Replace Piston Rings While Motor is Disassembled? - PeachParts Mercedes-Benz Forum

pragmatic · #1 02-28-2016, 06:26 PM

I'm doing a manual conversion on a new (to me) '82 300TD with approx. 230k miles. With bad motor mounts, a suspiciously slow starter motor and a leaking rear seal I decided to just pull the engine and tackle everything at once without crawling under the car for hours. I also figured I'd check out the condition of the internals.

So ... with the pan and head off, the block actually appears to be in decent shape. Cylinder bores mic out with .04-.06 piston to bore clearance, with taper of about .02-.03 from the top to middle of the bores - all within the service limit. Cross hatching is visible from about an inch below the top of ring travel. Before taking her apart, compression checked out between 320 (#5) and 340-360 across the rest of the cylinders. Not tip top, but nothing to sneeze at (valves adjusted first, etc., etc.).

So my question for the diesel experts is whether I should go ahead and hone and re-ring her while I have the motor apart? That #5 piston seems to be headed south, though I reckon that even so it has a lot of life left. I've also heard the horror stories of the new rings not seating even with a good hone and the motor ending up eating oil.

That said, my time is valuable and I don't want to tear down this engine again. So what would the experts do? Drop a $130 for relatively cheap preventative maintenance, though with a risk, or let sleeping dogs lie?

leathermang · #2 02-28-2016, 06:33 PM

Do you have a FSM for the 617.95 engine ? I assume it is the turbo engine ?
Is this a wagon ?
Have you already taken the pistons out of the block ?
That number five piston.. tell us why you feel that way about it .....?
Where do you get this $ 130 figure ?

pragmatic · #3 02-28-2016, 06:41 PM

Turbo wagon, yes sir. I have the link to the online CD with the right specs and the Haynes manual.

I've seen ring sets in that neighborhood, what is that too high/low? #5 seems to be wearing more than 1-4 and that matches the lower compression.

leathermang · #4 02-28-2016, 06:53 PM

If you want your rebuilt engine to last a long time... you need to go by the specs in the FSM... they are very specific...
The idea of getting that far into the engine...
and only honing and replacing the rings .... on an American engine.. sure... but not on these machines... that is the sort of thing which causes the ' oil eater' stories to get started...
You need to mic each of your pistons according to the FSM..
I suggest you read the section on installing sleeves.... and find the threads in the archives on that.. use my name for the search...

ah-kay · #5 02-29-2016, 12:28 AM

If I were you, I would at least take the piston out and take a look. I would clean out all the carbon in the grooves and on rings. There will be a lot of carbon deposit with 200k+ miles on the engine. You can replace the rings if you wanted to but some people reuse them. It is your call. It is possible that you may need to replace the oil rings as they are more brittle and may break when remove. Rings are cheap but the labor to get to them is expensive. You have a chance to do it for cheap.

leathermang · #6 02-29-2016, 01:28 AM

It is standard practice to replace the rings if they have been taken out of the engine... they are manufactured with a degree of roughness designed to wear to fit the bore. That is gone from already installed rings.

t walgamuth · #7 02-29-2016, 02:47 AM

With that mileage if it has been cared for the bores might meet spec as is but as LeathermanG says, check the FSM. If its apart I would as a minimum hone and replace rings.

leathermang · #8 02-29-2016, 09:14 AM

I can't really tell from his first post the extent of the take apart yet... he gave a piston to bore spec... but that can only be determined by having he piston out and measuring it... and why he thinks that number five piston is not looking as good as the rest without having it out and measuring it.... I don't know... so more answers are needed... but with the engine this far apart if the pistons are taken out and one wants it to last a long time and serve well... I lean (as per the FSM ) towards installing new sleeves in the block... these would then be honed to fit THE EXACT PISTON going into that bore...that is how proper FSM specs get accomplished. Not as extreme as it sounds and is how these fine machines have the potential for serving so many miles.

pragmatic · #9 02-29-2016, 10:35 AM

Thanks for the replies. The head, pan and all accessories are off the block and the crank will be coming out shortly to get at the rear seal. Clearance numbers are based on the actual measured bore diameter less the piston diameter stamped on the top of the piston (ie, corresponding to group 0, 1, or 2). I will obviously mic the pistons individually if I decide to remove them from the bores to do anything else ... which I am leaning toward doing ... but the pistons would have to be pretty far out of spec from their designated group for that to make a huge difference.

Off the top of my head the FSM specifies a service limit of .05 for taper and .12 for piston bore clearance. Based on these preliminary measurements the engine is still within spec (no more than .06 clearance and no more than .04 taper) with #5 being worse than the others for some reason. Again, I can still see most of the factory cross hatch from the original hone with no scoring.

So my options are to (a) resleeve and rering, assuming the pistons are in good shape $$$, (b) hone and rering $$, or (c) fix the rear seal, inspect bearings and declare victory $.

