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. Where a manufacturer has recommended tensions it may not always be understood why this can be so important.

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. The magic question: what did you use to tension up these flywheel bolts. There were four bolts holding the flywheel onto the crank. 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. 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 the approach of “good practice” we removed the original ring from the crankshaft and then we had the answer. 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. This indicated that only one-way fitting would be correct. A close inspection revealed that this steel ring had an inner chamfer at one end and an outer chamfer at the other.

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. The alloy carrier had also sealed against this plate, and dry, so that was not a problem either.

The large gasket at this joint was secure and dry. The plate between the block and the transmission was dry. With the engine out, we removed the flywheel and clearly sighted oil at the lower area of the neoprene seal. 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. 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. Authorisation was given to pull the engine.

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

6.7 cummins rear main seal leak