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Any number of items can be different on your engine, requiring you to use a different pushrod length. You should double check this when installing: new heads, camshaft, rockers. As cam profiles get more aggressive and valve spring pressures increase, the importance of selecting the right pushrods has never been more critical. COMP Cams® carries a wide selection of precision pushrods to handle any application from stock replacement to hardcore race.
What is the correct length pushrod for your application? The one that produces correct valve train geometry. What is correct valve train geometry? When the rocker arm roller tip rolls from the intake side of the valve tip, across the center of the tip (at approximately mid-lift), to the exhaust side of the valve tip (at full lift) and back. See Pic.
With the lifter on the base circle of the camshaft, tighten the rocker-arm nut to obtain "zero lash," meaning that there's no clearance between the valve tip and the rocker arm. Ideally on LS engines you want to be able to turn the ratchet another 1.5 to 2 turns more (See Video below).
Comp Cams offers a Hi-Tech Master Pushrod Checking Kit # 7705 that includes four different adjustable pushrod-length checking tools (1ea #7701, #7702, #7703, #7704) for pushrod lengths ranging between 5.800–9.800 inches. Master Pushrod Checking Kit # 7900 contains 1 Each: #7901, #7902, #7903, #7904 & #7905 for lengths 6.125”-11.5”.