Poly 318 vs. LA 318

LA and poly cams will physically fit in either block, but the cam for one won't properly operate the valves for the other. For one thing, poly intake and exhaust ports are evenly spaced; LA center center ports are adjacent, so put the cam in the wrong motor and some of the intake lobes would be operating on exhaust valves and vice versa. Also, the valve angles are different from one motor to the other, so put the wrong cam in and even the lobes working the right valve wouldn't do so at the right time.

Timing chains and distributors, however, do interchange between poly and LA. As do rods and post-61 cranks. In fact, the first 273 LA motors used the 318 poly crank, rods, timing cover, timing chain, damper, fan and crank pulleys and alternator bracket. (Early 273 alternator brackets had two sets of mounting holes; one set for the 273, the other for the 318 poly. Same bracket for both motors.)