Lost TDC changing exhaust cam

8 replies to this topic
  • crazydirthead

Posted July 18, 2010 - 05:03 PM


Ok, heres the deal. I bought a 2000 YZ426 that I wanted to put the autodecomp exhaust cam in it, so I watched the movies, read the directions and went for it. Somehow, in the process I lost TDC. In my stupidity I tried to get it back without the exhaust cam on, in the process I just lost TDC even more and got the chain caught down in the bottom. I freed the chain and can put the exhaust cam back on it now, but I don't know how to find TDC in order to line up the cams in the right spots using the dots. Any help on this would be GREATLY appreciated. Also, the clearance was .254mm before I started taking it apart, and now when I check the clearance, it is weird because I can put a .762mm gauge in, but I actually see the lifter move when I put it in. Is it normal to be able to move the lifter while checking clearance with the gauges? I am pushing fairly hard to get it in there. Thanks for any help!

  • grayracer513

Posted July 18, 2010 - 06:44 PM


I'm guessing that you have the flywheel off. Remove the spark plug and find TDC using a screwdriver to feel the piston.

And yes, if you push hard enough, you can push a gauge under the cam that's bigger then the actual clearance, lifting the valve off its seat in the process.

  • crazydirthead

Posted July 18, 2010 - 09:11 PM


Thanks for your reply grayrider. I don't have the flywheel off, will I need to take it off in order to reset to TDC? Is there a way to do it without taking the flywheel off? I guess I'll need to order the flywheel puller if there isnt a way. If you can't tell, I havent spent much time inside of a bike. I'm not stupid, I just havent been in there before so I'm making rookie mistakes :smirk: Once I put it back together i'll feel alot better.

  • davcon

Posted July 19, 2010 - 05:56 AM


You dont have to pull the flywheel. As Gray said "Remove the spark plug and find TDC using a screwdriver to feel the piston." ie Hold onto the chain and allow it to slide thru your hand while turning the motor with your socket and find the point where the screwdriver is at its highest position. That will be TDC. And you'll see the marks on your flywheel too. You'll see. Good Luck :smirk:

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  • crazydirthead

Posted July 19, 2010 - 07:31 AM


Thanks, I just wanted to be sure I didn't have to take it off, because it seems like when it's at about TDC it throws itself past the mark on the flywheel. Maybe I'll reinstall the chain tensioner and try it then. Another thing im not sure about is, since it's an autodecomp cam, do I need to take out the manual decomp or can I leave it in until I get a plug? Will it still run with the manual decomp in there? Also, do I go with Hot Cams clearance specs of .17-.22 or Yamaha's specs of .25-.30? I see in other forums people did both, but I want to know which one is the best way to do it? Should I aim for the middle like .235? Thanks again for all your help.

Edited by crazydirthead, July 19, 2010 - 07:48 AM.

  • tech24

Posted July 19, 2010 - 11:28 AM


I think gray assemed the flywheel was off cause you were asking how to find TDC. If you haven't just reove the timing plug out above the flywheel bolt plug and spin the crank until the mark lines up.

Can't help you with the valves never used HCs. Probably be ok either way or in between. There specs may be for absolute maximum performance.

  • crazydirthead

Posted July 19, 2010 - 05:41 PM


Ok, I got it, thanks to your guys' help. instead of sticking a screwdriver on the piston head this is what I did. I went to schucks and bought a spark plug for 2$ and drilled a hole through the middle of it and attached a hose to the end of it. I then tied a loop knot in it but didnt pull it tight. I had some oil in the end of the hose and when I turned the flywheel with the socket the oil would raise or lower if the piston was going up or down. I think this is alot more accurate than the screwdriver because the liquid in the hose was alot more reactive to displacement then a screwdriver on the cylinder head. The reason for the knot was to keep the oil out of the cylinder, kind of like a P-trap. I got it all put together with the new Hot Cams exhaust cam and it started first kick with 1/2 the effort. I then rode it for about 15 minutes and shut it off and it restarted 1st kick with me having to touch the red lever or the decomp lever. Highly recommended upgrade for the 426's and highly recommended tool to find TDC.

  • grayracer513

Posted July 20, 2010 - 06:32 AM


Clever, but far too complicated. The idea that the fluid is more reactive than a rod sitting directly on the piston crown is a bit far fetched, too.

With either method there is a zone of a few degrees either side of TDC where there is so little movement that the indicator can't actually locate TDC. To do it that precisely, a degree wheel and a piston stop pin are used. The piston is stopped a few degrees short of TDC and the degree wheel is set to zero. Then the rotation is reversed until the piston hits the stop again. The number of degrees shown on the wheel is divided by two, and the wheel is re-zeroed by that amount to find TDC.

As a practical matter, where timing the cams is concerned, you can get it well within the required level of precision just by feel once you get close to TDC. The pitch of the cam sprockets is such that you'd have to be 22.5 degrees off to move the cams a full tooth.

  • skthom2320

Posted July 21, 2010 - 09:29 PM


I just use a straw instead of a screwdriver!! :smirk:

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