For many years, nearly all automotive camshaft bearings were manufactured with a babbitt-metal lining.
Babbitt metal is a soft metal comprised mostly of tin (86 percent for Ford Motor Co. bearings, according to Wikipedia), with small amounts of lead or copper and antimony.
Babbitt’s soft and slippery composition is similar to solder. As a bearing surface layer, babbitt has the physical properties necessary to survive under adverse conditions such as foreign particle contamination, misalignment, and marginal lubrication on start-up, according to the bearing experts at Mahle.
Modern engines are making more power, which produces higher operating temperatures and puts greater demand on the valvetrain. Babbitt metal is prone to fatigue under these intensified conditions.
“Fatigue can be identified by craters in the bearing surface where sections of lining material have flaked out,” according to Mahle’s engine bearing installation tips.
Aluminum Alloy Replaces Babbitt Metal Bearing Lining for Hi-Po Engines
Bearing manufacturers have turned to aluminum alloy as a babbitt metal replacement, as it can withstand several times the load that will cause babbitt to fatigue or extrude, Mahle officials say.
Like most things in life, there’s a trade-off.
While aluminum alloy is much harder and stronger, it is less compatible with dirt, or misalignment, or marginal lubrication.
As such, when performance cam bearings are necessary, greater precision is required to maintain bearing performance and reliability, Mahle officials say.
Conditions such as cleanliness, alignment, clearances, journal surface finishes, and lubrication all must be controlled more closely.
Following are Mahle’s recommendations for optimizing performance when using aluminum alloy camshaft bearings.
1. Ensure Sufficient Clearance During Installation
These stronger bearings will not wear in rapidly to make their own clearance like softer materials. Minimum clearance should be .002″ for stock engines and .003″ for high performance. Optimum clearance range for high performance applications is .003″ to .004″. Because of the stack up of tolerances on the block, shaft and bearing it is impossible to control clearance to this range in the manufacture of the bearing alone. Clearances must be measured at installation.
Honing the IDs of cam bearings to increase clearance is not recommended because hone grit may become embedded in bearing surfaces which will cause shaft wear.
Bearing IDs may be reamed, but the most practical means is to adjust camshaft journal diameters by grinding. Even if not ground to provide additional clearance, camshaft bearing journals should be polished to a surface finish of 10 micro-inches Ra or better with the camshaft rotating in the same direction it will rotate in the engine.
2. Precision Alignment is Crucial
Any engine block that required correction to its main bearing bore alignment because of distortion is likely to have experienced cam bearing bore distortion as well.
Adequate clearance will help compensate for minor misalignment of less than .001″. Special cam bearing sets with oversize outside diameters are available for small block Chevrolet engines to allow align boring the camshaft bearing housing bores in these engines. SH-1352S Clevite bearings include five bearings which are all the same size. Blocks must be bored to 2.030/2.031″ (.010″ larger than the original #1 position) in all five positions. Similarly, SH-1401S offers a .020 oversize (2.040/2.041″ housing bore).
A third special cam bearing set available for the small block Chevy is the SH-1528S. These are special TriMetal™ bearings with .010″ oversize OD for blocks align bored to 2.030/2.031″.
These premium parts are priced higher than the aluminum alloy parts but offer the added advantage of a thin electro-plated babbitt overlay for improved bearing surface properties in combination with high strength.
3. Be Careful to Avoid Shaving Metal From the Bearings Into the Block During Installation
This galling action may cause a build-up of metal between the bearing OD and the housing bore, which will result in clearance reduction.
To prevent galling, check housing bores for a proper 25- to 30-degree lead-in chamfer before installing cam bearings.
On blocks without grooves behind the cam bearings, care must be taken to ensure that the oil holes line up between the bearings and the block. Where the block has a groove behind the bearing, the bearing should be installed with the oil hole at the 2 o’clock position when viewed from the front for normal clockwise camshaft rotation. This will introduce oil into the clearance space outside of the loaded area and allow shaft rotation to build an oil film ahead of the load.
4. Thoroughly Clean Engine Block, Crankshaft & Camshaft
Hot water and detergent are best for cleaning blocks, crankshafts, and camshafts to remove grit from honing, grinding and polishing.
After cleaning, blow dry and coat with oil immediately to prevent rusting.
Coat all bearing surfaces with Clevite Bearing Guard and all camshaft lobes and lifter faces with Cam Guard to provide a pre-lubricant to these critical surfaces. It is also recommended that engines be primed by externally driving the oil pump or pressuring the system externally through the pressure-sensing port before initial start up.
Engines should be operated at approximately 1,500-2,000 rpm for the first 15 minutes to ensure proper lubrication during the initial stages of break in.