jaetee

For those not familiar with my situation, I've got a 90k mile 2000 MkI which passed PPI with flying colors, came with full papers, but had also been driven less than 300 miles over a six year span. It turns out that after putting roughly 2k problem-free miles on the car myself I spun a rod bearing.... Ugh! Once we got into the engine, we found lots of visible pitting on the polished crankshaft journals, which apparently was grinding away at the bearings.... Ugh. Anyway, now I'm going through a full rebuild (with Darton sleeves) to make it a 3.6, put in a LWFW and have the whole rotating assembly dynamically balanced as well. If gonna spend the money, might as well get something extra for it.

After numerous delays, most notably an almost 3-month wait for Woessner to deliver a new set of oversize pistons and rings, my engine block is finally getting sorted... On a whim, yesterday I asked my engine builder to measure the IMS shaft to make sure it is nice and straight. I got an almost immediate reply that they found it has .004" of run out, which was enough to make it visible to the naked eye as it turns in the machine.

Somewhere along the way here on RL I read that there may have been a sizable lot of slightly bent IMS shafts that made their way into the production lines and that may be the primary silent culprit behind our infamous IMSB issue... This might even explain why some cars we've read about here have had repeated IMSB failure, while others with same bearing type have done just fine regardless of mileage and bearing type. Stands to reason that a single row bearing (higher failure rate) would be more prone to failure from a wobbly IMS shaft compared to the earlier dual row bearings with the much lower failure rate..

Considering it's application, is .004" of run out enough to be of concern on the IMS shaft? Personally, I find it a little bit concerning. Wondering now if I should swap that shaft, and if I do what are the chances of finding an IMS shaft that is straight? Considering the only way to find out is to disassemble the engine completely and measure in a tool that's not too easy to come by, I'm not expecting many answers here.... but you don't know if you don't ask, right?

My question is.... has anyone else doing a full rebuild tested their IMS shaft for run out, and if so was it perfectly straight or did it have any measurable run out? If so, do you remember how much?

Or did you just use what you had and move on with life?

Paul Waterloo

I do not know what the acceptance criteria is for the IMS runout, but I can tell you that 0.004" is WAY TOO MUCH. I'm involved with rotating equipment repair my whole life and considering the size, length and operating RPM, I would consider anything more than 0.001" excessive. I would prefer to have something closer to 0.0005". I would not put it back in the car with 0.004" runout. That's just going to stress the heck out of the bearings on each end.

Coopduc

I think you are saying that wear debris from the IMSB made it's way to the crankshaft/connecting rod bearings and caused the rod bearing to spin. Did you inspect the IMSB to verify, or have someway to trace the wear debris to the IMSB ?
I'm having trouble connecting the intermediate shaft runnout to crankshaft/connecting rod bearing failure. I'd be curious to know how the runnout was checked and where the runnout was measured, and with respect to what datums.
Also wondering how you are addressing the pitting on the con rod journal. To my knowledge there are no undersize bearings available for these, maybe I'm mistaken.

jaetee

Agreed. My machinist/engine builder says he does not want to re-use it... He said basically the same thing you did.

IMSB or IMS Shaft had nothing to do with my failure. What my mechanic and I believe to have caused my issue is a combination of a couple of things. First, corrosive pitting on the crankshaft, which comes from the car sitting very long periods of time between use, which gave gravity time to pull the oil completely off of the journals and allow condensation to build and eventually cause corrosion pits on the polished journals (you can see them with the naked eye). Also, I was on a rather spirited drive when the bearing spun, so there may have possibility been a moment of momentary oil starvation going through some twisties that might have contributed to the issue as well. I did see oil light come on 1 second before the engine stopped, but at that point I was "cooling down" and on straight road behind another car.... I think the oil light I saw was due to the failure happening then, not necessarily the cause, as it happened so suddenly afterwards. But I must also say that this was the first time I ever spun a rod bearing in my life, so I just don't know...

