Help rebuilding a stock k27 turbo (745i/m106)

[CorvallisBMW]

When I bought the k27 and related parts from the previous owner, he included an ebay rebuild kit, told me the seals were bad, and that it should be rebuilt.



After taking it apart, it was definitely due for new seals. I doubt it had ever come apart in it's 30+ years on this earth, so getting the turbine shaft our took a bit of persuasion. When it finally did come out, all of the bearings and washer came out too, so I wasn't able to see the exact order they were in. When it came time to put it back together, I tried to follow this exploded view diagram that I found for the Porsche-model k27s that came in 930 cars and other similar-era models:

http://garage.ideola.com/downloads/k26-explode-legend.jpg

The problem is that my parts aren't exactly the same as them, and I can't figure out how to make it go back together correctly. The bearings and bearing clips (#1 and #2) go in just fine:


So does the thrust washer:


And the thrust bearing:


And the splash guard:



But after that, it's really tricky. The diagram says I should have some kind of spacer, and it kind of looks like this piece with the broken seal ring:


Followed by this stepped washer that they call a Mating Ring:



But if I install them in that order, it just doesn't look right at all. It also doesn't fit!


However given the shiny areas you can see on the two parts, it does look like they may have fit against each other before disassembly:


Plus the diagram indicates there are 2 more sealing rings (#8) that are supposed to go on the compressor side, but where? I see absolutely nowhere to install any sealing rings, and none came off the turbo when I pulled it apart.

I know that if I try and put it all together the way the diagram indicates, I cannot get the backing plate on at all. But if I swap the order of the spacer and the mating ring, so that the mating ring is on the bottom and sitting inside the splash guard, it actually looks very good. But if I put on the backing plate and tighten the compressor wheel nut, the whole shaft binds and won't spin. I think it's because the spacer with the sealing ring protrudes past the backing plate:



So clearly I'm doing something wrong, but what? Am I missing a part somehow? I've been wrestling with this for several days now and can't figure out what I'm doing wrong. Help!

[citizen_insane]

From my reply on your other thread, hopefully this helps:

Disclaimer:

I understand that you're just trying to put everything back together, but that's not really good enough. You NEED to core balance the entire core assembly. It is not good enough to just individually balance the parts, they need to be balanced together as an assembly and that's just not something you can DIY.

From what I can see, it looks like your thrust washer is installed upside down (orientation is matters). The coking on the upper side in your pictures looks like it should be pointing down toward the core. I wouldn't expect to see that on the side that was in contact with the thrust bearing.

The little plug with the two sealing rings on it should be poking through the seal plate and the CW should run up against it. I'm thinking you're missing a spacer that goes under the large mating ring. The smaller side of the mating ring should go through the splash guard, but not be compressed against the thrust bearing.

However, without seeing an OEM assembly print of this turbo I can't say for sure how it's supposed to be assembled.

[CorvallisBMW]

Here's what I don't understand; why does the CHRA have to be balanced as a complete assembly? That could lead to problems if I ever have to remove the compressor wheel as it would no longer be balanced unless I got the orientation spot-on.

If you balance the pieces individually, as the shop I took it to did, orientation no longer matters. Since each rotating piece is in balance I don't have to worry about things becoming unbalanced if I happen to rotate a component.

So why is assembly balancing required and individual balancing not good enough? That seems completely opposite to me.

Just for the record this is the shop that did the balancing. They rebuild hundreds of turbos a year, it's not some kind of shadetree shit. I think they know what they're doing: http://www.dfispdx.com/

[citizen_insane]

Here's what I know: every manufacturer balances all their individual components, and then they balance as an assembly. No turbo manufacturer would ever send out a core that had not been core balanced.

We can assume your wheels are not perfect, they have some finite amount of imbalance in them as they exist in the real world. Additionally, the other parts in the assembly have some amount of imbalance as well (turbine wheel, shaft, compressor wheel and compressor wheel nut). When you combine the parts, if the imbalance stacks up in any way other than 180° out of phase, it will cause a harmonic mode that can be excited by the spinning of the assembly.

When you're spinning at 200krpm, it's hard to prevent those modes from being excited, especially with something as long and spindly as a a turbine shaft. Is it possible you'll get lucky and the assembly will be within the acceptable tolerance? Yes. Is it likely? Probably not, but then again I don't know to what specification your wheels were balanced (did they provide a cal sheet with your parts? I'd be interested in how/to what level they were balanced). I just know that no one in the turbo industry doesn't core balance and it's for a good reason.

[CorvallisBMW]

I understand the idea of tolerance stacking, but I dropped off the turbo fully-assembled to the shop and they took it apart to balance. This makes me think they trust individual component balancing over assembly balancing. Maybe I shouldn't just "assume" they know what they're doing, but considering they've been around for several decades and rebuilt thousands of turbos, I'm inclined to trust their methods.

[citizen_insane]

They very well may get good results from balancing individual components, it just depends on how good a job they do. The main reason for balancing in industry is for NVH considerations, but once you reach the maximum limits of NVH imbalance you can have shaft motion issues where the shaft can push through the oil film and touch the bearing, causing failure.

If they can balance all the parts, to within that specification, they should be fine. I just worry that with as many parts as you have (turbine wheel/shaft assy, thrust washer, spacer, mating ring, plug, compressor wheel, and compressor nut) going into the stack, you might have issues down the road with premature wear and or turbo failure. I'm not saying it can't be done, just that you will get optimum results from doing it the way it was originally intended.

Regardless, I think we've beat the issue to death. On the reassembly front I think you'll need to find the spacer that goes between the thrust washer and the mating ring so that you're not clamping the thrust washer between the parts. In the turbos I'm used to seeing, all three parts (spacer, mating ring and seal plug) are all one piece.

See the attached sketch. The white line is the compressive force from the compressor nut through the stack of parts on the shaft.

[CorvallisBMW]

^That drawing is hugely helpful, thank you!

[CorvallisBMW]

Yesterday I spent several hours on the phone with Melett (www.melett.com) and Central Motive Power (www.centralmotivepower.com). We were trying to figure out what rebuild kit is correct for the BMW-spec k27 and verify that it would come with all the parts needed.

I can't thank the guys at Central Motive Power enough, they were beyond helpful. Their turbo tech Mac Flynn spent an hour on the phone with me cross-referencing parts between KKK, Garrett/Honeywell and even BorgWarner. We found conflicting info from all 3 and it took some real digging to come up with the correct answers, but in the end we succeeded! So without further ado, here's the info you'll need to rebuild a BMW--spec k27!

This is the best exploded-view diagram I could find for the K27. It clearly shows the 2 different versions of the "Flinger ring" assembly (parts 6 & 11), as well as the correct order and orientation for assembly of all components:

Note that the BMW-spec k27 has the "single seal" flinger ring design. You can see the seals listed as part #11 in the diagram above. Many Porsches use the "double seal" flinger ring design. To get the right rebuild parts you need to ID which type of flinger ring your turbo has.


Now to get rebuild parts, you'll need to go through Melett as BorgWarner stopped making parts for these turbos in 2009. Switching to the Melett diagram:
- For the "single seal" design, you'll need Melett part number 1301-027-751
- For the "double seal" design, you need 1301-027-751 plus 1301-027-760



Shop around! Melett only sells through distributors, you cannot order from them directly. When I called a local distributor here in OR they wanted $175 for the rebuild kit!! Central Motive Power in Denver charged me $45 for the exact same kit!

I hope this information is useful to people in the future :)

[tschultz]

Nice to hear good things about Central Motive Power. I'll be going there probably tomorrow for some gaskets and it sounds like they are the place to go.