thicker head gasket, 84.50mm bore (0.5mm oversize) 75mm stroke so it is just a bit over the 10.5:1 with flat top pistons.

I doubt you need to check it. No idea what Jared is on about with regards oil pressure. Given that the compression was good and it still ran fine, you are almost certainly in good shape right?

I think he was on about blowing the air back up and out of the bores, trouble being all of those holes and oil ways would make it very tricky.
Not to mention that the compression rings are designed for pressure from the top down. not the other way, so you wouldn't get a true reading.

I have a comparison of the 2 pistons below:



Guess I was a bit confused about the size of the ring lands, the oil scraper ring is 3 times the thickness and the compression rings are marginally thicker too.

I'm kinda wishing that i fitted the 8.3:1 set now as it would be ready for my supercharger but I'll just have to save that for a spare engine and do a full project build on that one.
I'm a little concerned that all my eggs are in one basket at the moment. I need to get a second car to use as a daily driver so if and when I blow this one up I'm not stuck for transport!


I have had a closer inspection of the head today in the better lighting in my workshop, there are some signs of erosion around the area that the fire ring seats, so to play it safe I'm having the head skimmed again.

See below:

[Fred]

10.5:1 on 37 with pump gas and you reckon you might have had a little knock? lol. Lesson well learned.

Yeah, you can see where the crease in the firing ring has pressed into the head slightly. Definitely wants a shallow skim doesn't it. The little pin holes are interesting too. I think what you need are some carnage pics of someone elses to make you feel better.



There ya go :-)

Fred.

[davebmw]

ooooooh! fuck!! I bet that made some really interesting metallic knocking noises! I got off really lucky!!

[davebmw]

OK I had a day off work today and I have taken the head up to the machine shop. The engineer has taken a look and said it will require a skim.
Quite fortunately all I had to do is take the cams out to close all the valves and he'll do a very fine skim to clear off the erosion.
He is also getting me new head gasket and bolts should be ready tomorrow afternoon so I should be ready to start putting it all back together in the evening if the weather holds out.

It will give me a chance to clean out the hydraulic lifter valves again though

[davebmw]

Update:

I have got the head back with a new set of bolts. Unfortunately the dumbass motor factors conglomerate of dickheadery has provided the wrong head gasket.
I ordered one for a M50B25TU 84.50mm (0.5mm oversize) and 0.3mm thicker.

They sent an oversized one for a 2 litre!

so anyway i have flushed and blow dried the water and oil ways, popped, cleaned and primed the hydraulic actuators refitted into the bearing ledges and refitted to the head with the cams.

Now I wait for the gasket before I can go any further, just hope the weather holds out. I just need 6 hours clear weather to be able to put it all back together again is that too much to ask??

[Fred]

In the UK? Maybe :-p Actually, the last few days have been crisp and clear in London at least. Good luck with that!

Fred.