Sep 10, 2010

Intermediate Shaft (IMS) Bearing Info and Fixes

LN Engineering upgraded dual row IMS bearing and flange.
If you're considering a Boxster and have done your homework, you've no doubt raised an eyebrow at stout warnings regarding intermediate shaft (IMS) bearing failures sending engines to the scrap heap. No doubt, one is shaky to pull the trigger on a Boxster when presented with "there's this thing called an IMS bearing that can fail and blow up an engine that you can't afford to replace." On the other hand, if you already own a Boxster, you've probably turned the key at least a few times and thought "is my engine going to turn into an eligible candidate for an enthusiast's coffee table?"

Fortunately, there are kits available to replace the IMS bearing and prevent your engine from spending the rest of its life in a living room. What's more, they're not extremely expensive nor are they extremely hard to install. The kits cost ~$600-$700 depending on what type of bearing you have.

Click the link to read more.

Here I'll present the most current and pertinent information (as of the publishing of this post) on the subject and a look at what a DIY replacement job consists of.

So first:

What is an intermediate shaft and what does it do?
In short, the intermediate shaft times the engine. More specifically, it turns the camshafts on the heads and also turns the oil pump. It's necessary because the camshaft sprockets for each head are located on opposite ends of the engine block. I speculate that this is done so that the same head casting can be used for both heads, i.e. the head on one cylinder bank is the same as the head on the other, just rotated 180 degrees about the vertical axis.

The IMS sits just below the crankshaft. There is a chain on the flywheel side of the block that connects the IMS to the crankshaft, another chain on the flywheel side that connects the cams on the passenger side head to the IMS, and then on the accessory belt side of the block there is a chain that connects the cams on the driver's side head and a sprocket that turns the oil pump.

This illustration makes it clearer. The closer side is the accessory belt side, the farther side is the flywheel side:

(right click > view image or show image for larger size on any pics)

How common are these failures?
It's generally believed to be a small percentage, but this figure is meaningless as it is likely to depend on mileage etc. There are still plenty of low mileage Boxsters out there; these cars could suffer an IMS failure later down the road when they get into the 100k-200k range.

It's not something that predictably affects every single car like VW's 1.8T timing belt issues. There are cars that have 100-200k or more on the original IMS bearing. However, it DOES HAPPEN. Some choose to blow off the failure as if it's not going to happen to them, but there is no reason to believe it won't. In the 3 months or so that I spent looking for a Boxster, I saw at least 3-4 cars with IMS failures in my area.

A used replacement engine runs anywhere from $5,000-7,000 which is approaching the value of the entire car in some cases. Mechanically totaling a car is not something one wants to play the odds on.

So what, exactly, fails?
IMS failures are almost always caused by something involving the bearing on the flywheel side of the IMS (the bearing on the other side is different and doesn't suffer failures).

The bearing is actually located inside the end of the intermediate shaft. The outer race of the bearing is snugly fit into the end of the hollow intermediate shaft. The inner race of the bearing then sits on a flange, which is bolted to the crankcase. A stud that runs through the middle of the bearing holds the bearing in place on this flange.

This pic shows the flange, held on by three bolts, and the central stud that holds the bearing onto the flange:

Usually, one of two things will happen to cause an IMS failure. Either the bearing will wear down to the point where it disintegrates or the central stud will snap. If the stud snaps off there's a chance that the IMS will stay in place and not cause any damage, however if the bearing disintegrates the IMS is pretty much guaranteed to fall out of place. If the IMS does fall out of place, the timing chains skip timing. From there, pistons hit valves and can swiftly destroy the heads, cylinder walls and other things that one wouldn't want to pay for replacing. After a failure, a rebuild or replacement is the only option.

What causes the bearing to fail?
There's much speculation regarding this. The main issue seems to be that the bearing is a sealed bearing, which is an odd choice for an oily environment. The bearing has seals on either side of it that are supposed to keep it sealed from engine oil. However, these seals don't seals don't do their job very well; engine oil gets inside the bearing, cleans out its grease and often gets trapped inside the hollow intermediate shaft where it slowly deteriorates. With its factory lubrication gone and a pool of slowly rotting synthetic dinosaur soup sloshing around inside it, the bearing starts to wear rapidly.

