Can I just Flexhone these scored cylinders?

Hi folks.
Seeking advice on a newly rebuilt 2006 Sprinter turbodiesel engine that suffered an improper engine break -in. Please bear with me as I throw a lot of info at y'all.

This is the Mercedes 647 engine, 5 cyl. 2.7l common rail with variable gate turbo used in small box trucks and RV's.. not a hot rod.

The newly rebuilt engine had an airlock, so only had 1/2 the coolant in it during the 1st break-in drive. When I saw the temperature guage shoot up after 4 miles of driving, I shut off the engine, but the pistons seized as engine was shutting down. Later upon adding coolant, and completing the break-in, engine ran good, and did not seem to have too much blowby, and had good oil analysis samples. But used way too much oil. Scan tool compression check seemed good. 25,000 miles later, head gasket leak pressurizes radiator, so engine is torn down.

Tops of pistons were carboned up; good cyl. crosshatching; except for what I believe is a little top bore polishing from the carbon. Pistons are good.The problem is some very thin vertical scores from the pistons grabbing. Piston to cyl clearance is about 0.0027". These are almost brand new oversize Mahle pistons/rings, so I want to just flexhone the cylinders for new rings. I know the worst scores will remain. I am HOPING with a proper engine break-in the scores will not cause bad oil consumption. Because the blowby didnt seem too bad earlier. My THEORY is that the oil consumption was mainly caused by the improper ring break-in, not the scores, but I have NO experience in this matter. This is just a WAG.

Some of the TDI volkswagon folks use a 240 grit flexhone with the chromed rings in the cast iron block, for a re-ring job and new connecting rod bearings/bolts and head fix. That is my plan. Because it will save a lot of money. What do you folks think? Are these scores so bad a flexhone and rings won't solve the oil consumption problem?

Shown are 2 photos per piston. Only the scores that will catch a fingernail are denoted with arrows. Photo 1b of Cyl#1 has the deepest score. Thanks for your comments and experiences with this kind of re-ring job.

Helluva first post. Welcome sir, hope we can help you out of your pickle.

How deep would you estimate the scratches are?

Looks very minor, and a quick ball hone you should be fine. I know nothing about your particular engine, but .0027 is pretty tight, wouldn't hurt anything at all to hone the cylinder a thousandth to clean everything up. It was tight enough it already stuck a piston getting hot, surprised they look as good as they do after seizing up.

Yup. Run a ball hone through, pay attention to angle of drill and use lube, ATF is decent. And try to keep track of time and speed you feed the hone. And clean up GOOD when done.

Are the cylinders true and round now or is it a couple thou out of round in series with the head fastener holes? Could it be a machining error from not having a hone plate running that tight of tolerances?

Check the spec. on max bore size, it's gotta be at least .001-.002" more than what your measuring now. Drop the block off at the machine shop and ask them to MACHINE hone .001-.0015" out of the cyls. That's if you want it by the book. Honestly, in my experience I go by the old school machinist rule from back in the day: If you can't catch a fingernail on it, run it! A few swipes with a ball hone will fix er right up.

Thanks for all the quick replies. I wasnt expecting such a response. I'll try to address the comments:
The scores that can be felt by a fingernail are marked by the black arrows. There are 6 in all. The photos are 2 per cylinder, so the order of the photos is 1a, 1b, 2a, 2b, 3a, 3b, 4a, 4b, 5a, 5b. The cylinders are true and round, based on the the appearance of the cross-hatching. Only 25k on the engine. It was a VERY SHORT BRIEF overheat, but apparently screwed up the ring break-in.

The score in 1b is the deepest. You could never tell by looking at it. Some scores are not as deep... but this is all by "feel". The bottom 1.25" of the cylinders, where the most scoring is, are not swept by any of the rings. It's just where the skirts seized the most. The cylinders are 5 7/8" tall.

Almost nobody has a torque plate for the 5 cylinder Mercedes. So if a rigid or trueing hone is used, it has to have a torque plate. That is why I was thinking of doing it myself with a flexhone, to just deglaze for the new rings. And gamble on the oil consumption.

One machinist said sometimes it takes 0.005-0.010" hone to take out scores... which is totally way beyond the allowed piston clearance... these things are designed for tight clearance. I am quessing my scores are not that deep... but have no way to measure.

Some people have used the new abradable piston coatings on the pistons to take up the clearance with an over-hone. Maybe to a max of 0.010" might be honed out. Also fit oversized rings and file the endgaps.
I cant find any articles on this, and are not sure I want to be a guinea pig on this approach. But apparently it works. It has been done on the similar volkswagon TDI engines. And saves not having to buy new pistons.

I am thinking about hitting it with the flexhone, and see if the minor scratches are removed. Most likely the 6 "bad" scores will remain, as the flexhone wont remove hardly any material. If it still looks bad, maybe take it to the nearest machine shop with a torque plate... in Colorado! There is another one is Ohio...! Can't find one in Dallas area....

Any stories from people who have re-ringed with some scoring will be appreciated!

I do not have a specification for the max. bore size for these pistons. These are graphite coated pistons on the skirts. The tops of the pistons are marked by Mahle for 0.07mm (0.0028") larger bore dia. than the piston dia. stamped on the piston. Mercedes seems to make everything a secret... the only info I have is from Mahle, lol! Some of the graphite did get worn off by the seizure, and a few scores on the piston skirts, but doesnt look bad to me.

Run a ball hone through them and see what ya got?

If you're not comfortable doing it then don't. Take it to a good shop. If it's out of round, or worse tapered, you'll likely make it even worse.

Chassisman said:

If you're not comfortable doing it then don't. Take it to a good shop. If it's out of round, or worse tapered, you'll likely make it even worse.

Click to expand...

I hear you.
But I believe the bores are fine, except for the scoring. The whole problem I am having is how much oil consumption can occur because of 6 scores, marked by the black arrows.
This is because the cross-section area of the scores has to be tiny, say 0.003" x 0.003" as a guess. So let me assume after I flexhone, I am left with 6 scores that each have a 0.000009 sq. in. cross-section area for hot gas and oil to pass through. This will be about 1 score per cylinder.
Now compare the score area to the open area left when the ring end gaps close up. The area of the ring gaps has to be huge compared to the area of the scores.
Because of this, and the cylinders are under constant positive pressure all the time (no vacuum 'cause it's a turbo diesel), this lends me to believe that any oil consumption from the rings OR scores has to be because of some HYDRAULIC EFFECT (that I dont understand).
AND/OR the scores can pass hot gas (!) which turns into blowby; which passes through a CCV filter; but will still carry some oil into the intake manifold to get combusted. Which is maybe where the oil consumption can occur.
Sorry for the dissertation here; I am arguing with myself, lol, but the purists all say to rebuild these engines perfectly, but I am having a hard time believing 6 scores like this are going to cause significant oil consumption.
If i'm wrong,it will be a very expensive mistake. I am hoping somebody that did a re-ring job similar to this will see this post and chime in on their experience.
These engines use 1/2-1 qt. in 10,000 miles when everything is perfect. This thing was using almost 6 qts in 10,000 miles! (turbo was dry).
I know some engineer has written a paper on this; but I can't find it on the internet.