What Is a Camshaft Thrust Button?by Richard Rowe
A camshaft thrust button is one of those little parts that you'll often find in newer engines, but it may not be familiar to mechanics who are used to working on older engines. Camshaft buttons don't come into use because of the roller camshaft, but rather because of hot-rodders who retrofit roller camshafts to non-roller engines.
Many traditional flat-tappet camshafts use lobes with a very slight angle around the lobe surface. This very slight angle puts a very slight side-load on the camshaft, helping to push it back toward the distributor to compensate for the camshaft's inherent tendency to unscrew itself from the engine block. The roller lifters that came into use during the 1980s allowed manufacturers to make their camshafts more aggressive, but didn't allow for lobe angles to keep the cam in the block. This isn't really a problem for engines that use a front-mount distributor, but can cause serious problems for engines without one.
A cam button screws into the threaded hole in the front of the camshaft, and serves to take up the space between the front of the cam and the timing cover. Cam buttons come in three basic types: nylon, aluminum and rollerized. Nylon and aluminum buttons will work fine as long as they remain well-lubricated, but will eventually wear down to a nub against the hard timing cover. Rollerized cam buttons contain roller bearings that will help the cam button to live a far longer and less friction-filled life.
The primary result of cam button wear is excessive cam end-play, or "cam walk." Cam walk often manifests as changes in ignition timing, often leading to a retardation of the ignition advance. Timing retard happens when the distributor gears impart a side-load against the camshaft, pushing it away and changing the relationship of the distributor to the camshaft. This loss of timing advance can happen very suddenly, but more often it occurs over time. If your roller-cam-equipped engine loses a couple of degrees of timing advance per month or per 1,000 miles, your cam button may have begun to wear against the timing cover.
A rollerized cam button is cheap insurance against cam walk, but if you opt for a cheaper unit then make sure it's of softer material than the timing cover. Aluminum buttons are fine if you're using a stamped steel cover, but use a nylon button if your cover is made of aluminum. you may also need to upgrade your timing cover to something a bit stronger, since cheaper stamped steel covers can flex under the force of cam button impact. A flexing timing gear cover can lead to a loss of timing at high rpm, but will appear normal at low rpm. Having bronze bushings installed in your lifter bores can also help to reduce camshaft side-loading, but a quality camshaft button will reduce or eliminate camshaft walk in most cases. If you're going to sleeve the lifter bores, a set of SAE 660 bronze sleeves will work, but graphite-impregnated bushings will last longer on a street-driven vehicle.
- "Best of Hot Rod Series: High Performance Camshafts"; R.M. Clarke; 2000
Richard Rowe has been writing professionally since 2007, specializing in automotive topics. He has worked as a tractor-trailer driver and mechanic, a rigger at a fire engine factory and as a race-car driver and builder. Rowe studied engineering, philosophy and American literature at Central Florida Community College.