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Old 08-08-2008, 11:41 PM   #18
Scooby Specialist
Member#: 14141
Join Date: Jan 2002
Location: 13.239@102.85 @ 5800 ft on 13T
2002 Impreza WRX


It depends a bit on which engine and which valve you are talking about. You can generally run less clearance on the intake side than on the exhaust side.

When the intake valve makes closest approach to the piston the piston is moving away from the valve and the valve is trying to catch it. The intake valve is under acceleration so there is no slack in the valve train. On the domestic V-8's they typically run 0.050 as a safe side clearance, but can go much lower than that if they trust the build. One rule of thumb is on the intake side run .005 - 0.010 more clearance on the valve to piston that you have between the piston and cylinder head. If you do that the piston will smack the head before it tags the intake valve.

On the exhaust you need more clearance since the Piston is coming up the bore as the exhaust valve is closing, and over taking it at closest approach. If the exhaust valve floats at all it is more extended when the piston gets to its closest approach than under static conditions, and the piston will catch up to it and smack it closed. Also if the exhaust valve bounces off the seat due to an over rev and loss of control by the valve spring it can bounce back as much as 1/3 the max lift.

On the 2 valve heads that means absolute minimum clearance on the exhaust/piston distance of 0.065 - 0.070 if things are stable and 0.090 - 0.100 clearance if you want a good safe build in case you buzz the engine on a down shift etc.

I would be inclined to safe valve clearances since it gets very expensive very fast if you tag them. It simply is not worth the risk and you gain very little increased compression by running tight and you lose very little compression if you deepen a valve pocket to make it safe. The actual point of closest approach is not the whole valve but usually the lower edge of the valve and the edge of the valve relief cutaway on the piston. You can only determine where that point is by using clay and mocking up the engine build and doing some measurements. Once you find the crankshaft angle where closest approach is, then you can set the crank there, and push the valve down until it pushes through the clay and makes contact with the piston. That will give you a witness mark showing the actual point of most likely contact. Sometimes all you need to do to greatly increase the piston valve clearance is to slightly move or enlarge the valve relief cutout in the piston not change the deck clearance of the whole piston.

On our engines we have a large bore (over square) which increases potential for piston rock at TDC and with an OHC configuration you have potential for belt stretch that causes the cam center line to move about a bit under rapidly changing engine rpm conditions. Don't pay much attention to piston to valve clearance at max valve lift as the piston is not near the valves at that time. Closest approach between piston and Intake valve is typically around 10 degrees ATDC. Rule of thumb closest approach between the piston and valves occur at about 8-12 degrees BTDC for exhaust and ATDC for intake. If you plan on using adjustable cam gears better give yourself a bit more piston/valve clearance as your final cam timing may not match the cam locations when you measure your clearances.

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Last edited by hotrod; 08-10-2008 at 01:05 AM.
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