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M42 Head Gasket Blown, Project Thread

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    This is the borescope I used. $200 isn't cheap, but it's super handy to be able to aim it. Professional ones are a LOT more expensive. Also, the little diagrams were made by me...the borescope does not come with software to do that.

    I will be stripping the head (maybe the whole engine) over the next week or so, and I will get a look at the ports then. Fingers are crossed that there are no cracks!

    I'll get a look at the injector wiring to see if anything is amiss there. It's unlikely, but worth looking. Earlier in the year I was doing a lot of data logging to get the tune dialed in, and I had EGT probes for each cylinder. As far as I could tell, they were all within 5% of one another. I am having Sssquid clean and flow match the injectors anyway, though.
    Last edited by bmwman91; 11-06-2019, 10:16 PM.


      Almost looks like its burning oil
      89 E30 325is Lachs Silber - currently M20B31, M20B33 in the works, stroked to the hilt...

      new build thread


        Yeah, it is a bit oily in there. I assume that some of it is from cranking it a bunch running the initial compression test. Fingers are crossed that it is just seepage from the badly compromised head gasket. I know for a fact that it was running pretty rich for a while during initial tuning work (Sssquid and I were doing it remotely), so the carbon buildup on the pistons is not unexpected. However, the other less-ideal culprits for oily residue are:

        Bad rings
        The engine has never overheated or had a lubrication issue. Rings were initially seated per MM's recommended procedure with conventional oil for 50 miles, then 500 miles, after which it has all been Mobil1 15W-50 full synthetic. When I pull the head and look at the cylinder walls, can those alone indicate excessive ring wear? Obviously, if there are vertical striations then I'd see that as a strong indication. But if the honing marks are still well intact, does that make ring wear less likely? None of walls showed visible vertical wear on the borescope. Back when the second valve spring went (before MM replaced all of the light single springs with a more OEM double spring setup), one intake valve in cylinder 3 did make contact with the piston, making a small mark on the edge of the relief which I smoothed out. It is unknown if this distorted the piston in any other way.

        I assume that the only way that ring sealing can be evaluated is to put it all back together with a new HG and run compression + leak-down tests? The other option is to just pull the bottom end apart, re-hone and replace rings just to be safe. Swapping in a new HG only to find that compression / leak-down is no good would be frustrating. Thoughts?

        Bad valve guides
        Again, lubrication has always been maintained, so I'd assume that these would be OK unless they were all honed improperly from the start. All of the intake valves seem to look wet / oily, which is either seepage from the HG, bad valves, or maybe excessive compression leaking into the crankcase and forcing a lot of oil into the intake. I'll inspect the intake manifolds and ports once the manifolds are off.

        Since the head is coming apart no matter what when I get it resurfaced, so the shop should be able to check guide clearances. A pressure test to detect cracks and valve sealing issues is also part of the plan.

        The car has been consuming around 1qt of oil per 2000 miles. Some of that has been due to leaks from the timing cover / profile gasket butt-joint. Also, the amount of oil leaking from the timing case was always a bit surprising, so I am thinking that a long-term HG leak has been pressurizing the crankcase. It could also explain why I could periodically smell a PCV leak while it was running, despite never being able to find a leak and replacing all of the hoses and gaskets.

        I assume that the rest of the oil consumption has been from excessive blow-by because the car has mostly been driven for short around-town type stuff and short joy-rides, so the warm-up duty cycle is high. I only ever once took it on a long drive, where it achieved 26.5MPG with me deliberately trying to get the best fuel economy possible (a few hundred miles on the highway late at night locked at 65MPH).

        Still, if this has all been wishful thinking on my part and the oil consumption is way off, then I guess there's a lot more work to do.
        Last edited by bmwman91; 11-07-2019, 11:55 AM.


          [2013 content]

          Here are some photos from back in 2013
          when I was putting the head back on after MM repaired it from the valve spring failure.

          Upper combustion chamber bits.

          Piston #3 before I smoothed out the raised lip. Clearly there was also contact with the larger bottom surface of the valve. It is unknown whether this distorted the rest of the piston since, at the time, I was ready to roll the thing off of a cliff rather than deal with shipping the engine back to MM for a bottom-end inspection.

          The valve that made contact.

          The block deck when I was partially done cleaning it.

          M44 +0.3mm HG in place with whatever spray coating MM had applied to it. I assume they went with +0.3 since they had milled the head 3 times by this point (once for the initial build, again when the first valve spring went, and a third time when the second valve spring went and they rebuilt the whole top end for me).

