Page 14 - Summer 2018 GMC Vintage RVing

Technical Corner MAINTENANCE, PARTS, TIPS, INNOVATIONS AND MOREThe Three Way “Keyway” QuestionDick Paterson | 1976 Eleganza II | 705-325-4554 | Springfield Ignition | springfield-ignition.comIn response to a question from a GMC owner, replacing his timing chain, about which keyway slot should be used for the crank gear.This is a common query, as the new crank gear has three available keyways: quality replacement Olds timing chain sets, made in USA, from Cloyes or Speed-Pro.The directions in the box are detailed but can be confusing to a person not familiar with those parts.If the camshaft is not being replaced–only the gears and chain, then the crank gear, the keyway slot marked plus 4 should be used. Stock cams and chains sets were “retarded” and benefit from adding advance. (If a new cam is going in, then set the crank gear on zero.)Reason is: Almost all cams have four degrees advance built in, adding in another four–sets the cam at +eight degrees advance– killing the upper end of the torque band.Wide lobe centers with less overlap favor low rpm power– advancing or retarding the cam, relocates the overlap phase at TDC (top dead center) with a significant impact on output.Getting back to the gear set replacement only. The crank would be turned so that its plus four timing tooth would be–straight up (at 12 o’clock) position.The cam gear has a single “0” mark on it. This gear with the chain draped over it, is held up to the front of the cam so that it willNOTE: The shop manual states the job CANNOT be done without removing the motor.Three Keyway Crank Sprocket SetThe six numbers stamped into the crank sprocket are as follows:0 indicates which tooth to use for timing0 indicates which keyway to install key in +4 indicates which tooth to use for timing +4 indicates which keyway to install key in-4 indicates which tooth to use for timing -4 indicates which keyway to install key inSetting your cam timing at the standard (stock) setting:1. Install the crankshaft key in the keyway marked o.2. Use the tooth marked 0 to align with the cam sprocket for timing.Setting your cam timing at the +4° (4 advanced):1. Install the crankshaft key in the keyway marked +4.2. Use the tooth marked +4 to align with the cam sprocket for timing.Setting your cam timing at the -4° (4 retard):1. Install the crankshaft key in the keyway marked -4.2. Use the tooth marked -4 to align with the cam sprocket for timing.0° Timing Tooth+4° Keyway0° Keyway+4° Timing Tooth-4° Keyway-4° Timing Tooth14 GMC VINTAGE RVING MAGAZINE | GMCMI.COM+4-4+4-4