M20 head bolt torque query/discrepancy

Post Thu Jul 21, 2016 8:55 am

Hi all,

I've recently rebuilt my M20B28, and on investigating a coolant leak at the throttle body noticed a lot of water in the inlet manifold, so whipped the rocker cover off and found a load of mayo in the head (as well as on the dipstick).

I'm guessing the HG has gone (disappointing, given it's only been running for 60 minutes at most). The head was pressure tested before being fitted so I'll be truly gutted if that's gone/cracked. (I noticed one of the rockers has lost its roller/clearance thing, so I'm already gutted about that as it means a sump off job to retrieve the debris as I can locate all of the parts in the head except for the retaining nut.)

I've seen conflicting advise regarding the torque for the torx head bolts. Some say 22lb/ft + 90deg + 90deg. Others say 22lb/ft + 90deg + 90deg when warm.

I did the first, as I tightened it all up on a stand when I couldn't run the engine. Could this be the cause of the failure? Should they have been given the final 90deg at running temp? I've seen conflicting advise on this, and don't know which is correct...

Cheers,
Joe

Post Thu Jul 21, 2016 11:08 am

should be fine how you did it does not need to be warm with the Torque to yield torx bolts.

Post Sun Jul 24, 2016 11:57 am

Ok, thanks.

I took the head off yesterday, and the gasket looks fine. I'm doubtful the head has got hot enough to crack as the car hasn't been running continuously for long.

I did notice that some of the head bolts took a lot of effort/cracking noises to remove, whereas some didn't. I'm hoping it's these less tight ones that have caused the water leaking after a few heat cycles...

Post Sun Jul 24, 2016 12:01 pm

Did you use new head bolts. Once there torqued they can't be used again.

Post Sun Jul 24, 2016 12:50 pm

did you clean the bolt holes in the block? If not this could be a cause and the threads are full of shit and tighening the head down could cause a crack in the block. All the bolt holes are surrounded by coolant

An air lock in the head could of casued a crack in the head also. M20s don't do headgasket failts is always a crack 99.9% of the time

Post Sun Jul 24, 2016 2:01 pm

Unless they had specifically been told to test for a water jacket to cam carrier crack, then they would have been unlikely to have picked it up. The standard test is for combustion chamber cracks.
Bring the engine up to temp., whip the cam cover off, and watch the rear two cam bearings carefully. If you see a little ball emerge and scoot across the head deck, then there's your crack.

Post Sun Jul 24, 2016 6:12 pm

Yes, new bolts (torx style). Bolt holes were cleaned out too.

Thanks guys. It was maggspower on here who did the work, so I assume he tested according to the 885 head weaknesses.

Either way, I've found the cause of the problem, and it's exactly what I was hoping it wasn't!

I noticed there was coolant/oil in the throttle body a couple of days ago, so took the rocker cover off to find this:

(Part failure, or badly fitted, I don't know.)

I was extremely hopeful that the mayo and the broken rocker were coincidental. (Hope springs and all that!)

It seems not!

This definitely was not there when I built it, so I'm sure it's a result of the rocker failure and not an oversight by maggs/Byron.

Something (rocker cam, nut, or bolt - I'm guessing the cam as that's shattered) seems to have been crushed between the cam and the head, causing a nice big crack... I'll strip it down and inspect the damage - hopefully it's repairable, but I don't really know what I'm looking for! (I can bolt things together, but something like this is beyond me at the moment.

Is there much leeway on what damage these can take before they're no longer repairable? I know that photo is hard to make out (it's just as hard in person with all the valve gear in the way), but are they scrap once there's a 1mm crack, or can they come back from a few cm worth of welding?

Talk about gutting after having just rebuilt it all

If this head is dead, guess it's time to strip down my other one (also suffered a fault of some sort) and see what state that's in.

Post Sun Jul 24, 2016 6:28 pm

I'm guessing it's this bit just under the journal, and above what I assume is a water channel that's broken? If so, I guess that can be welded without too much of a problem? It's not like it's in the airflow channel or anything?

Post Mon Jul 25, 2016 4:03 am

welding the cam tunnel area only provides a limited service life and its not worth the hassle you will end up getting a crack in the HAZ somewhere down the track. the alloy is difficult to weld and its hard to get enough penetration and not introduce further micro flaws from the welding process. then you have to have a good enough fixture so that the cam bores and shafts are inline after you distort it from the welding process. might need to stress relieve and all sorts of things that are just not worth it. the only thing worse than cast aluminium is welded cast aluminium.

you'll come across people who say they can weld it but they just dont last so id source an alternate

Post Mon Jul 25, 2016 9:02 am

Thanks - appreciate the information (but it's not what I wanted to hear!