As far as the stud failures, the stud is fairly small and therefore has a tendency to break.

Can I prevent the failure easily or detect an impending failure?
Some speculate that changing oil more often or removing the seals on the bearing so it gets fresh oil can help prevent the failure.  I wouldn't count on either saving an engine from the scrap heap as there's no way to gauge whether the bearing is already deteriorating or how long it will last.

There are no signs of impending failure. Upon failure, many report an obnoxious rattling noise as the bearing disintegrates; at this point, it's too late.

So what kits are out there?
Currently, the only IMS bearing replacement kit on the market in the US is made by LN engineering and consists of an upgraded bearing and new flange.
LN Engineering IMS Retrofits

The LN bearing is superior to the OEM one in that it uses ceramic balls, which are several times more durable (and expensive) than their metal counterparts. Clearly, balls of steel simply aren't good enough to last in a Porsche engine.

The kits cost $520 for the single row bearing version and $600 for the dual row bearing version. Which version is required depends on what bearing is currently installed in the car. The custom bearing extractor (which is an essential for removing a dual row bearing) is another $70. Their page gives some background info about IMS failures etc as well.

This is what you get with a dual row bearing kit + bearing puller:

Pelican Parts is developing a cheaper kit that consists of a brand new OEM bearing with improved seals and a bigger stud. The kit will cost around $150 when it's released; according to Wayne Dempsey (owner of Pelican Parts, writer of all the tech articles) it is being tested now and should be out in December or January. While it's far cheaper, it's for the most part the same bearing, albeit with upgraded seals and stud.

In my opinion, if going  through the expense/trouble to have the bearing replaced, one might as well go with the superior LN bearing. What's spending an extra $500 to potentially having to spend over $5,000 on a new engine? On the other hand, when I removed my OEM dual row bearing after ~56,000 miles, it was in very good shape. The OEM bearing does have the potential to last, but some of them have been known to fail very early on.

Wayne has stated that the Pelican Parts kits will require bearing replacement every 50k. LN Engineering recommends that their bearing be inspected every 50k as a precaution. Here's a quote direct from Charles Navarro of LN:

We're recommending to check the bearing in four to five years or 50-60k miles as a precaution, as we've only had our ceramic bearings in service for about three years now. To date, we've had zero issues or failures of the ceramic bearings or any retrofit kits and prior evidence would suggest that there should be zero issues with the bearings provided.

It is our plan to re-evaluate this recommendation in another year or two and we expect our IMS solutions to be permanent fixes not requiring replacement during a reasonable lifetime of an M96/M97 engine.

Does my car have a dual row or single row bearing?
Since IMS bearings come in both dual row and single row versions, one has to choose the right type of replacement bearing kit to match what's in the car. There's generally a split somewhere around 2000-2001where all models previously had dual row and all models after have single row. The only way to tell for sure is to look at the IMS flange, i.e. take off the transmission/clutch etc and have a look. LN Engineering's site details the differences between dual row and single row flanges.

According to PET (Porsche parts catalog software), engines up to a certain engine code had dual row and afterwards had single row. The specific engine codes are available on LN Engineering's page. This can't be determined by the VIN, only by an engine code stamped on the engine itself.
The engine code is located on a small stamped plate on the passenger side of the block near the oil pan. Often it's too dirty to read, so it may require some cleaning first:

This is not always 100% accurate though, so it's a better idea to order the bearing once the car is apart and one is absolutely certain of what's in it. LN also recommends that an uncertain buyer can buy both types and return whichever one they don't end up using.

What's involved in replacing the bearing? I've heard the engine has to be taken completely apart.
The engine does not have to be taken apart! This used to be the case with dual row bearings as they are held in place in the IMS with an internal circlip that required lots of force to remove, putting excessive stress on generic bearing pullers and the crankcase. Now, LN has their custom bearing puller that rests on the intermediate shaft itself and makes pulling the dual row bearing out a non-issue. No engine disassembly required!

