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IMS Failure of the M96 Engine

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IMS Failure of the M96 Engine

The new water cooled engine design of the 996 Carrera and 986 Boxster, meant there were fewer machining operations and parts to make, transport and assemble, which was all part efforts to streamline finances and quicken production times.

But like most mass produced products, the low cost design would make internal repairs more expensive.

The most reported issues for most 996/986 owner is the possible failure of the IMS or Intermediate Shaft, a chain drive system that provides a camshaft speed reduction without needing huge sprockets on the camshaft ends. It runs at 2/3 of the camshaft speed which is ideal to drive an oil pump, which it is also connected to.

One end is a solid steel shaft running in a hole in the aluminum casing and is trouble free. The other end has a grease filled ball bearing that seals on both sides, which provides heavy interference and makes some bearings run tighter than others.

Although the design spec of the bearing is adequate, small particles of metal are rubbed off the bearings internals when running and have nowhere to escape to. So they mix with the grease and form a fine grinding paste.

With no flow of cooling oil, the bearing runs hot and expands. If the clearance becomes too tight it strains the casing until it fractures from metal fatigue. Or it can wear down the balls until they cracked the cage and destroy the bearing, after which the debris circulates through the engine which can score bores, damage a cam chain and bend valves.

To add to this, the spindle has grooves machined into an already small diameter, creating a stress point that can easily snap.

The issue can be easily resolved by fitting the thicker spindle and changing the bearings from double row to bigger single row. Simple but expensive. This preventative measure could mean the difference between many happy miles or a blown engine.