OM 617 Rebuild - Piston not perfectly aligned in Bore - PeachParts Mercedes-Benz Forum

pragmatic · #1 08-07-2016, 02:38 PM

I am reassembling a '82 300td block and may (or may not) have run into a snag. I installed new sleeves and had them bored and honed by a reputable machine shop to FSM specs. I did not replace wrist pins or pistons, but installed new rings and bearings,

On the #1 piston, there appears to be more of a gap to the front of the engine, and the piston, after torquing down the rod bearing cap, sits a tad proud toward the front of the engine (see pics). I removed the piston and reinstalled. No change. None of the other pistons do this and everything rotates freely without binding.

I noticed a bit of uneven wear on the piston, so for all I know this isn't new to the engine. Have any of you run into this issue before? What are the likely causes? I *think* I can eliminate poor alignment when boring - as best I can measure, the bore is square in the middle of the new sleeve.

I'm thinking a slightly bent rod or wrist pin. Or is this something that may align itself as the engine wears in?

Blaxmith · #2 08-07-2016, 04:29 PM

i would agree that something is slightly bent. I wouldn't be confident running with that for too long. maybe the sleeve isn't in right.

leathermang · #3 08-07-2016, 04:39 PM

Did the machine shop have the MB FSM to follow when doing their work or did they apply ' standard ' procedures to it ?

97 SL320 · #4 08-07-2016, 04:45 PM

The connecting rod is bent / small and big ends not parallel.

Have the machine shop check this rod for straightness.

leathermang · #5 08-07-2016, 04:51 PM

What is strange is that given that the piston pin is parallel to the longitudinal axis of the engine that a rod bent that much does not cause noticeable binding...it clearly will not fix itself... checking the rods it usually something the machine shop doing the work would be in charge of checking...

vstech · #6 08-07-2016, 05:10 PM

I believe if you pull the pistons out you will find that this piston is smaller than the rest...

If the bores are all identical... there is your problem.

Mb engines do NOT have all the same size pistons.
Cylinders need to be honed to fit the pistons...

compress ignite · #7 08-07-2016, 05:13 PM

Yeah " '97" and Leathermang are correct...

Running with this "Slightly" bent rod will end up "wallowing out" the new liner.

Other "Benefits" may run to excessive piston wear,Etc.,Etc.

pragmatic · #8 08-07-2016, 06:16 PM

The machine shop bored and honed each cylinder to fit the coresponding piston, so I don't think it is a size issue. I used a mic and bore gauge to double check the measurements of each piston/bore when I got the block back and found it to be spot on.

Well crud. I was hoping this was a well-known anomaly. I guess I will take the rod in to be checked for alignment and install a new wrist pin while I am at it. I'm hoping that is it and I don't have a warped crank.

leathermang · #9 08-07-2016, 06:23 PM

Quote:

Originally Posted by pragmatic

The machine shop bored and honed each cylinder to fit the coresponding piston, so I don't think it is a size issue. I used a mic and bore gauge to double check the measurements of each piston/bore when I got the block back and found it to be spot on.

Well crud. I was hoping this was a well-known anomaly. I guess I will take the rod in to be checked for alignment and install a new wrist pin while I am at it. I'm hoping that is it and I don't have a warped crank.

I will ask this again... did the shop have the engine FSM when they were doing the work ?

This is a separate item from the piston 's lack of being level with the top of the block..

jt20 · #10 08-07-2016, 06:25 PM

a 'warped crank' to that degree would not spin freely - assuming the bearing clearances were in spec.

might not need a wrist pin bushing, but I think most shops would prefer to do it once they straighten a rod.

It is important that you are upfront with the shop and tell them everything.

If you asked for a decked block and liners, they must have had other parts during this process (rods and pistons). I am surprised they didnt suggest checking them.

Frank Reiner · #11 08-07-2016, 07:31 PM

Quote:

Originally Posted by pragmatic

The machine shop bored and honed each cylinder to fit the coresponding piston, so I don't think it is a size issue. I used a mic and bore gauge to double check the measurements of each piston/bore when I got the block back and found it to be spot on.

Well crud. I was hoping this was a well-known anomaly. I guess I will take the rod in to be checked for alignment and install a new wrist pin while I am at it. I'm hoping that is it and I don't have a warped crank.