I will post up the actual measurements and some pics of the bores in a day or two, but for now I was mainly looking for general advice from the veteran diesel engine builders out there as to whether to dig deeper or leave it the heck alone.

ollo · #10 02-29-2016, 10:38 AM

+1 on replacing the liners, all of them, when doing an overhaul. During one overhaul, my machinist told me that 1 of the 5 cylinders was within specs and didn't require a new liner, so I saved a few $$. While the engine ran fine and and I had no complaints power wise compared to before overhaul, the cylinder with the old liner read 10 to 15 pounds lower when compression was checked. I'm sure when miles got high enough that cylinder would be the 1st to have cold start problems. Don

leathermang · #11 02-29-2016, 10:55 AM

+2
Ok... we now know you have been reading the FSM.... a huge advantage...
With all you are doing.... these engines are made so that you can go back to no taper and wear in a straight forward manner.... sleeves...
that is such an advantage over other designs.... that with your engine that far opened up... I think that is the prudent thing to do...

Dan Stokes · #12 02-29-2016, 11:02 AM

As an Old Fart with a lot of miles on me I'd say "Do it all or leave it alone". I'd either run it as is (probably good for another 200K or so) or do rings, sleeves, etc.

I did the rear seal w/o removing the crank and it's working fine. There's a thread on here about how to do that and I just followed it. So you CAN leave the engine as is and run it for many more happy miles.

BTW - your compression difference is not, in my experience, significant. There's a high probability that if you reran the test say, 100 miles from now, you would get slightly different results and #5 might test just fine. A tiny bit of carbon on a valve face or any number of other minor issues can make the difference you saw and (unless you ran the test several times over a period of time) I wouldn't worry much about it. Is it perfect and "as built"? No. But is it significant? African cabs run forever with worse specs than you have! My race engine has more variance than what you're seeing and I don't worry a bit about it.

Dan

97 SL320 · #13 02-29-2016, 05:44 PM

I've built / patched up / experimented with too many engines to count though for full disclosure, I have not built your particular engine.

A mini engine rebuild is possible for low cost and will extend the overall life of an engine.

The country of origin matters little to if the rings will seal or not. Pistons can be removed from the bores and reinstalled without issue. ( Just be sure to clean the carbon from the top of bore first. ) However, at 230 K , don't reuse rings if they have been removed from the pistons as expanding them could cause them to break.

If you replace the rings, use a hone that keeps the stones square to the bore, the center pivot ones don't do such a good job of straightening up the bore.

Ring break in is dictated on the engine load seen immediately after the rebuild. You don't want high RPM but you do want moderate to 3/4 loading in spurts. This forces the rings into the bores scrubbing off the high spots. If the engine is run at idle / very light load, the cross hatch fills with carbon and the high spots won't be shaved off any time soon.

Don't soak the piston in oil either, the oil will become trapped between the rings and break a land when the engine is run hard. I wipe a bit of oil on the skirts and that's it.

What is the condition of the rest of the car? ( Rust mostly ) It makes little sense to spend $ 5,000 on a rebuild if the engine will outlast the car. ( But if you had a spare body shell then a $ 5 K rebuild may be worth it. ) How many miles do you plan on driving the car?

My rebuilds vary from full on everything to new spec to "I know this engine won't blow up but might start to get tired in 50 K miles but by that point the car will be rusted away and that sure beats buying a worn out salvage yard motor".

A fresh set of piston rings , rod and main bearings , valve grind will do wonders for an engine. The trick is knowing where to stop adding parts otherwise you will end up with a full on rebuild.

The factory spec limits assume that the engine needs to run another X thousand miles untouched. I've used parts that were beyond the reuse limit but won't skimp on connecting rod oil clearance, this is the number one area that will cause a engine to fail.

leathermang · #14 02-29-2016, 06:03 PM

If you use oil when putting the engine together and no delays happen... sleep, work, accidents, etc... that is one thing... but normal engine rebuilding involves Assembly Grease... as talked about below... and most likely in the FSM...

Break-In Oils and Assembly Lube Needs - Engine Builder Magazine
..."""", if there’s not enough lubrication when the engine is first started, or if the motor oil that’s used during the break-in process fails to protect the cam and lifters or doesn’t allow the rings to seat.

A lot of things can go wrong during those first few minutes following the initial start-up of a freshly built engine. The engine has to build oil pressure quickly so all of the critical wear surfaces will receive lubrication.

Assembly lube is designed to cling to surfaces better than ordinary motor oil so there will be a protective film of lubricant until the engine is cranked over and started.

From that point on, oil pressure should take over and flush away most of the assembly lube. That’s why priming the oil system prior to starting it is so important. Priming prevents a dry start and reduces the lag time for oil to reach the bearings, cam and upper valvetrain components.

Ordinary motor oil can be used to lightly lubricate cylinder walls, lifter bores, wrist pins, piston rings, timing chains and bearing surfaces.

But if the engine sits for more than a few days, much of the protective oil film will trickle back into the crankcase."""""

leathermang · #15 02-29-2016, 06:08 PM

The ridge at the top of the bore is most often not just carbon.. but metal actually gets moved up to there... which calls for this tool... a ridge reamer with a carbide cutter..

TOOLS - Engine ridge reamer