The procedure will be covered in depth later - but essentially it's the same procedure as doing a clutch job but with an hour or two of extra labor for removing/replacing the bearing. Since the bearing flange sits behind the flywheel, a clutch job is the perfect opportunity to replace the bearing. The bearing replacement procedure itself is not much more involved than pulling out the old bearing and tapping the new one into place.

What shops can do this for me? Can I do it myself?
LN Engineering has a list of approved shops for the installation - some Porsche dealers can do it too, but obviously it'll be expensive through them.

It can be a DIY job too! One just has to be careful to not mess up the engine timing while doing it. A good, experienced DIY mechanic that is not intimidated by the idea of doing a clutch job on the Boxster can most likely do the IMS bearing replacement themselves.

How hard is it to mess up the timing? What if I do?
This depends on the engine. Boxsters up until 2003 had "five chain" timing systems, which are harder to skip timing on as the chains have quite a lot of tension on them. 2003 and up Boxsters have "three chain" engines, which are more vulnerable.

Overall, as long as one is careful to set the engine at TDC, loosen the chain tensioners and not do something stupid like turn the engine over while the IMS flange is off, the timing should stay put.

If the timing does jump, best bet is probably to have the car towed to a shop to re-time it. It can be a DIY job, but the specific Porsche timing tools required cost hundreds of dollars.

I want to do the install myself, where can I find DIYs, instructions, etc?
The LN kits actually come with written instructions. I will be sharing some pics/tips from my DIY replacement job as well, but there's already numerous DIY articles detailing the process as well as threads by people who have done it previously.

I spent time reading through numerous articles and DIYs so I'd know what to's some that I consulted:

Pelican parts has an IMS article, as well as lots of other articles detailing transmission removal etc that are needed in order to get to the IMS bearing.

Flat 6 Innovations also has a (somewhat outdated) article. Notice they're using an old puller, not the new custom one included with LN kits.

Here's some various threads from people that have done it themselves:

How long does it take?
An hour or two on top of a normal clutch job. It's hard for me to say absolutely as I did it in conjunction with a ton of other maintenance/mods on the car on a "few hours after work a day" basis. I think with two people working, it could be done in a weekend, but it'd require a lot of work each day. Random setbacks like stripped bolts etc can really slow the process down too. There's just a lot of nuts and bolts etc that have to be taken off to get to the IMS, which can take a while if they strip, or if you've only got hand tools, or if you've only got one pair of hands working.

Basic Overview of an IMS Retrofit Installation:

As always, be careful and work on your car at your own risk!

This isn't intended to be a set of DIY instructions, but rather just an inside look at what one might expect to see/deal with on the car. Always helps to know what you might run into.

This is what I started with:

I put the car up on four jack stands for more room and to facilitate removing the transmission more easily. I put the wheels under the car as an added safety measure.

First step is to remove the chassis bracing, which consists of two big diagonal braces and a thinner metal plate:

From here, the swaybar needs to be removed (easy). When reinstalling it, apply some grease to the bushings; mine were completely devoid of grease. I was previously tempted to put a big orange triangle on the back of the car as it was making squeaking noises like it had Amish buggy suspension technology.

Next step is removing the muffler. The clamps on my exhaust were hopelessly rusted and the muffler/exhaust were all seized together:

Since nothing would come apart, I decided to remove the muffler, midpipes and headers all as one unit. Removing the rear bumper makes it 100x easier to remove the muffler and will make accessing the back of the car way easier throughout the whole process, so now is a good time to take it off.

With the exhaust and bumper out of the way, there's easy access to the transmission:

Here's all the stuff that's been removed so far. Nothing like ferrous lawn ornaments to brighten up the yard.

Next is removing the axles, which requires great care. The axle bolts are made of a metal that bears the structural properties of peanut butter. I stripped three or four of them from not having my bit seated right. Perhaps I should have used a spoon.

