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Thread: finding TDC with a 336 cam

  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
    Administrator 20774's Avatar
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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 <---
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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
    Administrator 20774's Avatar
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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.

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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

  7. #7
    Superkraut typ181r90's Avatar
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    The 336 cam increases valve lift (IIRC close to 1.5mm greater than stock) and duration/overlap, the latter which effectively lowers compression from the stock 308. TDC is TDC, the valves should both be fully closed at this point as it's the top of the compression stroke, the 336 cam doesn't change that.

    Assuming your flywheel is bolted on correctly, set it to OT (ober totpunkt), the side that's intake valve opens and closes before OT is the side you'll need to adjust first as this will be on the compression stroke. When you want to do the other side just rotate it again a single time to reach OT and then the other side will be on the compression stroke. There weren't any additional valve clearance settings using the 336, I believe they should still be set at .006" intake, .008" exhaust. If you believe the flywheel is bolted on incorrectly, use Kurt's method of finding TDC
    // 1975 BMW R90/6 (cafe'd) // 1957 BMW R60 (in pieces) // 1967 Aermacchi/H-D Sprint 250 SS (custom special) // 1973 VW Type 181 Custom SOLD )

    http://symphonyofshrapnel.blogspot.com

  8. #8
    Superkraut typ181r90's Avatar
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    If you want the 336 lift specs I'll get them to you after my night shift, the BMW technical bulletin is sitting on my bookshelf, next to the brand new 336 cam I bought a couple years ago, 9.5:1 pistons, and Dellortos that I still haven't put in - the bike is running way too well to justify taking it apart again!
    // 1975 BMW R90/6 (cafe'd) // 1957 BMW R60 (in pieces) // 1967 Aermacchi/H-D Sprint 250 SS (custom special) // 1973 VW Type 181 Custom SOLD )

    http://symphonyofshrapnel.blogspot.com

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    All the performance cams that I have installed or read about had the valve lash adjusted at TDC, the same as a standard cam. I can't see a situation where that would change, but asking people who have experience with the 336 cam makes sense. There are some folks on the advrider.com site who have the 336 cam installed. That may be a good site to ask such a question about setting the valve lash. Here is but one thread on the 336 cam: http://www.advrider.com/forums/showthread.php?t=703435
    Last edited by Stan_R80/7; 11-14-2012 at 02:27 AM. Reason: Edit: Typ181R90 sounds like he can help you out with the 336 details.
    Stan

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  10. #10
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    As others have stated, top dead center is top dead center. Got a '74 R90S with 336 cam, wouldn't have it any other way. Valve clearance is the same. Have fun.

  11. #11
    jimmy armour
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    336 sports cam

    hi guys, I agree tdc is the same,years ago I installed a 336 sports cam and new timing chain,in my 1977 R100S, 20 odd years ago, ,on starting I heard a clicking in the left cylinder?,it went up and down with the revs,I was advised to enlarge the valve clearences from 4 and 8 thou, to 10 thou,noise went away,the valves were touching each other on over lap !!,a comon thing on the big valve motors I was told, I ran the bike over the weekend on a trip with the wife all seemed well, power came in later,but I decided if the valves were hitting at 4&8, a missed gear could be disasterious,so removed it again, traded it with the dealer for heated grips,hope this assists jimmy

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    thanks all for your replies and insight it is greatly appreciated I found it was easiest to do as 20774 said and used a straw in spark plug hole and measure the difference from where it stops to where it starts to move and take half. I think the fly wheel may have been put back on incorrect as the marking OT is no where near where I determined TDC to be.

  13. #13
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    The thread title is "233 cam"...what is that? Do you mean a 336 cam? I'd like to change the thread title for future searches.
    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!

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    #2 I messed up and I thought I corrected the thread title. No problem change it

  15. #15
    James.A
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    It's just this simple....

    1/2,...one half of a turn....past Top Dead Center,....TDC..., with both valves closed,....is the power stroke....

    That's 90 degrees of flywheel turn.

    So,....at least half of that 90 degrees, ...probably slightly more,.... both valves will be closed.

    Any point in that first 45+ degrees of flywheel turn,....that would be 1/4...one quarter of a turn.... after Top Dead Center, (TDC), the valve train will be slack.

    There lies your opportunity to set your valves with the cam lobes out of play, and the valve train totally slack.

    ...period.

    Watch the rocker arms. When the intake valve closes, you are on your way to TDC on the compression stroke. The power stroke is next.

    Actually,..any point in motor rotation after the intake valve closes and before the exhaust valve starts to open is fine for setting valve clearance.
    Last edited by James.A; 11-16-2012 at 08:54 AM.

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