Plastigage and M11 Rod Bolts

[ATB88]

This question seems important enough to make a thread rather than put in the dumb questions thread.

When doing my rod bearing job, my plan was to plastigage all 6 cylinders to assure correct clearances. Reading up on the S54 rod bearing DIYs where they explain the use of plastigage, it seems like they say to complete a full torque sequence on the rod bolts with the plastigage on the bearing, and then undo and measure.

This seems all well and good if you've got the later M10 bolts where you actually do the torque sequence (5nm->30nm->105 degrees) three times -- you can plastigage on the first sequence, remove the plastigage, and then do the remaining two sequences.

But what about if you have old style M11 bolts? The torque sequence is a single application (5nm->30nm->70 degrees). My concern here is that people seem to think that the M11 bolts only have so many times that they can be torqued before they're no longer reliable. If I have to apply the full torque sequence to get an accurate plastigage reading, then that means I'll end up torquing each rod bolt twice in the course of doing the job.

I know that the rod bearings in my engine were already done twice under both service actions back in the 2000s, so assuming original bolts, they've all already been torqued three times so far.

So my questions are:

1) do I really need to apply the full torque sequence to get an accurate plastigage reading?
2) If so, considering that my rod bolts have already been torqued three times, should I consider forgoing the plastigage step in order to avoid torquing my M11 bolts more times than necessary?

[bigjae46]

You're not getting an accurate reading unless you follow to the torque sequence.

I think plastigage is a good double check. BMW dealers have to do it for warranty purposes. The rest of us, my opinion here, can skip it under the assumption that the clearance is good since it is just a preventive replacement of the rod bearings.

[ATB88]

You're not getting an accurate reading unless you follow to the torque sequence.

I think plastigage is a good double check. BMW dealers have to do it for warranty purposes. The rest of us, my opinion here, can skip it under the assumption that the clearance is good since it is just a preventive replacement of the rod bearings.

Yeah, the OCD in me really wants to plastigage, but I agree that given that this is preventative, I have no reason to expect clearance would be out of spec unless I happen to have bought an out of spec bearing by bad luck. It just seems like good practice.

So, given that my rod bearings have been done twice already by BMW dealerships, it means that my bolts have, in the worst case, been torqued 5 times.

Now my questions are:

1) If my M11 bolts have been torqued 5 times already, should I seriously be considering not re-using them? (i.e. replacing them with ARP, which I would install via torque spec not stretch gauge) I've read sketchy things on both sides (using ARP bolts can result in ovaling the bore / re-using rod bolts is scary) but have no idea how much of a concern re-using the original rod bolts really is, and how many times is too many times.

2) If I do re-use them, is avoiding plastigage in order to avoid torquing them more times than necessary the safer play?

[maupineda]

You're not getting an accurate reading unless you follow to the torque sequence.

I think plastigage is a good double check. BMW dealers have to do it for warranty purposes. The rest of us, my opinion here, can skip it under the assumption that the clearance is good since it is just a preventive replacement of the rod bearings.

Plus, what will you do with the data, replace the crank or short block assembly? chances are you will roll with it, and as such, better to not know and create a mental issue that will keep you up at night. in this situation the most we can do is try different bearings to chase a better tolerance number, but very likely all will be the same.

[eacmen]

FWIW I have had no issues with ARP M10 or M11 bolts when torquing using their stretch gauge.

I think you would be safe plastigauging one rod then simply using that torque spec for the other 5.

[chris719]

Plus, what will you do with the data, replace the crank or short block assembly? chances are you will roll with it, and as such, better to not know and create a mental issue that will keep you up at night. in this situation the most we can do is try different bearings to chase a better tolerance number, but very likely all will be the same.

True, but it would let you identify an out-of-spec bearing before you installed it.

[bigjae46]

True, but it would let you identify an out-of-spec bearing before you installed it.

If you are doing the job because you have identified an issue or using aftermarket parts then yes, you probably should plastigage. If you are doing the job preventively using genuine BMW parts then why would there be an out of spec bearing that would cause an issue?

[chris719]

If you are doing the job because you have identified an issue or using aftermarket parts then yes, you probably should plastigage. If you are doing the job preventively using genuine BMW parts then why would there be an out of spec bearing that would cause an issue?

BMW has had to recall engines before due to supplier issues, including Glyco rod bearings. They don’t QC check every bearing. It’s unlikely but not impossible.

[bavarian3]

They're not stretch bolts, so you should be able to reuse without issue. Check the condition of the bolts/threads. I'm sure bolts can wear over time but probably from actual usage/heat cycles rather than torquing a few times.

ARP bolt bore ovaling apparently isn't an issue with m11's. If I was you I'd reuse the M11's if they look good otherwise ARP with a stretch gauge(do it right).

FWIW I resused my m11's, and the bearing were changed before.

[bigjae46]

BMW has had to recall engines before due to supplier issues, including Glyco rod bearings. They don’t QC check every bearing. It’s unlikely but not impossible.

The manufacturer I'm sure does some sort of QC check but I get your point and you are very correct. Still, plastigage isn't very accurate either. That's a chance you take with any part. What if an M10 rod bolt is out of spec and doesn't stretch or over stretches? Using BMW parts makes me feel better, not sure it is ALWAYS better. I'll stop there before I start giving everyone nightmares.

Again, a good double check if desired but not required. If I were doing this job for someone then I would absolutely take plastigage readings.

[Cubieman]

Im sure you have mentioned this but what type of bearings are you going to be using?

I have some ACL's, but since I won't be doing the plastiguage I wonder if OEM wouldn't be a better choice, but man are they expensive, so is a damaged crank.

[sapote]

Plus, what will you do with the data, replace the crank or short block assembly? chances are you will roll with it, and as such, better to not know and create a mental issue that will keep you up at night. in this situation the most we can do is try different bearings to chase a better tolerance number, but very likely all will be the same.

I rather to know the clearance on the new bearings than worry about the QC of the bearings.

[sapote]

If you are doing the job because you have identified an issue or using aftermarket parts then yes, you probably should plastigage. If you are doing the job preventively using genuine BMW parts then why would there be an out of spec bearing that would cause an issue?

Just a human error that a wrong bearing half in the box. Murphy's law ways it's possible.

[ATB88]

Thanks everyone for the thoughts, it was helpful to hear perspectives.

In the end I kept my old M11 bolts, put new OE BMW bearings in, and only plastigaged cylinder two because the bearing didn't feel as "good" going into the rod cap (didn't pop in like the others), and the rod cap seemed a little harder to mate to the rest of the rod than on other cylinders. Plastigaged it to see if anything is amiss with how the bearing was fitting, and found that the clearance was on the tight end of the spec (0.0012" or 0.0013" if I had to guess), but in spec.

Drove 110 miles home from my friend's and so far so good! Old bearings actually didn't look bad at all, but with 90k miles on them it was time. I'll post photos when I get a chance.

Thanks again for the help everyone!