IMS is an acronym for intermediate shaft.  The intermediate shaft is part of the valve timing mechanism, which links the camshafts to the crankshaft, to ensure that valves open and close at the right points in the combustion cycle.  The problem – such as it is – relates to the IMS bearing, rather than the IMS itself, and it is the Achilles heel of Porsche’s M96 series of engines, used in 986 series Boxsters, 996 series 911’s and the very first cars in the 987 and 997 series.
 
Over the life of the M96 engine design, two different IMS bearings were used.  Early cars had a double row bearing and later cars had a single row bearing.  There is no way, short of inspecting the bearing, to know whether a particular car has a single row or double row bearing.  It’s generally safe to assume that cars from 1998 – 2000 will have a double row bearing and cars from 2003 – 2004 will have a single row bearing, but it seems that the crossover was a fuzzy one.  Even the car’s VIN will not provide a definitive answer.
 
If IMS bearing failure is not detected as a rumbling in its early stages, the bearing breaks up, taking the engine with it.  Unfortunately it’s impossible to know exactly what ‘some cars’ really means.  Trawling the internet will reveal estimated failure rates of anywhere from 1% upwards, but it seems that nobody really knows what the failure rate is.  It does seem that cars with the earlier double row IMS bearing are less susceptible to this failure and that cars that are driven enthusiastically are less likely to have an IMS bearing failure than cars that are mollycoddled.
 
There are various aftermarket solutions out there, and the market leader is the L&N Engineering IMS Solution.  The recommendation is that this part is treated as a consumable, and replaced when the clutch is replaced.  Replacing the IMS bearing with the clutch lets you save on workshop labour cost (or time if you are tackling the job yourself).   Given that most of these cars are likely to be used as weekend playthings, in practical terms this means every 5-10 years.

Important Caveat: Installing an IMS bearing is a non-trivial task.  Get it wrong, and the consequences will be serious, immediate and expensive to remedy!  Give very strong consideration to letting a Porsche specialist workshop tackle the installation.
 
With the flywheel removed, you should also inspect the rear main seal.  If the seal is weeping or leaking, it needs to be replaced.  If the area around the rear main seal is dry of oil, you will need to make a call as to whether it is worth replacing anyway.  The advantage to replacing a still-dry seal with a new one is that the old seal is likely to be brittle and may be nearing the end of its life.  Best advice is to replace the rear main seal while you’re there.  Oil the new seal as part of installation, and ensure that it is installed straight.
 
The question of whether to go the upgrade route is best answered by Clint Eastwood’s famous question “Are ya feeling lucky?”  From a rational, statistical standpoint, it may not be worth doing.  However, as the actual failure rate is unknown, and the costs of failure are high, it’s worth doing for the peace of mind.  Installing an uprated IMS bearing allows peace of mind by mitigating a potentially very expensive risk.  (It’s the mirror image of buying a Lotto ticket, where the costs of buying are small but the potential gains are huge, which trumps the “idiot tax” aspect of gambling.)
 
The next question is whether you should be tackling this job yourself.  And the likely answer is “probably not”.  It’s a procedure with enormous potential for expensive and heartbreaking error.  It’s the sort of job that should only be done by somebody who’s done it before and understands what needs to be done to avoid destroying your engine’s valve timing.  Master Parts newsletter subscriber Pamela Talaue opted to entrust the work to her chosen Porsche independent workshop, and you can read her story here . 
 
If you’ve read this far and you want a modern 911, Boxster or Cayman, but don’t want to worry about IMS bearings, all is not lost.  Apart from the very first cars off the production line, the first generation 997 and 987 engines used a beefier IMS bearing, which seems more reliable than that used in 996 and 986 series cars.  The second generation 997 and 987 cars had a totally new engine design, often referred to as the DFI (oh yay, another acronym!) engines.  DFI stands for direct fuel injection; in these engines fuel is injected directly into the cylinder instead of into a plenum.  These engines do away with the intermediate shaft completely.
 
And there is another way…   The engine used in 996 and 997 Turbo, GT3 and GT2 models is a totally different design.  This engine is known as the Mezger engine, after the man who designed it.  The Mezger engine traces its lineage from the original air cooled flat 6 engines through to the watercooled endurance race car engines of the 90's.  It's a good way to avoid the potential issues associated with IMS bearings, but it's an expensive option.
 
Parts Needed
IMS Solution – Double Row or Single Row
IMS Toolkit – For removal of old IMS bearing and correct installation of new bearing.  Note that this tool kit does not lock the valve timing.
8 Flywheel Bolts (because the flywheel needs to be removed to gain access to the IMS cover)
Clutch Kit
The following are available:
Boxster 2.5 Clutch Kit
Boxster 2.7 Clutch Kit
Boxster S Clutch Kit
996 Carrera/Carrera 4 Clutch Kit 1999 – 2001
996 Carrera/Carrera 4 Clutch Kit 2002 – 2004
Flywheel for Boxster 2.5/2.7
Flywheel for Boxster S
Flywheel for 996 Carrera/Carrera4/C4S
Rear Main Seal