Engine Building Fundamentals: Measuring Thrust Clearance
Measuring Thrust Clearance
- While, technically, it's possible to measure the thrust bearing width and the width between the thrust surfaces on the crankshaft and calculate the thrust clearance, I prefer to make this measurement with the crankshaft and bearings installed in the engine block.
This way, we're actually getting a realistic measurement of the end float under normal operating conditions.
The way we do this is to set up a dial gauge so that it's located on the nose of the crankshaft and directly in line with the crankshaft.
Now, using a pair of pry bars or large screwdrivers, the crankshaft can gently be manipulated back and forth in the engine block.
What we want to do is note the difference between the two extremes of movement on the dial gauge and this indicates our thrust clearance.
One key point to note, here, is that if your engine uses a full 360 degree thrust bearing where half of the thrust bearing is located in the engine block and the other half is located in the bearing cap or cradle, it's possible to artificially reduce your thrust clearance by offsetting the bearing cap or cradle slightly in relation to the block.
This, however, will become apparent when you go to measure the clearance.
A good example of this is the early Mistubishi 4G63 engines used in the EVO I to IV models which used a 360 degree thrust bearing and a full cradle.
Since there is some clearance between the diameter of the holes in the cradle that the bolts pass through and the actual diameter of the bolts, it's possible to offset the cradle either forwards or backwards relative to the block itself.
At either extreme, this would result in a severe reduction in end float and the thrust load would now also effectively be supported by only one half of the thrust bearings, too.