Checking and setting proper crankshaft end play is a vital engine-building step. We show you how to do the job in this tech segment.
Engine building isn’t difficult – as long as everything goes according to plan. The challenge for the engine builder is to anticipate problems before they occur. Much of the process of becoming a successful engine builder is checking all the clearances and custom setting them when they are out of tolerance.
Crankshafts are generally among the most abused components in an engine. One way to minimize this abuse and maximize the crankshaft’s opportunity to deliver a long service life is to make sure all the clearances are correct. For this story, we’ll be looking at thrust clearance or what is often called end play. This is the amount of clearance between the crankshaft’s thrust plate and the vertical surface of the main thrust bearing.
It’s worthwhile to discuss first why it’s important to have a thrust bearing. There are transmission loads that tend to force the crankshaft forward. With automatic transmissions, this can originate from the torque converter. This should never exceed a light forward pressure, but this load does exist and must be accommodated.
|
Engine |
Crankshaft End Play Clearance (inches) |
|
Small-block Chevy |
0.003 - 0.011 |
|
Big-Block Chevy |
0.006 - 0.010 |
|
GM LS Gen III / IV |
0.0015- 0.0078 |
|
302-351W Ford |
0.004 - 0.008 |
|
429 – 460 Ford |
0.004 - 0.008 |
|
Ford Modular 5.0L |
0.004 - 0.008 |
|
340 – 360 Mopar |
0.002 - 0.007 |
|
440 Mopar |
0.003 - 0.007 |
|
Mopar Gen III hemi |
0.002 - 0.011 |
Ideal clearance would be mid-point between these minimum and maximum clearances.
Manual transmission thrust can be excessive with pressure plates that generate high static loads. The most abusive of these are the three-finger style pressure plates that use internal coil springs. With the clutch pedal on the floor, a major portion of the load released by the clutch pedal is directed forward into the crankshaft. These pressure plates are most often used in race engines, explaining why it’s always best to start an engine with the transmission in neutral so that the crank spins with no forward load. Starting a cold engine (when most of the oil has drained from this area) with the clutch pedal on the floor places tremendous load on the thrust bearing. It’s best to avoid this by starting the engine with the transmission in neutral.
For this checking example, we will be using a K1 steel crankshaft in a Dart Little M cast iron small-block Chevy. It’s always best to test fit all clearances for a new engine before final installation in case modifications are necessary. For this application, we pre-assembled the rear main thrust along with the Number One main bearing, dropped in the crankshaft, and installed the main caps with the studs lightly tightened.
Before fully torquing the main studs, it’s necessary to align the two pieces of the thrust bearing. To do this lightly hit on the rear of the crank with a rubber or plastic mallet. This will ensure the thrust surfaces are even from the rear which is where all the force will originate. This ensures the paired bearings are parallel. With this accomplished, the main caps can be torqued to the proper spec.
Next, you will need a magnetic base and dial indicator. Align the dial indicator plunger parallel to the crank snout and lightly force the crank backward and zero the dial indicator. Now lightly force the crank forward and read the amount of movement on the dial indicator. Different engines demand varying specs. Generally speaking, keeping the thrust clearance at 0.004 to 0.005-inch is appropriate but it is best to check the recommended clearance. For example, late model engines prefer a slightly tighter clearance to minimize travel of the crank sensor reluctor wheel. We’ve included a chart listing factory endplay dimensions for some of the more popular performance engines.
If when assembling an engine you discover the clearance to be very tight, there is a simple way to increase the clearance. The generally accepted procedure is to clamp the two thrust bearings together with a hose clamp, making sure the thrust surfaces are aligned and flat. Also make sure the two halves are clamped as they sit in the engine – it’s possible to orient them incorrectly which will not produce the results you desire. Always position the two halves with the locating notches facing each other. Then place a full size sheet of 600-grit wet/dry sandpaper on a large plate of either plate glass or flat metal plate. Add a few drops of machine oil like Marvel Mystery oil to the sandpaper.
It’s best to sand only the leading edge side of the thrust bearing when increasing clearance. This way, the thickest portion will be the trailing side which is where any wear will occur. Measure the combined width of the thrust bearing across both wear surfaces with either a quality dial caliper or a micrometer. We generally see a slight difference in thickness of perhaps 0.001-inch across the thrust bearing face.
Record this dimension and then keep sanding until you gain the necessary clearance. Generally you may need to only increase the clearance by 0.002- or 0.003-inch, but you will be surprised at how much sanding this will require. Some engine builders will lightly dress the sanded face with 1000 grit paper to polish the surface once the proper clearance is achieved. Of course, a thorough cleaning with hot soapy water and a sponge followed by a wipe-down with rubbing alcohol and a white paper towel is necessary to ensure that all of the sanding grit is removed before the bearing is re-inserted into the engine to recheck the clearance.
Crankshafts that are discovered suffering from excessive thrust clearance are rare, assuming no damage has occurred to the crankshaft. Alternatives may be to try a different bearing manufacturer to see if the clearance will improve although this is unlikely. The only other solution is to have the crankshaft repaired to put the thrust thickness back to its stock thickness. This may cost nearly as much as the price of a new crankshaft.
Checking and setting clearances is all about stacking the longevity odds in your favor. The payoff is when that engine starts and runs properly and delivers a long, productive, and powerful life.