If the wrist pin were bent, it is not likely that you would have been able to assemble the rod/pin/piston.
A bent rod, as has been suggested above, is at the top of the list, but there is one other possible cause of "The Case of the Canted Piston". Were the piston to have been produced with a pin bore that is not square with the vertical axis of the piston, you would see the pictured situation.
The probability is very low, but it is possible.

pragmatic · #12 08-07-2016, 07:43 PM

Quote:

I will ask this again... did the shop have the engine FSM when they were doing the work ? The clearance on the first piston to bore is different than the others.. do not remember if the piston is smaller or the bore is to be taken out a little more than the others..

Yes, they had it, and no, unlike NA engines the the #1 bore on turbo models is finished to the same clearance as the rest. The pistons differ by group number - the #1 may be a different group, but once you account for the different piston sizes the final clearances are the same across all cylinders (though much larger than the NA).

https://www.startekinfo.com/StarTek/outside/12265/disc_2/program/Engine/617/03-316.pdf

Quote:

a 'warped crank' to that degree would not spin freely - assuming the bearing clearances were in spec. might not need a wrist pin bushing, but I think most shops would prefer to do it once they straighten a rod.

Hoping that the rod just needs to be straightened. Thanks.

leathermang · #13 08-07-2016, 07:44 PM

I do not know where the ' bent wrist pin ' came into the conversation... maybe I missed it... but there is no way a bent wrist pin can do this... to 'bend' a wrist pin the piston would have to be broken instead... The pin is very hard steel..... the piston is aluminum ....
and there is no way a crank throw is bent... this , if anything, is a twisted rod.

Swapped books.... all the cylinders for the turbo are the same clearances.... I was quoting from the NA manual... will never do that again....

pragmatic · #14 08-11-2016, 11:02 AM

So the rod is out. Using a machinist's straight edge (two, actually) I can tell the big and small ends are not parallel, and the error is in the direction that would explain the canted piston. For $60 bucks, I've bought a set of rods from a OM617 turbo parting out, and will find one that is the same weight and see if installing it causes the piston to sit properly. Other than matching the weight, any other advice would be appreciated.

But here is the mystery (to me at least). Along one side of the old rod are handwritten numbers that don't appear on any of the other rods. Every engine tells a story, and I'm wondering if this is normal (weight, maybe?) or if someone has been in this engine before, or if some "adjustment" was done at the factory. I have heard stories of manufacturers intentionally tweaking rods to account for boring errors, but that somehow doesn't seem like the Mercedes way. Also, the piston is slightly worn on the offset side, indicating that this has been the running condition of the engine for a while.

Any guesses?

pimpernell · #15 08-11-2016, 11:31 AM

Quote:

Originally Posted by pragmatic

So the rod is out. Using a machinist's straight edge (two, actually) I can tell the big and small ends are not parallel, and the error is in the direction that would explain the canted piston. For $60 bucks, I've bought a set of rods from a OM617 turbo parting out, and will find one that is the same weight and see if installing it causes the piston to sit properly. Other than matching the weight, any other advice would be appreciated.

But here is the mystery (to me at least). Along one side of the old rod are handwritten numbers that don't appear on any of the other rods. Every engine tells a story, and I'm wondering if this is normal (weight, maybe?) or if someone has been in this engine before, or if some "adjustment" was done at the factory. I have heard stories of manufacturers intentionally tweaking rods to account for boring errors, but that somehow doesn't seem like the Mercedes way. Also, the piston is slightly worn on the offset side, indicating that this has been the running condition of the engine for a while.

Any guesses?

When I purchased an Alfa Romeo Spider from a guy who had tried to rebuild the engine and failed, I got the car at a great price. Crank, rod bearings, pistons and sleeves were all brand new, and yet after about 100 miles of driving, the #2 rod bearing would start to knock. Upon inspection I found that the bearing was not wearing evenly, and would start to fail on the outside edge. Pulled the rod out and sure enough the bearing cap was not sitting square on the crank journal. Shop aligned the rod, and the car ran like a charm. If your have the old rod bearings, it might give you some indication regarding the bent rod. Good luck!!!!