By William C. “Bill” Anderson, P.E.
I often hear collector car owners complain about their car’s poor handling. Sometimes, their expectations are too high; expecting a decades-old car to handle like their modern car is unreasonable. But, more often, I find that they have not bothered to completely rebuild the suspension system, both front and rear. A worn rear suspension can adversely affect handling, even if the front is fully rebuilt.
A car’s suspension system is what ties the assembly to the ground. There are springs, bushings, bearings, axles and axle supports. After 30 or more years of driving, these components are generally tired, if not completely worn out. This is especially true of cars that are more than 50 years old. Sometimes, the components may even be bent.
This article will address one component of early suspension systems. King pins tie steerable front axles to the weight-carrying axle, such as in a Model A Ford, or to the control arms in those cars with independent front suspensions. King pins can be found in cars up to the mid 1950s, when ball joints replaced king pins.
Determining if the king pins require repair first requires adjusting the wheel bearings to remove any looseness. Then, grab the tire at the top and bottom and see if it can be twisted in and out. Alternatively, the wheel can be removed and any in-and-out deflection can be measured with a dial indicator. If there is play in the king pin, it must be rebuilt.
Rebuilding king pins starts with reviewing the shop manual for the specific procedures for your car. However, most steps are similar to the description in this article. With the front end supported on jack stands at the frame, remove all the brake components: drums, shoes, etc. (See Figure 1). Next, the axle support must be removed from the control arms. This step requires releasing the spring, so be careful: place a jack under the control arm to support the weight, remove the lower support pin and then carefully lower the jack.
Figure 2 shows a degreased axle and axle support after removal. To separate the axle from the axle support, remove the grease fittings, drive out the tapered horizontal lock pin and drill a hole in the upper end cap. Using an appropriate tool, pry out the upper end cap and drive out the king pin from the top, taking the lower end cap with it. With the pin removed, the axle can be separated from the axle support; note the position of the thrust bearing and any spacers.
Rebuilding kits typically include a new king pin, bushings for the axle, thrust bearings and shims, a new lock pin, and new end caps. The old bushings are pressed out and the new bushings pressed in. When installing the new bushings in the axle, make sure the holes for the grease fittings are properly aligned. In days long ago, it was necessary to burnish the bushings; that is, a special tool was used to force the bushings tight against the axle. Bushings now supplied with most kits are of the hard-shell type and do not require burnishing. The bushings must be reamed to provide the proper fit with the king pin (See Figure 3; the shop manual will provide the correct tolerance). Most machine shops can handle the reaming if you don’t have the necessary tools.
At this time, make sure the lower bushing in the axle support is in good condition or is replaced with a new one. Figure 4 shows an axle support from a recent project where the lower bushing and support was so badly worn that it fell out of the axle support. In this instance, another axle support in useable condition had to be located.
Figure 5 shows the various components in the complete front suspension assembly. It is advisable to make a trial assembly of the axle and thrust bearing to the support; note the replacement thrust bearing may be a different thickness than the old one, requiring additional machining. Assemble the axle to the axle support with the thrust bearing and any shims, as required, using the king pin. Make sure the notch in the king pin is aligned with the hole for the locking pin. Then, insert the tapered, horizontal locking pin — it only goes in one way — and drive it secure. Install the end caps, which are a tight fit and require staking with a center punch. I also place the round end of a ball-peen hammer in the center and hit it with another hammer to slightly expand the end cap to ensure a secure fit.
Figure 6 shows the completed assembly. Also, install the grease fittings.
The next step is to assemble the axle support to the control arms.
King pins are only one part of the suspension rebuild. To perform as the car did when new, all wearable components must be carefully examined and all worn components replaced with new.
A completely rebuilt suspension system that is properly adjusted and aligned will provide handling consistent with the system design when new. Remember, cars of the ’30s and ’40s were intended for driving at 40 to 50 mph; not today’s freeway speeds.
William C. “Bill” Anderson, P.E., is an author, magazine editor, car show judge, and professional engineer. Through Anderson Automotive Enterprises – www.andersonautomotiveenterprises.com – he restores and appraises cars.