yz450f engine question help?

:thumbsup: i have a yamaha yzf450 and honda 400ex anyway i am going to rip both of them apart to do the piston, rings and valves anyway my question:

im a old car guy, does it matter if the piston is on the compression stroke or exhaust stroke when i put it back togather? top dead center i mean? like will it be off 180 if i put it in the wrong way??? i know to mark up the timing mark on side and on cam sprockets but just cannot make sence of if it matters what stroke the bottom end is on when putting back togather? and shop manual does not tell... any help great. thanks

when you put it back together it doesn't matter as long as the piston is at the top of the stroke, timing mark is lined up on the flywheel and the cams are set up with the timing marks in the correct spot. The ignition doesn't know if the piston is on the exhaust stroke or compression stroke it fires on both (when I say both, it fires every time the piston makes top dead center regardless of which stroke)..

The only time that matters is when the ignition trigger device is driven at half speed, as the cams are, but independently of the cams, as they are in most automotive applications. But the cams are what decide which stroke is which, so they can't be backward by a full 180 (360 at the crank). Since the ignition is crank triggered, as MK pointed out, which stroke you start with doesn't matter; they're all the same until the cams go in.

thanks for the great info. thats what i was thinking but i did not knoe for sure. so line up crank line, line up cam lines, pop it togather and its ready to go. thinking of this all night had me up all night. thanks

Bear in mind that the marks on the cams will more than likely not line up perfectly. If you think they are off, visualize the timing set at the next tooth over from where you have it.

In the YZ, plan to replace the cam chain, and while a tensioner may not be needed, it isn't very expensive, either.

ok, well im going to do the 400ex first cause it only has 1 cam but im marking everything and taking pics and have a manual so i should be good to go.

i was just more worried about if i put it in somehow on the wrong stroke and the spark would not be there on the power stroke and it would be there on the exhaust stroke but since it fires on both i guess it dont matter

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