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    Need advice from experience for an M42 rebuild

    odometer shows almost 380K and the car just recently overheated and started blowing white smoke. i checked drained the oil to check, as expected it came out milky brown. so i'm guessing it's the head gasket or a cracked/warped head?

    either way with its high mileage, it's prolly best to tear it down and rebuild it? i don't plan on boosting so maybe a stock rebuild is best? i want it to be as reliable as possible.

    i have tried to research as much as i can, so here is where i can use some advice:

    -is this job more like a pull the head out, send it off to get machined, replace all gaskets and button it back up?

    -is it best/worth the the extra effort and money to pull the engine/transmission out as one piece?

    -when i remove the head, do i have to put oil back in before i start since i drained out to find out it was milky?

    -what steps in what order do i need to take to get the head off?

    -does the upper intake manifold need to come off?

    i don't mind putting in the time since it's not a DD and i would love to know this car and engine inside and out. so i'd like to do as much as i can without taking to a mechanic/professional (like resurfacing the head).

    any help is vastly appreciated. thanks in advance!

    #2
    I've replaced the head on my m42. For you my guess is bad head gasket or worse, cracked head.
    Don't crank the engine without oil in it - next time use the dipstick or oil cap to check oil quality; the "milkshake" will appear there too.
    With oil in the engine, do a compression test. This will tell you which cylinder(s) lost the seal.
    Replacing the head sucks, but there are articles - I believe pelican parts has a write-up, which is what I followed. Basically, though..
    - oil and coolant come out
    - fan & shroud off - taking fan clutch off crank pulley requires a $20 tool or a couple small wrenches, a bfwrench and a friend
    - belts & pulleys off
    - plugs & valve cover off
    - water pump, thermostat, timing cover off
    - intake incl manifolds off
    - exhaust manifold off the head
    - cams off - this is tricky because you have to be careful not to mess up timing.

    Things to do while there:
    - clean everything
    - new head gasket (I had 0/2 experience with Victor Reinz)
    - new gaskets and seals all around
    - upgrade to arp head-block fastening in case you want to boost sometime
    - new timing guides

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      #3
      awesome stuff. thanks for the quick response, slowmoe30. do I need to put back oil in and do a compression test still if I am going to remove the head anyways? i can do all this and rebuild, and leave the rest of the engine and transmission still in the car? thanks again, this info is invaluable!

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        #4
        7 times out of 10 on an M42 it's a cracked head between a valve seat and a coolant passage, usually cylinders 2 or 3.

        Either way you're stripping the engine down, so you might as well get started. In my experience a head swap with engine in place isn't a big deal, but if you want to spend the extra and rebuild now is the time. That said, a whole other working M42 to replace yours is probably less expensive than fixing your current engine.

        Order of operations for minimal work: radiator/shroud/fan+clutch out; belts off; entire intake manifold off; coil packs/plug wires off; valve cover off (cover with a clean cloth); exhaust manifold off; set to TDC 1, verify and lock; remove upper timing case/thermostat; disconnect any remaining coolant lines on head; remove chain tensioner; mark and possibly wire cam sprockets/chain together (just as east to not do so); with proper E torx tool the head can now be removed with cams in place.

        Better option is to remove the lower timing case and dampener as well, but some find that impractical.

        Pretty sure that covers it off the top of my head. Then you inspect the head gasket for failures and inspect the valve seats to coolant passages for visible cracks. If you find nothing then your profile gasket probably just shit the bed, but you'd need to pretty much do this whole job to replace it anyway.

        Even in that case you still want to have the head checked/decked, along with the upper timing case. Then you can let your machinist inspect your valvetrain and remove/install the cams/valves for you, even though this is another thing you could DIY if desired.

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          #5
          Originally posted by JonsE30 View Post
          awesome stuff. thanks for the quick response, slowmoe30. do I need to put back oil in and do a compression test still if I am going to remove the head anyways? i can do all this and rebuild, and leave the rest of the engine and transmission still in the car? thanks again, this info is invaluable!
          Nope, you will want to take the head out. Only compression/leak down test if you want to roll the dice that it might be the profile gasket, which can in theory be done without removing the head IIRC.

