Mornin' folks... 'er, maybe afternoon or good evening... whatever. I've had a few midnight e-mails, PMs and phone calls over the past few months from folks having "issues" with their first time inspecting/adjusting their valves on their LC4's and other bikes as well. I think once they are in the right place, everything goes fine... it's the "getting to the right place" part that seems to be a bit of a concept to grasp. So, bearing that in mind, I thought that a little overview of how a 4-stroke, single cylinder engine works and the relationship to valve lash inspection might be useful to a few peoples. If you already know how things work... move along, nothing to see here. This method of finding TDC-C applies to all 4 stroke singles. KTM, Husqvarna, Gas Gas, Sherco, Beta, Husaberg, HRC-Montesa... and all Japanese thumpsters too. ____________________________________________ TDCC is where you find it. A 4-stroke engine is composed of 4 strokes, 4-cycles, 4 events, 4 call them what you will... things where shit happens. A piston is attached to a connecting rod, which in turn is attached to a crankpin, which in turn is attached between two flywheels in an offset position from the centerline of those flywheels. As the flywheels turn round and round... the piston goes up and down. Everything is measured in degrees of flywheel rotation... as in one downward movement (stroke) of the piston equals 180° of flywheel rotation. A following upward movement of the piston equals another 180° of flywheel rotation... for a total of 360 degrees of rotation, or one complete turn of the flywheels. In a 4-stroke engine, there are 4 "events". I like to call them events sometimes because that reduces the confusion associated with the terms stroke and cycle. They are intake, compression, power and exhaust. In simple terms, each event occurs during an upward or downward stroke of the piston... the reason a 4-stroke engine is called a "four-stroke" engine. Each event requires 180 degrees of flywheel rotation, so for all four events to occur, the flywheels must rotate two full revolutions... 4 events X 180° = 720 degrees of rotation. When a piston reaches the exact top of its stroke, it's called Top Dead Center... when it reaches the bottom; you got it, Bottom Dead Center. Remember this... there'll be a test later. Now, let's bring the valve train into play. For a 4-stroke engine to work, we need fuel, compression and spark. We'll save the details of the spark part for another day. To get a combustible fuel mixture into the cylinder, and the combusted mixture out of the cylinder, we need valves. The valves have to open and close in time with the relative location of the piston... and like the piston itself, the opening and closing of the valves is measured in degrees of crankshaft rotation. In a simplified engine, it goes like this: 1. The intake valves open at TDC-Intake, on the intake stroke and closes at the bottom or BDC-Intake. There's 180°. 2. The valves are closed on the compression stroke. There's another 180°, and we've completed one revolution. 3. When the piston is near the top of the compression stroke, or TDC-Compression, a timed spark occurs and now we begin the power stroke. The valves continue to remain closed. 4. The power stroke moves thru its 180° of flywheel rotation. 5. At the bottom of the power stroke or BDC-Exhaust, the exhaust valves open and remain open all the way back up to TDC-Intake, where the whole thing starts over again... There's the last 180°, and now we've completed the 720° of crankshaft rotation in our simple 4-stroke engine. 6. Ta Da! Event numbers 2 and 3 are the ones you need to look at when locating TDC-C for the purposes of valve inspection and adjustment. In a real engine, valves do not open and close at TDC and BDC... they open before and after these reference points to accommodate the realities of pressure and velocity. It is for this reason you must be very near TDC-C to ensure that both intake and exhaust valves are at a complete rest and all valve lash is present and accounted for. Now, knowing what we know, let's find TDC-C. Remove the spark plug so that the engine can be turned over more easily. Remove your rocker or cam covers so that you can see the valve actuation as it occurs. Shift your transmission into top gear and lift your rear wheel off the ground. Turn your rear wheel slowly in the normal direction of rotation and watch the valve actuation. It is not necessary for you to continuously turn the rear wheel... you can stop any time you feel the need, and start again when ever the mood strikes. When you see the intake valve actuation... the valves pushed down into the combustion chamber, then allowed to close, you are at the end of the intake stroke and on the compression stoke. If you went too fast and too far, you went right past the TDC- Compression stroke and are now somewhere on the power stroke... oops. :huh How do you know you are on the compression stoke? This is where a soda straw comes in handy. Inserting a soda straw into the spark plug hole allows you to see the movement of the piston in relation to the valves. If the intake valves have closed and the soda straw is climbing out the spark plug hole, you are still moving up to TDC-C... good for you! Slowly bump the rear wheel and watch that soda straw, it rises bit by bit until the piston reaches TDC-C... when it stops moving upward, this is as close to TDC-C as you will ever need to be to inspect your valve lash. Oops... the soda straw is starting to go down the hole! :eek1 Can I back up the rear wheel a bit without going around again? Yes, you can... if you really have a good idea where you're at. If not, it's not going to kill you to go 'round one more time. Pull the soda straw out of the plug hole and reinsert it after the intake valves have just closed... unless you enjoy watching your engine eat soda straws. ... On most engines, for the purposes of valve adjustment, all you have to do is be close to TDC-C, not exactly right on it. If you read the description of the simplified engine, you see that after the intake valves close, the exhaust valves don't open until near the end of the power stroke. If you can be within less than 8mm of piston travel, on either side of TDC-C, you should be good to go. I think that should do it. I hope I've been able to explain this in simple enough terms that anyone who owns a tool set and motorcycle, and wants to attempt their own valve lash inspection/adjustment can do so. I've linked this post to the original valve lash inspection/adjustment post. Creeper To print a copy of this guide, go to the top of the page and click on "Thread Tools" then click on "Show Printable Version" Oh... to what ever person did these illustrations on the website I stole them from 6 months ago... thanks, I knew I'd use 'em sooner or later.