When fitting aluminum cam bearings, it's important to allow sufficient bearing-to-camshaft clearance, since the harder aluminum alloy bearings won't wear in as rapidly as babbitt bearings to make their own clearance. Minimum clearance should be 0.002 inch for stock engines and 0.003-0.004 inch for high-performance engines.
Our Clevite bearing expert stresses that you should not apply oil to the block's camshaft bores or to the outer diameter of the cam bearings. The cam bearings must feature a dry installation at the block's cam bores. Schaerer also says to make sure you have the proper tools for installing the cam bearings. For example, he recommends using a dedicated cam bearing driver tool, which is available from any engine specialty tool supplier.
At this point, Schaerer says to apply a coating of lubricant to each of the cam bearing's exposed face (the inner diameter of the bearings where the cam will ride). You can use either clean engine oil or, preferably, dedicated engine assembly lube. "A quality assembly lube will cling to the bearing faces better than oil," explains Schaerer. "Granted, you'll be lubing the camshaft's journals prior to inserting the cam anyway, but it never hurts to protect the bearings while you have access to them. Think of it as a bit of added insurance, just in case you should forget to apply lube to one or more of the journals."
Ensuring Optimal Bearing FitWhen replacing cam bearings, it's a good idea to check camshaft-to-bearing fit, even if you purchased your car new, and this is the very first time you are replacing its cam bearings. Clevite's cam bearing expert explains why: "Many rebuilders and do-it-yourselfers are unaware that original equipment engine manufacturers (OEMs) may sometimes install unfinished-size cam bearings into the block, and then machine the cam bearing inside diameters in order to establish correct size, shape and alignment," says Schaerer. "In the industry, this is referred to as 'finished in place,' or FIP, cam bearing installation.
"During the rebuilding process, FIP bearings are removed and replaced with precision cam bearings that have already been manufactured to size," explains Schaerer. "While replacement cam bearings are manufactured to precise tolerances, it is still advisable to check camshaft-to-bearing fit so as to identify potential camshaft journal diameter variances among the replacement camshafts."
If cam fit is too tight, bearing material can be removed by reaming. Schaerer says that honing bearings to size is not recommended, because honing stone grit can easily be trapped in the bearing surface, and its presence there can lead to premature camshaft journal wear.
Another method of "shaving" cam bearings to accept the camshaft journals involves the use of a modified spare/old camshaft of the same application, provided that the old cam's journals are within original diameter specification. In this instance, you begin by cutting a diagonal groove across each camshaft journal, about 1/8-inch deep, then relieve the journal surface on one side of each groove, thereby creating a single cutting edge on one side of each groove.
This modified camshaft is then inserted and used as a reamer. Afterwards, you'll need to attach a used cam sprocket to the cam and carefully turn the cam by hand, in the direction of normal engine rotation, in order to cut clearance in the new bearings.
Clevite's bearing expert says there are also other variables to consider when replacing cam bearings. Casting shifts, for example, can occur in a cast-iron or cast- aluminum block as the block "seasons" during its initial use. The block can become distorted due to temperature extremes (overheating) and improper cylinder head installation where excessive or unequal cylinder head fastener tightening had occurred.