My looking into the IMS shaft was preemptive as I'd like for this engine to come back as good as close to perfect as possible. The IMS bearing looks fine and is LN dual row classic which was replaced by the shop at previous owner's request. It says so on the shop receipt, and when I called the shop who did the work the owner remembered the car (Millennium edition) and assured me that they have a strict policy of not replacing an IMSB that has already failed. The original IMSB did not fail, nor did this one. Unfortunately, the LN IMSB I have now only has 5k miles on it (3k previous owner, 2k by me) but it will be replaced because it was contaminated with oil laden with microscopic bits of non-ferrous metals from the bearings. We don't trust it anymore, and now we don't trust the shaft either.

Coopduc

Thanks Jaetee for the clarification. We are on the same page with respect to the shaft runnout, 0.004 is probably an order of magnitude too high.
Still interested in how you addressed the damage to the crank journal. Will just a light polish clean it up? If so you are very lucky!
FYI - I may be in a similar boat. A few months ago I bought a '99 roller with what sounds like a bad bearing knock. Back in the day we called it "rod rap". I plan to start my teardown next month, so will be following your progress.

jaetee

Oh, forgot to answer that one.... the pitting is evident on all of the crankshaft journals. And during my failure, the amount of clearance change that occurred when the bearing slipped allowed that piston to tap and bend the exhaust valves. Those valves have been replaced and the heads have been ground and overhauled. But when the machinist inspected my crank it was found to not have any cracks, but it did have 0.002" of rollout which he thinks came from that valve/piston contact. We decided to scrap it. Instead I went on the hunt and found a pristine 28k mile crank from a known ebay p-car parts seller whom I've done business with before. Although the parts came from a Boxster, I bought the replacement crank plus it's original basket because the part numbers for both matched exactly what came out of my car, for $1k shipped. I definitely got lucky.... And based on measurements after the crank arrived, we will be able to use standard size bearings for the build. That was another thing that contributed to my delays along the way...

Flat6 Innovations

And here we have something that we've been seeing as a constant with engines that lose one, two or three IMS bearings in their lives... People don't check for this runout, and the new bearing is killed as a result. We've been tossing shafts that have too much runout for years.

Things like this separate engine builders from the clowns that just bolt stuff together, assuming it'll "be ok" and then sell it for some cheap price.

Kudos for taking the time to check this.

The acceptable amount is a bit of a personal preference, and I am not going to stick my neck out with my specification, we'll see what Charles at LN says about this one.

Coopduc

Well done, nice find! I didn't realize 996 and boxster used the same cranks. Time will tell if my engine has the same issue...

jaetee

Pretty much got the idea from deep-diving into many threads where you made commentary. I think it was you who at one point said something along the lines of "you wouldn't believe the amount of runout we see on some of these IMS shafts when we do rebuilds." So, this comes back full circle to you...THANK YOU. Wish I could afford to have sent you my block, but I must admit that I have confidence in my guy too...

Flat6 Innovations

Sure...
Those were Mr. Raby's posts a few years ago. He's assembled a team to handle the parts of this business that he doesn't care for, and he is having fun staying away from forums, and out of the office completely.
Glad to see that you have someone local to handle this for you. Remember, we only build complete engines, and don't perform any machine work :-)

Ahsai

I would only trust one that's been inspected and pinned by LN.

Chris(MA)

if you search on here you will find Charles from LN comments on shaft run out, I believe yours at 4 thou is not that bad at all, if memory serves me correctly he said he's measured up to 12 thou

Are you using wet or dry liners?

jaetee

Thanks. It took a while, but I found this thread: https://rennlist.com/forums/996-foru...l#post14321136

According to those pics, measurements found to be as bad as 28 thou! My IMS shaft rollout was measured at the center of the shaft itself... I've sent the link to that thread to my machinist.

My block is fitted with dry liners, soon to undergo reassembly...

Charles Navarro

Typically anything more than .005" we will fail for use with ball or roller bearings. The IMS Solution was tested and had no problems with runout of up to .015".

Also, you can't just throw the ims into a 3 or 4 jaw chuck and expect accurate results. We have soft jaws that grab the ims tube (to prevent damage to the IMS tube) and the fixture is dialed in every time before any measuring or machining operations are carried out. We typically wait until we have a bunch of shafts to do at once because of the required setup time.

Ahsai

^Now you see why I only trust the IMS from LN