Jump to content
Sign in to follow this  

how to check if the piston is tdc?

Recommended Posts

Hi guys, I was checking the valve clearance on my xr250 2004 and I discovered that to do it I must have the piston in the top dead center, but what the heck is that?? and how can archive that?

Sorry if the question is so obvious but Im just starting to understand the mechanics of my motorcycle :thumbsup:

Thanks everyone.

Share this post


Link to post
Share on other sites

On a four cycle engine, the "top dead center" position is when your piston is at the top of its stroke. This occurs twice during a cycle--once at the top of the compression stroke and again at the top of the exhaust stroke. You set your valve clearances at the former, or compression stroke position. The latter will find the exhaust valves still held open by the camshaft. I assume the 250 is similar to the 400 like mine. There are two plugs on the side case. The big one in the center exposes the flywheel nut. The smaller one above it is where you will see the indicator mark for top dead center. Remember, this will appear also on the exhaust stroke. If you remove the valve adjustment caps, you can tell if the exhaust valves are tight or open indicating it is on the exhaust stroke. On the compression top dead center position, the valves should all be a little loose. When turning the flywheel nut, turn it counter-clockwise when finding the position. If you turn it clockwise, the automatic de-compressor will hold one of the exhaust valves open. I would recommend getting the Honda or aftermarket shop manual, You can find them on eBay.

Share this post


Link to post
Share on other sites

Top dead center(TDC) is when the piston is at it's highest in the cylinder bore. This happens 2 different times. Once on the compression stroke and once on the exhaust stroke. The valves are checked with it at TDC on the compression stroke. When you rotate the crank as suggested in the procedure I sent you, you see the T mark line up in the hole as shown in the picture. The piston is at TDC there. To determine if it's on the compression or exhaust stroke, take the 4 valve adjust caps off the head and put your fingers in the holes and grab the subrocker arm in each and see if you can wiggle it or feel a slight amount of play. If you can feel play in all 4, you're most likely on the compression stroke, right where you want to be. If no play, turn the crank 1 more revolution, line up the T mark and check again.

Glen beat me to it.

Share this post


Link to post
Share on other sites

Really thanks, I have the honda manual for the xr250 but much of the info is missing, or at least they assume I have previous information of mechanics, that in my case is not true, so more info was needed :bonk:

I really appreciate the help :thumbsup:

Share this post


Link to post
Share on other sites

Again, really thanks, this forum is one of the best I ever know, lot of knowledge and lot of disposition to help a 100:thumbsup: for all of you .

Share this post


Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Reply with:

Sign in to follow this  

×