I'd suggest seating the bit, hitting it with a hammer a few times to make sure it's COMPLETELY seated, then trying to loosen the bolt.

That said,  an angle grinder helps for bailing one's self out in the event that the bolts do strip.

Axles off. I was replacing the CV boots as well so I cut part of the old ones off for better access:

Now comes unclipping the shift linkage, unbolting the shifter bracket, unbolting the slave cylinder and unclipping the single wire connector at the back of the transmission...then time to tackle the bell housing bolts.

One of the bolts is a 12 point head which requires a 12 point bit (got mine at AutoZone) - the one I had was too long to fit in the space where the bolt was, so I cut it down with my trusty angle grinder:

The bolt in the back amongst the black oil residue is the one I'm talking about. The remains of some of the axle flange bolts I had to cut off are visible too.

Some of the bolts on the top of the bell housing must be removed through the top of the engine bay, which looks like it's been painted with dust (powder coated?)

The intake tube also has to be removed.

Now there's access:

I printed off this page from the Pelican Parts transmission removal DIY to help keep track of the bell housing bolts. Careful as the dimensions listed for each bolt are not 100% accurate for a Boxster S! I spent a while second guessing myself as I measured the bolts before reinstalling them and they didn't match the measurements listed.

$80 transmission jack from Harbor Freight going in now. I don't suggest trying to drop the transmission with a regular floor jack as the transmission seems like it would be prone to spilling like a big,  alkaline Earth metal cocktail. The jack has a ratchet strap that holds the transmission in place and prevents the spilling of expensive magnesium cast gearboxes and makes a nifty little dolly for wheeling the transmission around the garage.

The transmission actually has little fins on the bottom that correspond to the channel in this jack. Must be a dead knock off of a Snap-On or something.

I supported the engine on the main boss using a floor jack but a jack stand should be used. I didn't have a spare jack stand to use as I was using four to hold up the car. The engine actually has a steel cable that holds it to the frame and prevents it from falling so the engine can't be used for testing the tensile strength of the wiring harness.

After tweaking the jack and doing some major pulling, the transmission came off:

Unfortunately it can't be removed completely with the transmission mounts still in place so I had to remove one of them (ended up removing both to make the reinstall easier).


The pressure plate bolts are also made of a similar peanut butter alloy like the axle flange bolts; they strip easily and I'd suggest replacing them.

I decided to replace my throw out bearing, which sounded like the wheels on a Hotwheels toy:

Flywheel still looked usable, as did the pressure plate:

Clutch disc on the other hand...uhhhh:

I replaced that. Wasn't slipping or anything, but the clutch engages more briskly with the new one now.

Now, finally, the IMS flange is visible:

As you can see the car also has a leaky rear main seal. Now is the time to replace it (there's DIYs out there for this).

Yuck! I'm pretty sure one could make asphalt binder out of the gunk that's residing on the back of the crankcase.

Before proceeding farther, the engine must be rotated to top dead center (TDC). Removing the spark plugs makes the engine easier to turn. There's a mark on the crank accessory pulley indicating TDC. Getting at this pulley requires accessing the engine from the passenger compartment:

Also, removing the cam plugs allows one to see and mark the position of the cams. This is used later to verify that timing hasn't shifted. The cam plugs are rather tightly fit (the tool shown ended up not being strong enough to remove them) and are one time use only, so replacements are required ($10/each from the dealer!).

A 5-chain engine, such as mine, only requires removal of the exhaust (bottom) cam plug on each head as this is the cam that the timing chain attaches to. The intake cam is linked to the exhaust cam by a chain inside the head; therefore if the exhaust cam is in time, the intake one is as well.

Next, the chain tensioners must be loosened. They are big 32mm hex heads. There is one on the driver's side, oriented horizontally:

There's a corresponding one on the passenger side, oriented vertically. Don't have any pics of that one, sorry!

I loosened them about this much, as per the Pelican Parts article:

Now, the IMS flange can be removed to expose the bearing:

The puller tool ready:

The puller tool in place. I also positioned an oil pan underneath to catch the oil that's inside the IMS.

