Poly 318 Pulley and Harmonic Damper

(also applicable to 277, 301, 303, 313, 318, and 326)

Harmonic Damper

At the risk of being a semanticist and to clear up confusion out there, no 1956 – 1966 A-block ever came with a “harmonic balancer.” All A-blocks are internally balanced, and the part on the front of some A-block cranks is a neutrally balanced “harmonic damper” that absorbs engine harmonics and does nothing to balance the assembly. Harmonic balancers are used on externally balanced LA360 and cast-crank LA340 engines to balance the rotating assembly. It is important to never use an LA360 or cast-crank LA340 harmonic “balancer” on an A-block since doing so will throw the rotating assembly out of balance.

To add to the confusion, some poly A-blocks came without a harmonic damper and instead used a special extra-deep crank pulley. Mopar engineers decided to install harmonic dampers on most car A-blocks paired with automatic transmissions (Figure 1) but used the extra-long pulley on some engines paired with standard transmissions and in trucks (Figure 2a – 2b). Many 1950’s A-blocks regardless of car, truck, or transmission type used the long pulley and no damper. Regardless of these factory configurations, all A-block crankshafts accept a harmonic damper, and we now know it is wise to run a harmonic damper regardless of transmission to assist with, well, engine harmonics.

The neutrally balanced LA 273, 318, and 340 harmonic dampers interchange with all A-block engines, although the TDC timing mark is clocked on the left side versus the A-block’s right side. If replacing an A-block damper with an LA 273, 318, 340 unit, line up the old damper and pointer at TDC before removing the damper; after installing the new LA damper, remark the damper TDC with timing tape or by filing and painting a shallow groove into the damper. If starting off with no damper to align, you must degree the crank to where cylinder #1 is at true TDC in order to accurately mark the new damper.


Water pump pulleys depend on the year of the water pump. Factory 1956 – 1969 A-block and LA engines use an iron left-hand intake short-shaft pump, whereas 1970 – 1992 LA use an aluminum righthand intake long-shaft pump. The water pump pulley and pump needs to be paired in order to align (Figure 3).

A-block and LA crank pulleys interchange no matter the type of water pump and year, but keep in mind the LA damper and crank pulley bolt pattern changed slightly in 1970 where one pulley hole must be elongated on a 1970 onward pulley when mating it to a 1956 – 1969 A-block or LA damper.


Mopar poly 318 automatic damper
Figure 1: Automatic Harmonic Damper
Mopar poly 318 crank pulley damper
Figure 2a: 1962+ Manual Long Pulley
Mopar poly 318 crank pulley damper
Figure 2b: 1956 – 1961 Long Pulley
Mopar Poly 318 Pulleys
Figure 3: Water Pump Pulleys: 1956 – 1969 (left) 1970 onward (right)