03 9372 3111 info@engineaction.com

Rear Main Oil Seal Leak


Our customer reported an oil leak beneath his forklift ten minutes after starting up the diesel engine that we had just reconditioned. We went to the vehicle and agreed that the slow leak was most likely coming from the rear main seal. Authorisation was given to pull the engine.



This engine had a new crankshaft  fitted in the work that we did and the rear main seal  was the three part design that is usually reliable. That is: an alloy seal carrier, a two lipped seal with the spring loaded oil sealing lip and the second lip that acts as a dust seal, plus a substantial ring that presses over the crankshaft flange to provide a seal track surface. Bullet proof?

With the engine out, we removed the flywheel and clearly sighted oil at the lower area of the neoprene seal. The plate between the block and the transmission was dry. The large gasket at this joint was secure and dry. The alloy carrier had also sealed against this plate, and dry, so that was not a problem either. It had to be the seal?

We opened up the new seal from the store and carefully noted the direction in which the steel  ring had been assembled into the new seal. A close inspection revealed that this steel  ring  had an inner chamfer at one end and an outer chamfer at the other. This indicated that only one-way fitting would be correct. By the assembly drawing, the ring was assembled with the inner chamfer towards the block and the outside chamfer facing the assembler to guide the fitting.

Taking the approach of “good practice” we removed the original  ring from the crankshaft and then we had the answer.

Why, having wiped oil off the rear of the crankshaft before removing this ring, was the crankshaft and the inside surface of the old ring wet with oil? Being a size-for-size press fit we expected to find both surfaces dry.

Taking a close look at the inner surface of the ring we found four distinct marks equally spaced across the full width of the ring. There were four bolts holding the flywheel onto the crank.

The magic question: what did you use to tension up these flywheel bolts. The flywheel bolts had been over tightened and this swelled the outside diameter of the crankshaft flange, lifting the ring off the surface at the points where the flywheel bolts screwed in, allowing oil to pass between the two surfaces.



Where a manufacturer has recommended tensions it may not always be understood why this can be so important. In this instance, the manufacturer knows that the rear of the crankshaft has bolt threads that come within 3mm of the circumference of the outside diameter of the flange where the seal track ring fits. The manufacturer also knows that over-tightening of the flywheel bolts WILL DISTORT the circumference and thereby also the seal track ring and lead to an oil leak.

What was going on with this example is that oil was bleeding between the ring and the crankshaft which bypassed the function of the seal resulting from the use of a RATTLE GUN instead of a tension wrench.

It needs to be noted that other manufacturers, who do not use  a seperate seal track ring, also have this design issue. It is not an incorrect design. But the design is very vulnerable to installing mechanics who regularly use the rattle gun to fit flywheel bolts and thereby stretch the assembly by over-tensioning the bolts.

To complete this installation, we smeared grey silicon on the crank before pressing the new ring into place in case the crankshaft flange was by now permanently distorted.