Piston-Valve Clearance

[Guest Anonymous]

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[grumpyvette]

What is that minimum allowable clearance after any cam timing change as opposed to the original 0.100" settings???

 

IDEALLY you should HAVE GREATER THAN .100 clearance to start with so that changing the timing (ADVANCE/RETARD within about plus or minus 6 degrees from that SPLIT OVERLAP POINT still allows that .100 minimum piston to valve clearance.

BTW youll normally find that the valves reach their closest point to the pistons at about 10 degrees before and after TDC, NOT at TDC

Ive seen guys go as tight .060 but thats nuts in the long haul if your expecting that engine to live very long at high rpms

 

look carefully at this chart

 

http://www.iskycams.com/ART/techinfo/ncrank1.pdf

 

now look at this cam card

 

http://dab7.cranecams.com/SpecCard/DisplayCatalogCard.asp?PN=119681&B1=Display+Card

 

 

at two points those valves are damn close to the piston

if that lifter is .050 off the base at 11 degress BTDC the ROCKER ARMS 1.6 ration lifts the valve .080 thousands, the piston is only .035 thousands down the bore, with a common .025 deck and .021 head gasket that means the edge of the valve is .001 thousands froum touching the piston without valve relief cuts for additional clearance on a flat top piston. heat expansion allone would cause them to hit BEFORE the TIMEING EVER THOUGHT OF GETTING ADVANCED