Most people describe putting load on the puller then hearing a "pop" as the circlip collapses at which point the process gets much easier. For me, it was more like putting a ton of load on the puller and turning and eventually hearing a series of crunches...then it became easier.

The puller can be hard to turn at times; a big wrench comes in handy.

Here's a video of the final moments of bearing removal.

Bearing removed:

New one waiting to go in...notice the O-ring floating on the stud. This is a spare! Remove it! This is there just in case the one that's already on the stud gets damaged.

New bearing coupled with the installation tool:

Installation consists simply of tapping the bearing gently into place with a rubber hammer:

Here is the bearing installed including the spiro-loc. To get the spiro-loc in place, I pulled it apart a little until it looked like a spring then threaded it into the groove.

Now with the new flange installed, flange sealant on the nuts and bolts and green Loctite on the central stud:

From here, it's just a matter of reassembly.

Make sure to reassemble everything following torque specs and using Loctite where necessary!

Car finally seeing the light of day again:


  1. Wow, that was impressive. You could most definitely be considered a professional mechanic. Was considering this as a DIY on my 996 C4S, but after seeing your video, it actually made me realize NOT to do it myself (I know my limits). I will hire a pro to do this. Are you available? :)

    Thanks for the fantastic article...

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  2. Thank you!

    I hope more people consider buying the 986/996 after realizing that the IMS issue is not difficult to remedy.

  3. Thank you, so much! This is exactly the info I was looking for. I'm considering buying an '03 or '04 Boxster but only if I thought I could replace the ISB myself. Now (with this article) I know I can. Question: I'm new to Porsche and I'd like to know if the repair is essentially the same for a six cylinder "S" model.

  4. I don't own a Boxster yet but I plan on getting one so just doing the research beforehand.

    Thanks for the excellent write-up!

  5. Looks like I will be replacing the IMS on my '00 Boxster S. I, too, was thinking it would be a DIY project but too much for my little garage to handle. Now just have to find a reliable and reasonable mechanic. Oh the joy!
    Thanks for sharing. Great job. Simple yet precise.

  6. A great article, many thanks for taking the trouble to put it together.

  7. Great article. That looks like much more than I could handle. Does the 2011 Boxster have the same issues?

  8. Thanks so much for this article. As a professional tech and technical writer I can testify to your prowess. You may consider yourself a pro-mechanic. Thank you.

  9. This is the best writeup I've seen on how to tackle this job from end to end. Thnk you for taking the time to pull together so much useful info.

  10. Great information!

    I am more likely to buy a used Boxster with that information.

    Any other issues I should be aware of? RMS?

  11. The RMS can be done at the same time as the IMS bearing. I chose not to bother, but my car definitely has a leak from there. It's not serious enough yet to cause anything more than some oil drops on the garage floor though.

  12. I have the same problem please help do you remove the bearing if only the outer race is bearing..

  13. Great information. How long did it take you to do it and how much was the cost please?

  14. Hi,
    Shaft Seals are very essential component of any mechanical device that has rotating parts. Many devices that we use today has these types of seals. Devices like pumps, mixer, washing machines, blowers and many more devices that we use today contains this type of seals.

    Danny Morgan

  15. This comment has been removed by the author.

  16. thanks again, this was very helpful.

  17. There are cars that have 100-200k or more on the original IMS bearing. ...

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  20. Hello
    I made a big mess on my Boxster S 987 , 3.4L
    I remove the IMS Bearing flange to change the rubber seal of IMS without locking the Crankshaft
    and now the shaft is on a side.
    What should i do now :((
    Can you give me some advice .
    Thank you

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  26. Hello,

    So I have a question regarding my 99 Porsche Boxster IMS bearing. I noticed an oil leak under my car after a drive. I turned it on the next day and I heard a rattling sound so I turned off the car. Is it too late to replace IMS bearing? A mechanic is saying that even if the IMS is "gone" that he can still replace the bearing. How true is this, and is it worth paying for the job?

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