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Info Camshaft Timing

Discussion in 'Tech Tips' started by maelstrom, Jun 6, 2019.

  1. maelstrom

    maelstrom LiteTek Staff Member Premium Member 250cc Vendor Contributing Member

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    I am writing this because I have see too many articles saying "just line up the marks" etc. NO you do not 'just line up the marks'. The other give away is that people use the term Top Dead Centre (TDC), TDC what? If it is not a two stroke then it is either TDC compression or TDC exhaust, choose one. The only reason that you can be so ambiguous about this is because most multi-cylinder bikes use a crank driven ignition rotor and a redundant spark (fires on both compression and exhaust stroke). Otherwise you would have to rotate the crankshaft to the correct TDC (compression) as determined by the ignition firing position for cylinder one.

    For the purposes of this article I am going to assume that it is an inline 4 cylinder engine and cylinder No 1 (left most when sitting on the bike) is the position at which the manufacturer has chosen to use as alignment for the cam timing marks.

    Important:
    When you fit the camshafts to the engine you have to ensure that they are both in the correct position in relation to the crankshaft otherwise you run the risk of bending valves. If you have already moved the crankshaft into the TDC position for Cyl 1 then You MUST NOT rotate the cams to move them to the desired position or you will bend valves.

    Rotate crank to TDC Compression Cyl 1:
    With no camshafts fitted, rotate the crankshaft to the desired position. There will be a mark on your timing rotor or elsewhere for other engines that indicates TDC. You can check by holding a pencil through the spark plug hole against the top of the piston and gently rotate crank and you can 'feel' TDC.

    Fitting Camshafts:
    Now if you hold your two camshafts for Cyl 1 in the TDC Compression position (lobes at 10 and 2 o'clock) see image. The marks on your camchain sprockets/gears should also correspond with the information in your service manual. I added the green lines to highlight the rotation angle. If this does not match your service manual then something is seriously wrong and you need to find out what that is before going any further.
    TDC_Compression.jpg
    In many cases of non-crossplane engines the lobes for Cyl 4 would be in the TDC Exhaust position (lobes at 4 and 8 o'clock). This image shows the TDC Exhaust position but it is on Cyl 1. You can see how close the valves are to the piston and this is why you must NEVER ROTATE THE CAMSHAFT with any piston near TDC.
    TDC_Exhaust.jpg
    Okay so now you can fit each cam. They will not just drop in because it has to push some valves open. If you try to use your OEM cap screws to wind the caps down you run the risk of pulling the threads out of the cylinder head. I have used long screws in the past and gently tighten them, evenly of course, replacing them with shorter as needed until the caps are bedded.

    Of course this is just about one kind of engine, but if you understand what you are doing then you do not need any marks at all. You can do it with a degree wheel and a dial gauge.
    The pics come from this video



    Cheers
    Blair
     
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    Last edited: Jun 7, 2019

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