If you were lucky enough to take auto shop in high school, you might have been fortunate to be able to “rebuild” one of the class engines. In those courses, you merely disassembled the engine and then reversed the procedure. Hopefully for the first-timer, the engine started and ran happily and if so, you passed the test. However, for the true engine builder, assembly really is the easy part. It’s all the checking and clearancing that is essential before the engine is assembled, where all the real effort is located.
Crankshaft endplay never seems to garner the same level of attention as say, rod and main clearance, but even with its somewhat limited attention, this is still a crucial area. If nothing else, if crank endplay is not checked and verified, the end result will be disassembling the entire engine to repair the damage, because too loose is just as bad as too tight. This makes crank endplay a spec that cannot be overlooked, and that alone should motivate all but the laziest of engine builders.
We will be using both a small- and big-block Chevy for the images in this story, but the process is the same for all engines even when the thrust is located in the center main bearing as with Fords and the GM LS. The thrust bearing incorporates both the journal bearing as well as fore-aft thrust surfaces. It is this clearance we will be addressing in this article.
Setting The Stage
With a completely disassembled engine, the first step in this process is to ensure that the proper main bearings are used. This sounds simple, but in the case of a recent big-block Chevy, we used 0.010-under bearings for the first four main bearings, and due to a slightly undersized Number Five crank journal, this required a set of Federal-Mogul 0.011-inch-under bearings to set the clearance on Number Five. Once the proper main bearing halves were selected, only then can we test for endplay.
Thrust clearance is important since there is a significant amount of forward movement from both automatic and manual transmissions. Automatics are often blamed for burnt thrust bearings due to converter ballooning (expansion of the converter), and this does occur, but there are other causes. One cause that receives very little attention is also one that can be easily avoided.
Manual transmission cars with the older style three-finger Long or Borg & Beck clutch pressure plate designs, apply increasing pressure as the clutch pedal is pushed to the floor. This produces maximum forward thrust on the crankshaft while attempting to start the engine. Diaphragm pressure plates go “over center” at full release which applies only limited forward thrust. So the technique for a car equipped with a Long or Borg & Beck clutch is merely to start the engine in neutral. This eliminates potentially excessive wear on the thrust bearing.
There’s a simple technique for installing and measuring thrust bearings. For testing we start by installing the front and rear bearings. For a Ford with the thrust in the center, you will need three bearings – the thrust and both end main bearings. With the bearings and crank in place, set the main cap in place and lightly set the thrust cap with perhaps 10-15 lb-ft of torque on the bolts.
Using a rubber mallet, tap the crank forward. This will align the two thrust bearing halves at the rear. Now torque the main cap bolts/studs with the oil pump, if so equipped. Now we can measure the thrust clearance by first installing a dial indicator on the crank nose using a magnetic base. We pry back on crank using a long screwdriver, then zero the dial indicator and then pry the crank lightly forward to read the total clearance.
Each engine family has slightly different specs for crank endplay, but the majority of engine families fall in 0.004-0.008-inch range. We’ve created a chart listing the popular V8 engines endplay clearances expressed as a range. This puts the ideal clearance in the exact middle of the range. So for an engine with a range of 0.004 to 0.008-inch, the ideal spec would be 0.006-inch. Note that late-model engines attempt to limit the maximum endplay, because excess crank movement can cause crank sensor errors.
In the case of our big-block Chevy, with its new Number Five thrust bearing, we measured barely 0.002-inch of endplay. This will require us to thin the bearing, which is much easier than having the crankshaft flange milled. The most common way to trim the thrust bearing thickness is to sand them.
The bearing-thinning procedure starts by carefully placing a large stainless hose clamp around both thrust bearing halves with them sitting flat on a smooth surface. We use a 24-inch by 24-inch piece of scrap quarter-inch thick aluminum plate. We then measure the overall thickness of the thrust bearing end to end using a dial caliper. Most performance thrust bearings are now built with multiple small ramps that change the “height” of the thrust slightly so it’s important to measure in several areas to locate these ramps. We record the widest number for reference.
Then we place a large piece of wet/dry 400-grit sandpaper on the aluminum plate and cover it with light machine oil like Marvel Mystery Oil. We mark the rearward facing side of the thrust bearing for reference and then only sand the opposite or forward or front-facing side. This maintains the original thickness on the rear side that will receive the forward thrust from the clutch or torque converter. Sanding in a figure-eight motion, we will sand for a minute or so and then check our measurements. This whole process may take ten to twenty minutes or more to complete.
Once the desired overall thrust thickness is achieved, sanding the bearing with a lighter 1,000-grit or 1,500-grit paper can make the surface smoother. With all sanding complete, it’s best to remove the stainless steel clamp and then thoroughly clean the bearing halves with alcohol and a lint-free paper towel. This is crucially important since that oily slurry which is left over after sanding is ridiculously abrasive and we don’t want that going through the bearings.
After the bearing is clean, we reinstall the bearing and go through the endplay check another time. If you’ve done your math correctly, you should have an endplay clearance of somewhere near 0.006-inch as the number to shoot for.
In some cases where high loads might cause increased thrust bearing wear, as with constant acceleration/deceleration with road racing or circle track applications, or with drag cars with manual transmissions and high clutch loads, Mahle-Clevite suggests a minor modification to the rear thrust bearing. The accompanying photo shows the location of a minor 0.040-inch chamfer which can be placed in the upper bearing half at the parting line on the side of the bearing closest to the thrust.
Just like all other engine clearances, the thrust clearance is critical for proper engine performance. Even if sanding the thrust face is required, the task is not difficult or necessarily time consuming. The end result is a properly clearanced crankshaft, which ends up being its own reward.