          [/2013 content]

          I am strongly considering a Cometic or Athena MLS HG for this next rebuild. I need to call up Cometic for some input, particularly about surface prep for their product. I have read that MLS gaskets require much more meticulous surface prep than OEM composite ones, so I may need to get it machined in order to properly use a MLS type. This isn't a highly stressed forced induction or race engine, so another M44 HG would probably be fine since I suspect that my issues are all due to 2013 being my first HG replacement experience and not properly preparing the block. But I'd feel better at this point with a MLS one, especially since I may need to go a few thousands thicker if the head and/or block are machined again.


            What bore size are you running, 86 or 87 and are you use a stock head gasket?

            the reason for asking is on the m20 the stock head gasket approx 85.5 mm ID overhangs the bore 86 mm (MM pistons are chamfered to clear) so the fire ring is exposed to more heat among other things which is not ideal. now MM use 87 mm bores but a different piston (flat style) but not sure what gasket they use. i imagine with M42 the gasket also overhangs the bore

            I also agree a MLS is probably worth considering for the following reasons:
            - for the thinner gap between bores the MLS is stronger and can tolerate a less than ideal tune better, most OE use them these days so they are pretty good certainly need to be more meticulous though.
            - optimum piston to head clearance, you want the piston to head clearance (approx 1 mm) to generate good squish which if the gasket overhangs the bore is probably not happening. With a cometic MLS there are about 10 thicknesses you can get to set it up correctly with the piston close to the head but need to check P to V. looking at the pistons you should not have an issue though as pockets look deep enough for a hot street cam with 2-2.5mm lift at TDC.

            You can visually see if the bores are totally stuffed but not much more. Having a cross hatch is one thing but the characteristics of it and how much is left are in the microscopic scale. With the piston down the hole you could use a bore gauge to check but you’d want to know what it was at the start otherwise you might not learn much.

            You can do a leak down after the head gasket goes on before assembling the cam and whole thing to evaluate the rings
            89 E30 325is Lachs Silber - currently M20B31, M20B33 in the works, stroked to the hilt...

            new build thread


              Bores are 87mm. The head gasket is an OEM M44 +0.3mm thickness one, which I believe has ~85mm holes.

              Given some of the obvious abuse that occurred, I am heavily leaning towards a complete tear-down to make sure that everything in the bottom-end is in proper shape. I am going to need to locate a competent local builder, or maybe see what it would cost me to work with MM and spend a week out there observing and learning. If I am going to go with Cometic, I'll probably need to get the block into a shop one way or another to properly check the surface and (possibly) machine it. At that point I am probably taking the bottom end apart anyway.


                honestly pull the head and make an assessment before going "full bore" with rebuild. you can pull the pan off and rods out in the car if needed
                89 E30 325is Lachs Silber - currently M20B31, M20B33 in the works, stroked to the hilt...

                new build thread


                  So Jim from MM called me earlier. I guess it bugged him that the engine is out on its ass again. Despite repeated mechanical issues with the expensive product I purchased, I have maintained a good working relationship with them. I expressed to Jim that I don't really feel like I ever ended up with the product I paid for, even if the last HG install involved my own errors, and MM would like to try to make things right for good.

                  The short story is that I am pulling the engine and shipping it out to them. They'll tear it down, and I'll make the call about what to do or not from there. They aren't doing it for free, but it is at a reduced cost that I think is fair. This is somewhat of a "lazy" solution, but I just don't have sufficient time and energy to deal with this correctly myself right now. If MM was not willing to work out a deal then I might feel more motivated since I dislike the idea of throwing money at things, but they seem willing to stand behind their product.

                  Also, outsourcing the work gives me time to look into the other stuff that we have been discussing, such as an RHD flywheel, improved header and cams.


                    you know they will try and sell you stuff and still and convince you not to go down other routes lol
                    89 E30 325is Lachs Silber - currently M20B31, M20B33 in the works, stroked to the hilt...

                    new build thread


                      Of course, avoiding the up-sell is part of much of life. I just want the product that I expected in the first place, and that's what was agreed to. In terms of all the other non-MM improvements, I won't bother mentioning that stuff to them. While they do want to stand behind their product, they don't need any encouragement to try to sell their version of that stuff lol. I'll do my best to avoid a "you told me so" outcome, and this wasn't really in my budget planning anyway, which I also expressed to MM to make it clear that I am not in a position to blindly toss money at the situation.


                        yeah new gaskets, seals, bearings and a re-ring job if pistons are ok. fingers crossed the valve contact pinched a ring and you get a freebie set of
                        89 E30 325is Lachs Silber - currently M20B31, M20B33 in the works, stroked to the hilt...

                        new build thread


                          lol, I guess pistons decorating a wall are a step up from hubcaps.


                            bmwman91, what is your compression ratio?

                            BTW, this is the m42 content that I come to R3V for. Sub'd
                            Last edited by Victell; 11-08-2019, 04:08 PM. Reason: reasons


                              The builder specifies the static compression ratio at 11.5:1.

                              I'm happy to provide pictures of bad head