Changing oil seals

Colin Lowe decides to save some money and change the oil seal on his main crankshaft himself when a drip develops.

A continuous drip of oil from the rear of the diesel was an ugly sight for this technologically challenged skipper. Replacing the oil seal on the main crankshaft was daunting. However, on the bright side there were several factors in my favour – I had time, friends to call upon, a socket set, a workshop manual and a dash of optimism.

The engine was a 50hp Nissan four-cylinder diesel that was marinised as an auxiliary. I was spared the task of searching for parts because the previous owner had left a new main crank seal kit on board.

The job could be done in situ. Difficult, but the alternative was slipping to the hardstand and the employment of mechanics. In an ideal world the engine would be taken out and the whole operation performed on the turned-over engine on a support frame. 

I removed the oil sump pan and resealed it with a silicone gasket to see if that was the cause of the leak. Negative, so the rear oil seal appeared to be the culprit.

Mental factors

Philosophy 101 kicks in here. Listen up, Colin. While a mechanic has the hindsight of project knowledge, shortcuts and tricks of the trade, I would have to discover some of these as I went. Ambitious projects are made up of a series of incremental jobs, each by themselves quite achievable. I would have the luxury of time and equally challenged friends to analyse each step and what to do. The big difference with this job compared to a professional job would be the in-situ arrangement where the engine remained in place, saving the expense of outside assistance and machinery. The labour would be free.

The recipe

First, I removed the locking pin that tied the propeller shaft to the thrust bearing. This allowed me to slide the shaft aft and make room to take off the short length of shaft from the universal to the gearbox. Next, I made sure the engine was securely supported by constructing a wooden support that transferred the load down to the bilge sole. It was a simple arrangement of wood supports held tight using wedges. I could then remove the bolts that secured the engine feet to the gearbox assembly. The gearbox was removed from the bell housing by unscrewing six bolts.

After the starter motor was unwired and unbolted from the bell housing the bell housing itself was removed. I cleaned up and repainted any items while I had the opportunity. These were wire brushed, treated with rust neutraliser, then spray-painted with engine paint.

Removal of the shock-absorber plate exposed three countersunk Allen bolts fixing a mounting plate to the flywheel. One of the Allen bolts appeared to be seized tight, probably by some seizing solution applied during assembly. Judicial use of a gas blowtorch on that bolt and the surrounding area of the flywheel softened up the bolt-seizing compound sufficiently to allow me to release it. A pipe on a socket tool that held the Allen key gave me plenty of leverage. Separating the flywheel from the crankshaft flange was a relatively easy task.

The oil sump pan was unbolted to reveal the bottom bearing cap. The cap held one of the semicircular seals on to the crankshaft and was removed by undoing two bolts. The bottom seal was pushed into this cap making sure that a small section (1.5mm) stood proud in order to press up against the top seal when reassembled.

Fun and games

Now the real fun began. It was “fun” because in the bilge area, everything now was under the engine, and mirrors often had to be used. The old top seal was removed by pushing on one end, causing the semicircular section to slide around the crankshaft and drop out. I had difficulty in pushing the new one in because an internal oil port had sharp edges that caused the oil seal to catch on as it slid into position. I used the old oil seal to experiment by cutting minute bevels on the part of the seal that did not bear on the crankshaft. These bevels allowed the seal to slide past the oil ports without being caught up on the way. As mentioned, I left the ends of the seal proud so they would butt up against the ends of the bottom seal tightly, thus preventing leakage in that area. Some blue RTV silicone high-temperature-resistant sealer was applied to the bearing cap surfaces before I bolted it back to the cylinder block, making sure that the marks on the bearing cap and the crankcase lined up.

You can see now that the girls were often out on the town while the old man was occupied. Time stretched on.

The square-sectioned side oil seals were pushed up into their holes before the oil pan was sealed with silicone gasket then bolted back on.

All other items were then bolted back on, using the correct tightening torque. The supports beneath the engine could then be removed.

Clean as a whistle

To my astonishment and pleasure, when I first ran the motor after the repair, the drip zone in the bilge remained clean. The operation was a success. I will not divulge how long the job took, nor the number and variety of profanities uttered. Suffice to say, I enjoyed several Tuesday barbecues over the duration of the project.

What stops your engine from bleeding oil via the main crankshaft?

The rear oil seal is made up of two semi-circular metal reinforced rubber forms that fit around the circumference of the crankshaft. A flap of rubber slides against the crankshaft and the pressure of the oil ensures the seal is kept in place, thus preventing leakage of oil.

The top oil seal fits into a groove of the cylinder block and the bottom seal fits likewise into a groove in a solid metal block called the bearing cap. When the bearing cap fits snug up against the cylinder block, the ends of the two semicircular sections butt up against each other and thus form the complete circular seal around the crankshaft. Not surprisingly, the bearing cap carries one of the soft white metal main bearings on which the main crankshaft rotates. The bearing cap also has grooves on the two sides into which the square-sectioned rubber side oil seals fit.

The oil seal fails because the rubber surface in contact with the crankshaft wears away or perishes, allowing the oil to leak out. There is a good argument for keeping engine oil clean and refreshed, and filters in good condition. In an unused engine the seals can tend to bind to metal surfaces, breaking off a minute amount of rubber upon starting.