# This CAT 345C couldn't keep working like this! Replacing the rear crankshaft seal

> A Caterpillar 345C excavator leaking oil from behind the flywheel: removing the silencer, hydraulic pump and flywheel, a loose repair sleeve on the crankshaft, fitting the seal, sealing through-threads and checking valve clearance.

![This CAT 345C Couldn't Keep Working Like This! We Got to Work on the Engine](https://wibako.com/media/articles/article-846/izouinte8aq.webp)

A **Caterpillar 345C** excavator came to us with a leak that could no longer be ignored: **oil from behind the flywheel was escaping through the starter opening**. The cause was fairly obvious – a worn rear crankshaft seal on the flywheel side. Getting to it, however, meant taking a large part of the machine apart.

## The way to the seal

- draining the oil and disconnecting the hydraulic lines,
- removing the large silencer – the opening was plugged with rags so nothing could fall in,
- removing the hydraulic pump,
- removing the flywheel – behind it a heavy black oil leak was visible straight away, along with plenty of silicone from earlier sealing attempts.

## A surprise: a loose repair sleeve

After removing the old seal it turned out that the crankshaft already had a **repair sleeve** – a ring heat-fitted over the factory surface when that surface has been grooved by an old seal. The problem was that the sleeve was **loose – it could be removed by hand**, and the surface underneath was worn. A new seal could not work properly on such a base. After assessing the situation we used an **oversize seal** matched to the condition of the shaft, together with an additional O-ring seal in the flange.

## The detail that determines tightness

The threaded holes for the flywheel bolts are **through-holes** at this point. If they are tightened on oily threads – even with thread locker – oil can seep out along the thread. That is why we cleaned and degreased the threads thoroughly and fitted every bolt with sealing thread compound. Then we fitted the flexible coupling, the hydraulic pump and the remaining parts, filled the oil and bled the system.

## Valve clearance check

At the customer's request, as part of periodic maintenance, we also checked the **valve clearance** – on a cold engine, as adjustment is more accurate than on hot manifolds. The clearance was within specification and needed no adjustment. The valve cover was refitted with a new gasket.

## Result

After reassembly the machine was started and checked – all functions work and the engine is leak-free.

## Takeaways

- Oil coming out of the starter opening is a typical symptom of a leaking rear crankshaft seal.
- When replacing the seal, assess the crankshaft surface – a loose repair sleeve or a worn groove will defeat a new seal.
- Through-threads must always be cleaned, degreased and sealed, otherwise oil will find its way along the bolts.

See our [combustion engine overhauls](https://wibako.com/services/overhauls-of-combustion-engines) and [construction machinery service](https://wibako.com/services/construction-machinery-service).

**Watch the repair video:** [This CAT 345C Couldn't Keep Working Like This! We Got to Work on the Engine](https://wibako.com/about-us/video/846-this-cat-345c-couldn-t-keep-working-like-this-we-got-to-work-on-the-engine)

Published: 2026-09-04

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Source: https://wibako.com/about-us/news/847-this-cat-345c-couldn-t-keep-working-like-this-replacing-the-rear-crankshaft-seal
Contact: +48338704200, biuro@wibako.pl