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Old 04-10-2012, 01:32 PM   #1
jjrichar
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Project M54 Engine: Pistons and Conrods

Pistons and Conrods

Link to other parts of the project.
https://forum.e46fanatics.com/showthread.php?t=899347

Before I start here, I want to point out that this is the real business end of the engine. To test and rebuild the components requires a fair bit of knowledge, and some tools that most people don't have in their shed. This is just to work out what work needs to be done. Additionally you will need access to all the engine specs provided by BMW. Once you work out what needs to be done, then you will need to source a workshop that can complete the work for you.

This thread is just designed to give you an idea of what is down there, and a bit of an introduction into the types of testing of components that you would need to do. I'm not an expert. Not even close. So I'm not confident in putting stuff up here that could be completely wrong.

When the oil pump and oil splash guard are removed, this is what you will see. Each of the components go in one place only. Pistons only go in the place they came from, and orientated in the correct direction. Conrods only go back where they came, and correctly orientated. Conrods and their caps are numbered so they only go back together. Piston pins and conrods are matched to each other and can't be mixed. Take lots of photos, bag and number everything.











Removing the pistons is a pretty simple process. The BMW manual says to use two long tools that attach to the bottom of the conrod to help with its removal and installation. These are there just to stop someone who is really clumsy damaging the crankshaft journal or the cylinder bore. For a half careful person, not required.

Remove the two conrod bolts and pull off the cap. The cap might need a bit of a tap with a soft mallet to dislodge it. It can be stuck on with the oil that's inside. When the cap is off, push the conrod and piston up and out. Again, you may need to give the bottom of the conrod a bit of a tap with a soft mallet to dislodge it from the crankshaft. Be careful. You don't want to damage anything.

When the piston is out you can pull it apart. The bearing shells easily pop out by getting a pick tool and prying them out at the tang (bent in bit at the edge) of the bearing. Use your pick tool to remove the round circlips that hold in the piston pin.

With the piston and conrod out, this is what you will see.











Pulling everything apart is pretty simple. Once the circlips are removed (only one needs to be removed for the piston pin to come out), then the piston pin can be pushed out. Due to varnish build up, it may take a little time to work it out. Just keep on pushing it back and forth and eventually it will come free.

Once everything is apart, this is what you will see.





Removing and installing the rings is pretty simple by hand. I had available a ring installation tool, which is what it suggests in the BMW manual. I found it much easier to do by hand, especially after a bit of practice, and I found the risk of scratching the piston to be less as well. There are lots of videos on the web that show how to do it as well by hand. I also watched some videos of guys building high end race engines, and they did it by hand. There obviously isn't too much wrong with it.

I was warned by a bloke who had done this a whole lot that there was a risk of the ring snapping when removing, and consequent injury. This didn't happen to me, but to be safe, make sure you are wearing eye protection.

When installing the rings, don't have the gaps of each at the same place. Have the two compression rings gap at 90 degrees to the piston pin, but 180 degrees apart from each other. Have the oil ring somewhere between.











To install the piston in the block, put the whole piston arrangement with conrod back together but without the bearing cap. The bearing shell for the conrod half should be installed. Now get your piston ring compression tool and put the piston in the block. Ensure there is a light coating of oil on the cylinder wall, as well as on the inside of the tool. I was watching a video on the rebuilding of this type of engine (it was an S54 M3 engine), and the guy, who was clearly an expert, warned against using too much oil. Just the lightest amount is required. However, on the bearing shell you should have a good amount of oil.








When the conrod end is placed on the crankshaft now place on the cap with its bearing. Make sure the bearing is well lubricated. Try and keep oil out of the surfaces that are going to join. You will need to use new bolts for installation. Once used, and then removed, these stretch type bolts need replacing. Torque for these is: Initially 5Nm, then 20Nm, then an additional 70 degrees of turn.

Checking the piston and conrod

There are a number of checks on the piston, rings and conrod that determine what work or replacement needs to be done.

Checking the ring end clearance. Each ring's clearance is different.





Checking the ring axial clearance.





Checking the piston diameter. Once you have this, and afterwards measured the cylinder bore, you can then work out the piston tolerances.





Checking the piston pin





Here are all the tolerances for the above tests. Note these are only for the 2.5L engine.





