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Solved: Desperate for help with Cam Timing: piston hitting valve

Discussion in 'The Garage' started by zaphoid18, Aug 21, 2016.

  1. Hi Guys,

    I am getting REALLY frustrated with this one. I recently pulled the head and bores off my bike to do some freshening up and new head gasket. Now when buttoning her up as I turn the crank bolt the piston contacts the exhaust valve. I've probably redone these steps 7 or 8 times now.

    I've followed the Haynes and service manual to the letter and cannot wrap my head around what could possibly be wrong, especially as my bike was running before this.

    I set the left cylinder to LT
    aligned the sprocket pip to 12 o'clock
    aligned the camshaft pip to 12 o'clock
    lines on sprocket angle slightly retarded but within reason (pic)
    insert bolt and rotate install second sprocket bolt
    rotate back to LT confirm position of markings and lobes
    front cam chain blade in
    cam chain tensioner in
    rocker cover installed, bolts tightened gradually in star pattern, rocker arms are all centered on valve stems
    rotate engine slowly and feel soul crushing boop.


    -The pistons were never removed from the con rods so they aren't mixed up
    -the rockers and valves were never removed or disassembled so they're the same
    -pistons went back into bores nice and simple engine rotates smoothly without head
    -cam chain is not bound up and engine rotates smoothly without rocker cover.

    Help please totally at a loss here. Whats really frustrating is its all so simple and straightforward, Its a SOHC!

    Attached Files:

  2. 16VGTIDave

    16VGTIDave XS400 Guru Top Contributor

    I know little about SOHC engines, so take the following with that in mind!

    You can't be using the correct timing marks. I was under the impression that the "LT" mark was for setting the ignition timing. Which would be advanced relative to Top Dead Center. There should be a TDC (Top Dead Center) mark. This can also be verified by gently inserting something like a wooden pencil or dowel into the spark plug hole and resting on the piston. Gently turn the engine over while making sure the dowel doesn't bind, and you will be able to accurately determine when the piston is at the top.

    Good luck!
  3. BBS360

    BBS360 XS400 Guru Top Contributor

    You're on the right mark. LF for ignition timing, LT for left top dead center.

    I've managed to get the lines on my timing chain sprockets perfectly horizontal during the few top end reassemblies I've done.

    It's only a guess but I'd suspect maybe you're one tooth off.
  4. Jeff0133

    Jeff0133 XS400 Junkie Top Contributor

    I know nothing about timing but read somewhere that when rebuilding and engine there are spec measurements for the conrods ...could it stretch with mileage/abuse....just a thought...:umm:
  5. xschris

    xschris a lifestyle not a trend Top Contributor

    Confirm your at tdc on the felt piston like dave said. You may be 180* out.
  6. Thanks for the replies guys,

    I'm sure i've got the cam and sprocket positioned correctly ( pips up, lobes down) so that makes me question the crank.

    When i set the left cylinder to LT which is supposed to be TDC the piston has gone well past the top of its stroke and is travelling down again, pictured here image.jpeg

    Should i not trust the rotor markings and go by the pencil method as dave suggests? Or is this normal for our bikes?
  7. BBS360

    BBS360 XS400 Guru Top Contributor

    It's possible that the rotor with the timing marks may have been removed previously and reinstalled without the woodruff key to align it properly.
    Wouldn't have affected anything before if the timing was already correct before the rotor were reinstalled.

    You could get TDC close enough for reassembly by the pencil method but it'd be a good idea to figure out why the marks aren't correct.
    I'd say you could mark the rotor when you're at TDC and measure back 10° to get the LF mark, but if the woodruff key is missing the rotor might not stay in its current alignment.
  8. Greasey Fingers

    Greasey Fingers XS400 Guru Top Contributor

    Did you set the crank position before installing the cam/cam chain etc? (its a step that could easily be missed if you put it together in a hurry) You will need to open it up and set the crank position correctly if you didnt.
  9. HelloGentlemen,

    As it turns out the problem was the woodruff key. Somehow it came loose in the rotor housing and lodged itself in the starter clutch. This allowed the rotor to spin a bit and throw off all my efforts to time the crank.

    Thanks for your input. The best thing to learn is if what youre doing doesnt feel right, stop and revaluate. Sometimes the solution lies deeper than you thought. Theres a reason for everything

    Best of all shes a runner!
    Fired up like a champ. Went for a little rip and she runs great. No smoke or leaks. image.jpeg
    xschris and 16VGTIDave like this.
  10. HoughMade

    HoughMade XS400 Guru Top Contributor

  11. I'm about to dive down this same rabbit hole. Where is the woodruff key?

  12. BBS360

    BBS360 XS400 Guru Top Contributor

  13. WelderDave

    WelderDave XS400 Addict

    Quick question regarding your timing mark picture. I’m doing mine right now and I lined up with that line and it appears I’m slightly off maybe a half a tooth or less...
    I did have the head resurfaced slightly to make sure it’s flat. Any input?
    Last edited: May 20, 2020

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