          You would of course need fresh oil to compression test, but cheap stuff would do just fine.

          You can also rebuild the engine in the car, but it's much better (read easier) to remove it if you want to go all the way.

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            #6
            Originally posted by JonsE30 View Post
            awesome stuff. thanks for the quick response, slowmoe30. do I need to put back oil in and do a compression test still if I am going to remove the head anyways? i can do all this and rebuild, and leave the rest of the engine and transmission still in the car? thanks again, this info is invaluable!
            I did it engine-in-car. About compression test, it's just another way to diagnose; not part of the solution. If diagnosis is complete, on to fixin'

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              #7
              roguetoaster, yeah i immediately expect a cracked head and to get the head re-machined.


              how less expensive will a working m42 cost? maybe i can just swap that in and pull mine out and rebuild it on the side if the extra cost isn't too unreasonable.


              when you say a "whole other working m42" do you mean i just swap heads? or like the entire swap?

              Comment


                #8
                Originally posted by JonsE30 View Post
                roguetoaster, yeah i immediately expect a cracked head and to get the head re-machined.


                how less expensive will a working m42 cost? maybe i can just swap that in and pull mine out and rebuild it on the side if the extra cost isn't too unreasonable.


                when you say a "whole other working m42" do you mean i just swap heads? or like the entire swap?
                Machining the head won't fix a cracked head, you'll need a replacement if your is cracked. In theory it can be welded, but it's not economical.

                A whole engine swap is faster. Yes, you could swap heads, but I'd prefer to keep an intact, working engine as is and not monkey with it. Expect to pay $2-400 for a cylinder head, and $3-600 for an engine depending on you local market.

                If the whole engine needed nothing you'd save a few hundred or more over a head swap depending on how much maintenance you have to do once you tear yours down. However, chances are an engine that was pulled will need a bit of love at least.

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                  #9
                  oh. I didn't know that about the cracked head.
                  so i need to source a non cracked/warped head or a working m42 complete engine which then after I do the rebuild?

                  Comment


                    #10
                    If your head is cracked, yes.

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                      #11
                      gotcha. I'll try to source a whole engine for swap. thanks for the info.


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                        #12
                        If you do want to go the route of a rebuilt M42 its going to cost a ballpark of $2000-4000, depending on how much you do yourself and how far you go.

                        From a cost perspective its cheaper to do a complete swap but food for thought, my engine was 150k miles and needed rings at cylinder 1, had a cracked head, etc.

                        Just because its half the miles doesnt mean its half the wear. Make sure its running and see if you can get a compression test done before the engine is out. I dont know where I sit with the majority, but I feel like most M42 are a time-bomb with their timing rails and that should be at least reviewed with any work.

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                          #13
                          New head = more time, risky because you'll do a lot of the work. This can be managed if you are cautious and thorough.

                          Swap engine = less time, more risky IMO because the engine may be a wild card.

                          Since time isn't an issue for you, I'd recommend to take the head out, in-car. You'll learn a lot about your engine when you do this job. Take the time, be cautious and thorough.

                          Get the head out and get it inspected. Depending on what's wrong with it, best to worst, you'll get a new gasket, get your head machined, or get a new head.

                          Also, because you'll be working on aluminum parts, FOLLOW TORQUE ORDER AND TORQUE SPECS ON ALL FASTENERS.

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                            #14
                            Need advice from experience for an M42 rebuild

                            jaredmac11,

                            so if I go with an m50 swap it would be best that it is still in the car so I can do a compression test and if it's good have them take it out?

                            a lot of the ones I see on Craigslist are already out described as "pulled from wrecked car" that had XXX,000 miles. is there no way to test the health of the engine once it's out? or do I have to take the seller's word for it?



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                            Last edited by JonsE30; 07-23-2018, 08:49 AM.

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                              #15
                              slowmoe30,

                              that's exactly how I feel to about swapping an engine gambling on its health since your spending all that time for it to be a bad engine after it's all buttoned up then I'm back to square one.

                              I just don't see m42 heads for sale as often as m50's. i also don't know how much more complicated it would be to make it street/smog legal. CA has strict laws about this stuff, but I read getting it "BAR'd" is just like a more involved inspection, but I don't know much since I've never done it.


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