Now the conrod bearing clearance needs to be measured. Between the bearing and the journal there needs to be a small gap for lubrication. Too little and not enough oil can get in. Too much and it will just splash about everywhere, and possibly reduce the oil flow to other parts of the engine. This is called the conrod bearing radial clearance. For this engine it is 0.020 0.055 mm.

The task is done by the use of plastigauge. This stuff is pretty cheap and can be bought off ebay or most motor spares places. One piece of plastigauge is about enough to test your whole engine. Make sure you use your old bolts when testing. Only use the new ones when you are finally putting the engine back together. When you tighten the bolts to squash the plastigauge, be careful to not turn the crankshaft. You don't have any oil in there for the test, and you might damage the journal or bearing.

You can see from the bottom picture the clearance for this journal was about 0.038 mm. This is right in the middle of the quoted range. Interesting because the photos above showed what looked like significant wear on one of the bearings. Obviously what looked like a lot of wear really wasn't.








Now you need to measure the bore. It's done with an internal micrometer. You need to measure at the top, middle and bottom. Measure across and up and down for each of these positions. You will see that the bore is probably not perfectly round. You need to compare these measurements with the piston diameter. The maximum permissible tolerance between the piston and cylinder bore for this engine is 0.15mm. The worst I found was about 1/3 of this.





Here's an overview of what I found when measuring these.

RIng axial tolerance: All were fine.

Ring end tolerance:
Ring 1: All were fine. Some were close to the maximum, and some right in the middle.
Ring 2: Some were just at the maximum, some were beyond.
Ring 3 (oil scraper): All were well beyond the maximum tolerances. Some were nearly 1mm.

The measurements on the bore were not identical, but there was a constant pattern.
At the top, it was oval shaped. The longest part of the oval was across the engine, shortest along the length of the engine. This is what you would expect due to the rotation of the crankshaft and sideways force on the piston when the engine is running. The maximum amount was about 0.025mm difference between the two. Some were less than this. At the middle of the bore, most of this difference had gone, and at the bottom, there was no difference. Basically there was maximum wear of about 0.025mm on the side parts of each of the bores right at the top. After measuring each of the pistons, and the bores they came from, I had a maximum difference between piston and bore of about 0.045mm, which is well below the maximum 0.15mm.

This piston pins fitted like gloves. No wear.

Conrod bearing clearances were right in the middle.

Conclusions from the tests: The bore and pistons were slightly worn, but well inside the maximum tolerances. No replacements required.
Piston pins fine.
While some of the rings are within tolerances, I'd replace the lot.
I need some advice on the bearing shells. My gut feel is to replace them just because I've got the engine open. They look worn, but the radial tolerances are perfect. Some advice from an expert who has done this a lot before would be helpful.
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Old 01-24-2016, 10:31 AM   #2
white_335i
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Hi jjrichar, Thank you for another detailed post. I am replacing the oil pan gasket on my car and since I am in there, I am also replacing the oring and spring in the oil pressure reg and oring on the oil pick up tube. I am also replacing the rod bearings(120K miles on the motor). I have a question on the torque specs for the rod bolts.

You listed as 5nm, 20nm and 70 degrees. I went to the dealer and the service advisor gave me the specs from all data. They are 5nm,30nm and 70 degrees. I torqued all the bolts to the alldata specs. I did google after I torqued the bolts and found a link to TIS doc that has the specs you listed. I didn't find anything on bentley. Should I purchase new bolts and replace them with your specs?

Thanks
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Old 01-24-2016, 09:25 PM   #3
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Abi, I honestly don't think it would make much difference. The difference in turn between 20 and 30 deg will be minimal, and you have to put 70 deg after this. The tolerance on the turn angle is normally 10 deg, so I suspect that it will be still inside this. I'm not an expert at all, but that is my gut feeling.
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Old 02-05-2016, 03:19 PM   #4
latitude39
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Oil scraper rings wearing out faster than other components

jjrichar,
Thanks a million for putting together all these fabulously illustrated and informative threads. I've not seen anything like them anywhere.

Oil consumption
My '02 325xi with 160k miles has been starting to use oil, in the order 1 quart per 2k miles. I worry about the combustion chambers and O2 sensors getting fouled. I've been reading "oil consumption" threads, trying to determine a cause.

This morning I saw the new post here and I re-read your thread, looking especially at the ring measurements. This thread says the tolerances were in-spec...except for the oil rings, which showed substantial wear.

