Bottom Line
Maintenance Q&A

By Paul Abelson
senior technical editor


Q: I was reading the Maintenance Q&A in the February 2008 issue about the switch to synthetic oils.   Previous experience on a switch from regular oils in rear-end fluids to synthetics tells me the seals will start to fail. It has something to do with the compounds in the seal. If you start with conventional fluids, stay with them. If you plan a change in fluids, you may as well change the seals at the same time.My question: How many seals in the motor will I have to change once I’ve switched to synthetic motor oil?

A: You should not have to replace any seals. However, it should be noted that leaks were reported with early generation synthetics – although it’s a far less common occurrence now.

According to every refiner and blender of synthetic engine oils, the new formulas are totally compatible with all engines and all seals in those engines.

Back in the 1980s and early ’90s, oil seals had a greater content of natural rubbers. Those seals adjusted to the content of the oil that lubricated them. If there was a drastic change, seals would dry and crack, or swell and soften, depending on the changes.

Today, we use seals made of Viton and similar polymer materials that are far more tolerant of a broader range of hydrocarbons.

Q: Can you tell me which turns shorter – a 220-inch wheelbase Freightliner FLD with a set-back front axle; a 220-inch wheelbase Freightliner axle-forward Coronado; or any axle-forward truck with a 220-inch wheelbase?

A: It depends on two factors: the wheelbase and the amount of wheel cut at the steer axle.

For any given wheelbase, the tighter the wheels are turned, the shorter the circle will be. So, for any given maximum wheel cut on a shorter wheelbase, the tighter the turning circle. Conversely, the longer the wheelbase, the greater the circle.

Wheelbase is a measurement taken from the center of the steer axle to the center of a single drive axle or the midpoint of a tandem drive axle.

If the steer axle setback is, say, 40 inches from the bumper and the wheelbase is unchanged, the distance from the back of the cab to the center of the drive axle or tandem will be about 10 inches longer.

You would have to compare the location with a set-forward axle to determine the exact difference. If you want to maintain the same overall length but use a set-back axle, the wheelbase should be reduced by the extra 10 inches of setback.

If the wheelbase is constant at 222 inches, there will be no change in the turning circle. With a shorter 212-inch wheelbase (222 inches less 10 inches), the circle will be tighter.

I believe the Coronado has a greater available wheel cut than the FLD, but that could be based on options. If things are equal, both trucks will turn the same. If the Coronado has a greater wheel cut, it will turn tighter.

Q: A friend helped me change the bearings on the steer axle of my 2001 Century at about 650,000 miles. Now there is squeaking coming from the wheels. It’s only been three months since the repair, so it shouldn’t be the bearings. Any ideas?

A: For starters, the bearings themselves could be the problem. They are among the truck repair products most often found to be counterfeit. As with most counterfeits, the packages are designed to look almost identical to genuine products. Counterfeits use inferior heat-treating; cheap steel; and soft bearings and races that will not stand up.

If it’s not the bearings, you need to look at the installation.

Wheel bearing maintenance is one of the most delicate and demanding jobs you can take on. There are many individual steps, and many of them require precision measuring instruments and tools not found in many do-it-yourself shops.

To properly measure, you need dial indicators that read to 0.0001 of an inch and gauge blocks. For seals, installation tools are manufactured specifically for what you are using. One size does not fit all.

Once installed, bearings must be adjusted to exacting tolerances. The Technology & Maintenance Council has a number of Recommended Practices on bearings and seals. They should be read and understood before tackling such critical jobs. RP618 (Wheel Bearing Adjustment Procedures) describes adjustments for achieving desired endplay. It applies to conventional single-nut and double-nut systems.

Newer bearings require preload to maintain adjustment. SAE International RPJ2535 (Setting Preload in Heavy-Duty Wheel Bearings) describes a range of values for popular Class 7 and 8 truck and trailer applications.

The Technology & Maintenance Council’s

RP640A (Alternate Wheel Bearing Adjustment Systems) identifies unitized and pre-adjusted systems and their appropriate maintenance considerations. The TMC’s RP622 (Wheel Seal Bearing Removal, Installation and Maintenance) covers steps needed to avoid problems such as you may have experienced.

For your specific problem, also refer to TMC RP644 (Wheel End Conditions Analysis Guide). All RPs are included at no extra charge when you become a TMC owner-operator member. To join the Technology & Maintenance Council, call 703-838-1763. LL


Paul Abelson can be reached at