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Setting TDC in the M42

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    Setting TDC in the M42

    Hi friends, I'm currently doing the head gasket on my M42. I'm a bit confused about the proper way to set TDC. Everywhere I've searched (and in the Bentley, too), it says to use the flywheel locking pin. That's great, except my engine is on a stand without the flywheel mounted, so I can't use this pin. I know that the woodruff key at the front should be pointed straight up, but I'm not convinced that this visual test will be accurate (without another tool that I don't have). And how do I ensure that the distance between the cam sprockets and the crank sprocket isn't off by a tooth or two when I do this?

    Alternatively, should I take the engine off the stand and mount the flywheel before setting TDC? I'm not sure how this would work either, the Pelican guide doesn't say anything about aligning the flywheel to the crank (only the clutch and pressure plate to the flywheel), and the bolt pattern looks symmetric.

    Not really sure how to proceed, I'm in a bit over my head. I'd appreciate any advice.



    Side note, this engine has almost 270k miles and I still see hone marks on the cylinder walls. No other evidence that it was ever fully rebuilt (though turns out someone swapped an M44 timing case at some point), but I thought that was kinda neat.

    #2
    You can use the arrow on the oil filter housing to the dot/hash on one of the crank trigger wheel teeth for crank agreement. This should get you almost spot on. You can double check this once you have the FW back on, and worst case you'd have to remove cam gears to get everything centered. Also, just aim for close to centered in the cam "windows" until you reference with the locking pin. Cams should point up and towards each other at cyl 1, flats at rear should be square with each other. E marked cam goes on the intake side in case you didn't mark it.

    FW can only go on one way IIRC, pretty sure there is a pin or two that make that happen.

    Not sure if you took the front timing case off or not, but just be sure the chain doesn't bunch, and that you only rotate clockwise during your timing setting.

    It's a little bit fiddly, but really easy enough.

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      #3
      Originally posted by roguetoaster View Post
      You can use the arrow on the oil filter housing to the dot/hash on one of the crank trigger wheel teeth for crank agreement. This should get you almost spot on. You can double check this once you have the FW back on, and worst case you'd have to remove cam gears to get everything centered. Also, just aim for close to centered in the cam "windows" until you reference with the locking pin. Cams should point up and towards each other at cyl 1, flats at rear should be square with each other. E marked cam goes on the intake side in case you didn't mark it.

      FW can only go on one way IIRC, pretty sure there is a pin or two that make that happen.

      Not sure if you took the front timing case off or not, but just be sure the chain doesn't bunch, and that you only rotate clockwise during your timing setting.

      It's a little bit fiddly, but really easy enough.
      Got it, so the arrow can align with any of the trigger wheel teeth. I guess I can visually set TDC since it's only off by a degree or two probably and then turn the crank to get a tooth aligned. I was able to get the head off without removing the cams, but I noticed that on the square heads at the rear, only the vertical surfaces were machined. Is it close enough to just clamp a bar across the top surfaces, or is that unmachined surface going to be an issue? I was planning on cutting a small block of wood to fit in between the two to keep the vertical edges parallel, but clamping something across the top would be easier.

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        #4
        The arrow on the flywheel aligns with one and only one tooth on the trigger wheel. If you look closely, there is one tooth with a little notch in it, and that is the one.

        Similarly, the cams can be put into rough position by having the lobes for cylinder 1 point up and toward each other, and the precise position can be set by making the flat/square rear ends parallel. I believe that the faces which should be parallel at TDC also have some small holes pecked into them. I used a flat block of metal to lay across the ends and check for parallelism, and a wrench on one of the hexagonal parts in the middle of the cam to adjust it.

        It does not take much timing error to get pistons to hit valves. Get TDC set as best you can with the tensioner piston installed and released (fully extended). Turn the engine through a few full rotations using a socket on the crank bolt and re-check the timing.

        Comment


          #5
          Originally posted by sedrat View Post

          Got it, so the arrow can align with any of the trigger wheel teeth. I guess I can visually set TDC since it's only off by a degree or two probably and then turn the crank to get a tooth aligned. I was able to get the head off without removing the cams, but I noticed that on the square heads at the rear, only the vertical surfaces were machined. Is it close enough to just clamp a bar across the top surfaces, or is that unmachined surface going to be an issue? I was planning on cutting a small block of wood to fit in between the two to keep the vertical edges parallel, but clamping something across the top would be easier.

          Originally posted by roguetoaster View Post
          You can use the arrow on the oil filter housing to the dot/hash on one of the crank trigger wheel teeth for crank agreement
          The cams cannot be reliably locked with wood. Two adjustable wrenches will work, and then you may be able to clamp those together, but it's hard to also check flatness at the same time.

          Generally on the cam flats I'll get one to visually flat, bolt down the cam gear, adjust the other with a flat reference material, then I'll make small adjustments to one or both as needed to hit a flat agreement.

          Comment


            #6
            I see the mark on the trigger wheel tooth now, thanks for clearing that up. Wasn't able to get the head back on today, but I think I have a good idea of what to do now to avoid the pistons hitting the valves. Really appreciate the help guys!

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