This is the evidence I've been looking for: specific measurements of in-service piston rings. The tolerances for the oil rings are similar to the other rings:
  • .2 - .4mm

But the oil scraper gaps approach 1mm, which is far out of spec.

Do you know the actual mileage of this engine?
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Old 02-06-2016, 08:58 PM   #5
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I have no idea of of the history of the engine. I picked it up in an old warehouse that had a whole bunch of assorted engines and transmissions. The people who sold it to me had no idea about it or any of the other engines there, except that it had the word BMW on it. All I wanted to do was pull it apart and learn, so its history wasn't important.
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Old 02-06-2016, 09:37 PM   #6
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Thanks
I'm trying to determine what's the common cause of oil consumption since I've recently joined the oil consumer club. This thread has provided the best data--really the only data from a torn down engine--I've seen. The oil scraper ring looks like the prime suspect. Too bad it's so deep in there and can't be replaced (easily) as PM.
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Old 09-25-2016, 03:20 AM   #7
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I love your whole engine series, jjrichar. I'm planning a rebuild shortly. You already know how helpful your transmission videos were to me. Really just incredibly invaluable you've been to me and everyone else and I thank you again.

The only thing I want to add to this particular page is that I am reading that when you measure the ring gap, you're supposed to place them in the lower part of the bore, about 20mm from the bottom, such that the ring is in the area of the least amount of wear. I would be curious to know what the oil rings would have measured when squared up at the bottom of the bore rather than the top, as you first measured them. I suppose I will find out when I rebuild my engine.

Anyway, thanks again.

-Jason

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Old 09-29-2016, 04:31 PM   #8
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Jason, thanks for this. That makes perfect sense, to measure it on the bore where there is going to be minimal wear. My suspicion is that if I measured the rings like this, all compression rings would have been ok, but the oil rings would have still been well out of tolerances.
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Old 12-30-2016, 10:27 PM   #9
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It should be noted that the M54B25 con rods are cracked (fractured).

The M54B30 rods are NOT cracked (fractured)

That said, AWESOME series of write-ups! Very informative!
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Old 09-01-2017, 07:50 AM   #10
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im doing engine rebuild on m54b25 and dont know which piston rings to buy. there are 2 models, one with 0.26 extra something? what about head gasket? there is also some kind of 003 extra? 2 models also.
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Old 09-04-2017, 10:28 PM   #11
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Quote:
Originally Posted by rulerss View Post
im doing engine rebuild on m54b25 and dont know which piston rings to buy. there are 2 models, one with 0.26 extra something? what about head gasket? there is also some kind of 003 extra? 2 models also.
Unless you are having your engine overbored and buying oversized pistons, you need standard piston rings, not oversized (0.25mm and 0.5mm are the two larger sizes).

The standard head gasket is 0.7mm thick, the oversized one is 1mm thick and you only need the oversized one if the machine shop takes a lot of stock off the head when you have it resurfaced. For instance, they took about 0.008" off my head and 0.006" off my block for a total of 0.014" which is 0.36mm so I needed that thicker 1mm headgasket which is 0.3mm thicker than the stock one, because that made up for the material that was removed.

Also, during my rebuild I used the oil rings from the M52TU to solve the oil consumption problem and used the regular M54 compression rings. I found a piston ring supplier on eBay who was able to make up that custom ring set for me. This is the original eBay listing:

http://www.ebay.com/itm/131912802729

I would contact the seller directly to make sure you are getting exactly what you need.

If you want more information on why I did this:



-Jason

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Old 10-05-2017, 10:28 AM   #12
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I could read 10 pages of what you're making for us here. Great documentation and descriptions
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Old 11-10-2017, 09:55 PM   #13
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Could someone confirm that this is in fact correct specs for piston rings?


I got new piston rings and most have end clearance of 0.014", while two or three have clearance of 0.016" tight. NOt sure if I should leave them as is or file all rings to have clearance of 0.016"
0.014" = 0.3556mm
0.016" = 0.4064mm
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Old 03-01-2020, 08:25 PM   #14
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At the risk of digging up an old thread, I'm having the same issues with my B30 swap. My set of rings, 4 cylinders spec out to .406mm, and 2 are .25mm...since I'm running mostly stock, I'd like the tighter gap. Looks like I'll have to keep exchanging the sets until I get one that's pretty even across the board.
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