).

I'll strip my other head down and see what state that's in I guess. Hopefully they're both not scrap - if so, the hunt for another begins!

Post Mon Jul 25, 2016 9:36 am

Really sorry to hear of your woes

Its largely irrelevant now, but the head was 100 percent crack free, its fairly obvious that something has gone under the cam lobe and mashed it self into the cam valley, I have seen countless broken rockers but this is very unlucky. There has been some serious force to break the eccentric.

As said above, its possible for it to be welded, but the better option by far is another head.

I may have a head if you are stuck.

You need to find ALL the bits of that rocker, or your bottom end is at risk too.

Post Mon Jul 25, 2016 1:13 pm

Thanks - I'll see if I can get my old head tested off the original engine, but if not, if you've got one, that'd be great, esp. if it's all tested and ready to go. To be clear, I'm in no way blaming you for this - it's entirely my fault/part failure (just so people searching/reading don't get a bad impression of your work

).

I know - I've got this week, and next, off work, luckily, so the sump will be removed tomorrow so I can clean it all out properly. The entire nut is in there somewhere, so there's no way I'm willing to leave that rolling around in there waiting to get launched into something! Luckily the washer, the bolt, and most of the rest of the eccentric was in the head. There's a few bits of swarf I need to find/remove/air blast out, but largely it's all accounted for.

Post Tue Jul 26, 2016 1:44 pm

And for completeness, the extent of the damage. Quite some going! Chipped the rocker itself too on the valve...

Must have had some force behind it!

Post Tue Jul 26, 2016 1:52 pm

And the, thankfully, minimal damage to the camshaft. Should be fine to reuse (in my non-expert opinion)

Both photos are the same lobe - one to try and show the surface damage, the other to highlight the minimal depth of damage

Post Tue Jul 26, 2016 2:41 pm

I would not reuse that camshaft.

The damage will soon wear out the rocker and it may well be bent.

Ben

Post Tue Jul 26, 2016 11:55 pm

That cam is toast. If you take it to a cam grinder they might be able to regrind it?

Any idea about what happened? What let go first in the chain of events ?

Post Wed Jul 27, 2016 10:37 am

I'm going to see a friend later to pick up a valve spring compressor later, and on his site is a motorsport prep firm, so I'll ask them about the cam. I don't know if that's the sort of thing that they can do though. They do specialise in E30s though, apparently.

Why would a regrind fix it, but just filing the minute imperfections flat won't? The first photo overplays the damage - the second is a more accurate representation. (This is all new to me, so excuse my ignorance!)

No, not yet. When I drop the sump, if I find the retaining nut in one piece, then I'd guess it just worked its way loose leading to it all falling apart. I'm certain it was tight though, so I do find that surprising (although not impossible). If it's in pieces after being smashed then I'll be none the wiser really. I'd love to be able to prove that it was the eccentric that failed and that somehow lead to all of this. If I could get some sort of £££ back out of Febi for damages caused (given the rocker was brand new), that would be very useful right now!

Post Thu Jul 28, 2016 9:44 pm

Got a new head on eBay. It's a 2001 casting, and was used for a few hundred miles before the car was written off, so fingers crossed. The old head is with someone who builds race e30s and is in the same estate as the friend from whom I was going to borrow a valve spring compressor (there's was too small), and the new head is going there, so hopefully that'll be sorted ok. (swapping valves over)

In good news, the sump came off easily and what's this nestling there? Rejoice! It's the complete nut. Phew.

No need to go hunting for bits of nut, just a few bits of swarf from the eccentric.

Post Fri Jul 29, 2016 11:55 am

TriggerFish wrote:I'm going to see a friend later to pick up a valve spring compressor later, and on his site is a motorsport prep firm, so I'll ask them about the cam. I don't know if that's the sort of thing that they can do though. They do specialise in E30s though, apparently.

Why would a regrind fix it, but just filing the minute imperfections flat won't? The first photo overplays the damage - the second is a more accurate representation. (This is all new to me, so excuse my ignorance!)

No, not yet. When I drop the sump, if I find the retaining nut in one piece, then I'd guess it just worked its way loose leading to it all falling apart. I'm certain it was tight though, so I do find that surprising (although not impossible). If it's in pieces after being smashed then I'll be none the wiser really. I'd love to be able to prove that it was the eccentric that failed and that somehow lead to all of this. If I could get some sort of £££ back out of Febi for damages caused (given the rocker was brand new), that would be very useful right now!

the lobe is a complex profile, if you dont use specialist equipment you are likely to cause issues