Mercruiser Transom Assembly Replacement

Mercruiser transom assembly on white background

A leaking transom assembly is a common problem found on boats over 10 years old fitted with sterndrives, living permanently in saltwater.  Many popular makes such as Sea Ray, Riviera, Sunrunner and Mustang will all likely need a transom assembly replaced or repaired at some point.

But what’s all the fuss about?

After completing the process on both boats that I’ve owned, I saw this as an opportunity to document and photograph the entire process, and share it with other boaters.

Firstly, the most common reason the assemblies need replacing is because in most cases, the steering pin seal leaks. Once it starts to leak and is left untouched, it corrodes the surfaces where the seal sits. This means that installing a new seal won’t work because it won’t seal. The seal is located approximately where the arrow sits in the photo below. However, it can only be accessed internally when the engine is removed, without modification. It is possible to cut holes and install bungs, but in my case, the assembly was too heavily corroded.

Mercruiser Transom Assembly and Sterndrive exterior
Location of steering pin externally
Mercruiser steering pin leaking
Aftermath of leaking steering pin internally (engine removed)

When repairing my boat, unfortunately on the port side, the steering pin had been leaking for so long it had corroded a hole the entire way through the transom housing! If the water line had dropped an inch or so, the boat would be constantly taking on water and likely sink.

Corrosion Hole in Mercruiser Transom
Corrosion hole in Mercruiser transom

So what’s the process of replacement?

Day 1

The first day was spent slipping the boat, then removing the hatch. Once that was out of the way, I had access to disconnect the engines. Before disconnecting the engines, I removed both sterndrives. Upon removing the port side I found a shell blocking the water intake, which was the answer to an overheating problem I was also looking for. This is pretty common to find when a sterndrive boat hasn’t been slipped and legs split in over 12 months.

Blocked Mercruiser water intake
Blocked water intake
Sunrunner 3300 hatch removed
Hatch removed for access to engines

Now I was ready to disconnect the engines. This involves removing all the mounting bolts (2 per side and 2 at the rear), fuel connection, electrical harness, throttle, shift, trim and mercathode wires, power steering lines, exhaust droppers and rubbers, water inlet to sea pump, positive supply to starter, ground wires on belhousing and leg overflow lines on both engines. Sure enough, by early afternoon I was ready to crane the engines out.

Sunrunner engine removed
Starboard engine removed with exhaust horn detached
Two Mercruiser V6 engines on hard stand
Both engines removed and on the hard stand

I moved onto removing each transom assembly, getting the starboard side out but the port needed a little more persuasion. It was getting late, so I decided to call it a day and come back fresh in the morning.

Day 2

First up was removing the port transom assembly, which was the side with a hole in it. A large sledgehammer and about 20 minutes of banging – and sure enough out it came. The transom timbers were in good shape with no signs of rot, so there were no repairs required. Knowing the transoms had been leaking for such a long time, I anticipated that the exhaust horns would need replacing, and sourced two new (second hand) horns before organising the job. Lucky – because the old ones were heavily corroded.

Mercruiser bullhorn corrosion
Corrosion to bullhorn sealing surface

A few hours of scrubbing and vacuuming the water out, and sure enough the bilge was now clean. I then spent the rest of Day 2 fitting the new transom assemblies, exhaust horns and changing the exhaust manifolds and risers on the port motor.

Sunrunner 3300 bilge clean and new transom assemblies installed
Bilge clean and new transom assemblies installed

Day 3

The pressure is on – the boat was due to go back in the water in the afternoon and engines were still on the hardstand. The morning was spent tidying up the engines, getting them ready to be refitted. New thermostat and gasket was fitted on the starboard motor, then I cleaned as much corrosion as possible, and painted the motors. The port side exhaust manifold elbow also cracked, so a new one was sourced and fitted. By about 11am, the engines were getting craned back in. Fast-forward to 2pm, they were all bolted down and alignment done, ready for the sterndrives to be fitted. Once the sterndrives were fitted, the Marina was notified that we were ready to float. By about 4pm, the slings were being connected, and we were floating at 5:30pm.

Two Mercruiser V6 MPI motors fitted to Sunrunner 3300
Engines refitted

 A few leaks from the exhaust droppers were found, but an adjustment of the rubbers and a few new hose clamps and 30 minutes later it was ready to drive. I parked the boat at the marina overnight while all minor adjustments were made (leaks, shift cables etc.).

Day 4

The fourth and final day consisted of cleaning the now extremely dirty rear deck, and refitting all seating and upholstery. After this, a test run was done, finding a few more small leaks (more hose clamps needed replacement), top up of oils and we were good to go!

So there you have it – that was my 4 days of replacing a pair of Mercruiser transom assemblies. I will also note I inspected the boat before taking on this job, to ensure engine compressions were okay and fit to reuse, as the job wouldn’t be viable if there was more engine work to do. Find out how much a marine survey costs and what entails a good pre-purchase marine survey here.

Feel free to contact me with any questions you might have. I no longer offer mechanical repairs, but am happy to chat all things boating and repairs if you need help or a second opinion. You can reach me at or call on 02 9188 5182.

How long do Mercruiser transom assemblies last?

This depends on the usage, if installed on a moored boat that lives permanently in the water, and the waterline is close to or above to steering seal, it’s not uncommon to get 10-15 years before replacement is required. If the waterline is lower than the seal, it’s possible to get even longer.

Can you repair a transom assembly?

Yes, while it is possible to repair, it will depend on how long it has been leaking for. In some cases the engine will need to be removed to assess first, and if it has been leaking for a long period there can be other items that need replacement too, for example the exhaust droppers.

Do Volvo transom assemblies leak?

Yes, it is possible for Volvo transom assemblies to leak in a similar manner. Volvo uses a different seal setup, has a number of different transom assembly designs, and as a result are less prone to leaking than a Mercruiser.


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