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Thread: finding TDC with a 336 cam

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  1. #1
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    finding TDC with a 336 cam

    I just want to start out by saying I have been a lurker for about 8 months now and have found really all the info I needed with out having to ask. But now ran into a possible issue. I have a R75/6 frame with a R90/6 engine and some previous owner installed a 336 cam in the late 90's. The bike sat for 2 years cause the guy I bought it from wanted to cafe it out and cut too many wires and couldnt figure out how to make it right again.
    I am trying to check valve clearance but I am unsure if I can still use the marking (OT) on the fly wheel to find TDC. From my reading the 336 cam prolongs the compression stroke so will that also change how to find TDC. Also while I am thinking of it, is the timing affected by the cam also. If so what do you all recommend I set the timing to.

    Thanks ahead of time
    Last edited by Mortie; 11-13-2012 at 10:31 PM.

  2. #2
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    Mortie -

    Welcome! Finding TDC should be pretty easy. With the spark plugs out, rotate the engine with something like a straw poked into the hole. Find the point at which the straw stops moving out. There will be a range where the straw doesn't move at all while the engine continues to turn. The exact TDC will be midway between when the straw stops moving out and begins moving in. They make devices that screw into the spark plug hole to do the precise measurements.

    There are of course two TDCs, one on the compression stroke and one on the exhaust stroke. You set the valves on the TDC on compression. While turning the engine, watch the valve/rocker action. When you see the intake valve open then close, you're heading to TDC on compression.
    Kurt -- Forum Administrator ---> Resources and Links Thread <---
    '78 R100/7 & '69 R69S & '52 R25/2
    mine-ineye-deatheah-pielayah-jooa-kalayus. oolah-minane-hay-meeriah-kal-oyus-algay-a-thaykin', buddy!

  3. #3
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    Not to be too rudimentary, but the four stroke engine has four cycles: intake, compression, power, exhaust. The intake valve opens on the intake stroke and closes during the compression, the top of the compression stroke is TDC, then the power stroke and movement of the piston back to the head with the exhaust valve opening finishing the exhaust stroke with the piston at TDC but with the exhaust valve open. The point to this discourse is that you don't need some specific marks on the flywheel to determine TDC.

    Regarding timing, the ignition occurs before TDC. The standard marks on the flywheel give a number of degrees before TDC (BTDC). Basically, once TDC is determined from looking at the valves and piston position then the offset from TCD can be used for the timing mark offset.

    All the above taken into consideration, the flywheel is connected to the crankshaft. Any changes in the cam will not change the piston TDC location on the flywheel but with a new cam it would be wise to check the valve cycle and make sure both valves are closed at the flywheel TCD mark. The same goes for the timing mark as it will be the standard BTDC in degrees, but that BTDC may not be the best for that cam. However, it is a good place to start and may work ok.

    Other than excessive (pre) detonation, an engine can't be damaged by setting the timing wrong. Also, after some trial and error it becomes clear where the timing should be by adjusting the timing plate. Using the standard timing mark on the flywheel with a light bulb to determine when the points open should get close so the engine will run. Good luck!
    Stan

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  4. #4
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    What if the flywheel was put on wrong? I think that with evenly spaced mounting bolts, the timing marks could be 72 degrees or more off.
    Kurt -- Forum Administrator ---> Resources and Links Thread <---
    '78 R100/7 & '69 R69S & '52 R25/2
    mine-ineye-deatheah-pielayah-jooa-kalayus. oolah-minane-hay-meeriah-kal-oyus-algay-a-thaykin', buddy!

  5. #5
    Registered User amiles's Avatar
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    Your answer #2 provides TDC independent of the timing marks.

  6. #6
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    I think you are missing the point of Mortie's question which could have been worded better.

    He indicates he is trying to set valve clearance with an unusual non standard cam which may lift the pushrods at a different time than the standard stock BMW setup

    TDC is top dead center as others have explained and may be easily determined as described regardless of timing marks or flywheel position

    Mortie's problem though is that TDC may not be the correct piston position to measure valve clearance with the wierd cam

    So to get proper clearance do as follows:

    set clearance at TDC and then see if it gets any greater with slight rotation of the engine and if it does reset at that point

    To put it another way you are trying to set clearance at the point in the cam's rotation where the tappets are farthest into the block and you may have to find this point by trial and error

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