Why Do We Adjust Valves With the Piston at TDC of the Compression Stroke?

[Mighty_mix]

Hello Moses,

I have recently purchased an XR650r and have stumbled upon your website when looking for information on valve clearance adjustments. I just want to say I really admire your dedication and passion; this website is a treasure trove of mechanical knowledge.

I have a conceptual question regarding valve clearance adjustments, specifically on a single cylinder dirt bike. Why does the piston have to be set at TDC on the compression stroke? will the valve adjustment be different if set at TDC of the power stroke, or any position within the power/compression bands where the valves are completely closed?

Ahmed

[Moses Ludel]

Thanks for the feedback, Ahmed...I have started a new YouTube channel and will be doing tech in video form there.  I would value your participation, please join us and subscribe to the channel at:  https://youtube.com/@motorcycletechandtravel.

Our objective when adjusting a valve's clearance is to have the camshaft lobe on its heel.  Visualize the camshaft as an end view (below).  The heel is the lowest point in the lobe with the shortest distance from the camshaft centerline.  The lobe's toe (peak) is the high point on the lobe.  A rocker arm follows the contour of the lobe as the camshaft rotates.  Here is a general drawing of this viewpoint (courtesy of Comp Cams):

Illustration above shows the base circle.  This is the point where all four XR650R valves can be adjusted.  Clearance ramps are the take-up points or "ramps".  Ramp is factored into camshaft lift specifications to serve as a stable reference point.  Comp Cams and others use a 0.050" ramp figure in lift specifications.  Valve clearance cannot be measured or adjusted at the clearance ramps.

Above is a photo of the HotCams Stage 1 camshaft for the four-valve XR650R (courtesy of HotCams at Amazon).  Center lobe opens two valves simultaneously.  Outer two lobes also open two valves simultaneously.

With our four-valve, single cylinder thumpers, the two exhaust valves open simultaneously.  The two intake valves also open simultaneously.  The crankshaft rotates twice for each one rotation of the camshaft.  (The piston rises to TDC twice during each camshaft rotation.)  When setting valve clearance, we want the rocker arms on the heels (lowest point) of the camshaft lobes while adjusting each valve.  Here is what the valve/rocker arm and camshaft lobe events look like for our 4-valve, single cylinder, OHC Honda XR650R engines:

1) To your question, the power stroke is a downward movement of the piston to BDC.  The crankshaft moves 180 degrees while the camshaft rotates 90 degrees.  At BDC on the power stroke, the valve clearance cannot be adjusted. Piston at bottom dead center of the power stroke also does not work for adjusting all valves at the same time.  At BDC of the power stroke, the exhaust valves are beginning to open. 

2) With the piston at top dead center of the exhaust stroke , the rocker arms are not all on the lobe heels of the camshaft.  Why?  Because we have partial opening of each valve at this TDC position.  The exhaust stroke ends (TDC) with the exhaust valve still closing (not seated yet) as the intake valve begins to open (valve unseated).  We commonly refer to this as "valve overlap".  Neither the exhaust valves nor the intake valves are fully seated.  The camshaft lobes are not at their heels.  This is not a position to check valve clearances.

2) Piston at bottom dead center of the intake stroke also does not work for adjusting all valves at the same time.  At BDC of the intake stroke, the intake valves are still closing.  Both the intake and exhaust valves could not be checked for clearance.  Visualize the piston rising and dropping in the cylinder while the camshaft lobes rotate through each of the four strokes.  Again, during the four cycles of the engine, the camshaft rotates 90 degrees for each 180 degree movement of the crankshaft.  The piston moves from TDC to BDC (180 degrees crank rotation);  BDC to TDC (180 degrees crank rotation); TDC to BDC (180 degrees crank rotation) and back up to TDC (180 degrees of crank rotation).

4) With the piston at top dead center of the compression stroke, the rocker arms for the intake and exhaust valves (all four) are on the heels of their camshaft lobes.  In the illustration, this is the "base circle", a safe place to accurately adjust all valves.  Each rocker arm of the XR650R engine is on the base circle (heel) of the camshaft lobes.

5) Note that there are other places on the camshaft lobe where either the intake valves or exhaust valves can be adjusted.  This is always within the lobe base circle with the rocker arm on the camshaft lobe's heel or lowest point.  To adjust the valves this way requires positioning the intake rocker arms carefully within the lobe base circle, ideally directly opposite the lobe's toe or peak.  Here, the valve clearance can be checked and adjusted for the intakes valves.  Then the crankshaft/camshaft must be rotated until the two exhaust rocker arms are on their lobe heels within the base circle, ideally directly opposite the lobe's toe or peak.  Exhaust valve clearance can be checked and adjusted at this position.

Adding to this, the rocker box is an enclosure with the rocker arms and adjusters visible at each valve cap opening.  Without a degree wheel on the crankshaft and precise camshaft specifications, finding the lobe heel centerline is somewhat obscure.  You can eliminate guesswork and time by simply placing the piston at TDC on its compression stroke.  At that precise point there is no concern for valve overlap degrees or the valves being either partially open or not completely closed—which would be the case with the piston at TDC on the exhaust stroke or BDC on the power stroke or intake stroke.  TDC of the compression stroke (piston rising upward to the firing point) has all four valves closed and the rocker arms riding on the base circle heels (lowest point) of the camshaft's lobes.  

Summing up, you can adjust the intake or exhaust clearance by rotating the crankshaft and camshaft to place a pair of valves (either the intake or exhaust pair) on its camshaft lobe heels.  You could disregard whether other valves are open or closed.  The piston's position in the cylinder would not be a concern.  This, however, is time consuming and requires a clear view of the rocker arms at the camshaft lobes.  Determining the lobe's heel (lowest point) by eye is not always simple.  On a four-valve engine, you would need to rotate the crankshaft twice to center each pair of rocker arms on its camshaft lobe heel.

So why not use an accurate way to set all four valves at the same time with a simple way to locate the piston in the cylinder?  We have a timing mark for TDC.  We simply need to distinguish the compression stroke from the exhaust stroke.  In both cases the piston is rising in the cylinder.  The piston rising on the exhaust stroke has the exhaust valves open and is pushing spent/burned fuel out of the cylinder.  The compression stroke, with all valves closed, is compressing the mixture into the combustion chamber.  With the spark plug removed and a finger over the spark plug hole, we can feel or hear compressed air as the piston rises on its compression stroke.  We rotate the crankshaft in its normal direction of rotation and stop right at the TDC mark—at the top of the compression stroke.  Here, we can check and adjust clearance on all four valves.

This is the quickest and most reliable way to adjust the four valves on a Honda XR650R's single cylinder, OHC engine.  The piston is at TDC on its compression stroke.  All four rocker arms are at the base circle and heel of their camshaft lobes.  Valve clearance can be accurately and confidently adjusted at this piston and camshaft position.

Moses