Spot the issue(s)

[stickandrudderman]

We sold this car (230) to a customer a few years back and sold it cheap on the understanding that it was a very sound car that needed further investment and it was a car previously unknown to us so we couldn't vouch for it.
One of the things that we've since noticed is excessive oil leaks from the sump and water stains down the left side of the block from the head gasket area.
Having removed the head I was very surprised to see that the pistons look like they were fitted last week; they are quite obviously new but more worryingly they are not Mahle pistons.
We've also noticed something rather strange going on with the head. Who can see it? (Professionals are disallowed form commenting for 48 hours!)

[WRe]

Hi,
some valve guides look strange, some are corroded (old, burned) some are braas shining (new) and different.
...WRe

[waltklatt]

As WRe says, but also how is the oil getting to the top.
Seems the opening for the oil coming from the sump is blocked by a plug or something there at the base of the cam tower at #1 cylnder.
The others I cannot make out.


Also, are you working on this car now.  Theought you sold it and that was the last of it.
Walter
1967 220SL-diesel

[stickandrudderman]

Sales is not my core business; service/repair/restoration is. We rarely sell a car a that we don't expect to see again. It is the return of cars for their on-going maintenance needs that is my primary focus. I'm happy to take a minimal profit on a sale as long as I get to make sure that the cars eventually become the cars they deserve to be.
This car was sold at a reduced price as a "driver" with a list, not definitive, of known issues that we expect the customer to employ us to resolve going forward.
The customer is happy with that arrangement and this work is part of that process.

[Benz Dr.]

I'm not cheating - this is a general question. Why would someone make after market pistons and step them? You will loose about 15 - 20 PSI of compression with those things right out of the box.

[Tyler S]

I'm not cheating - this is a general question. Why would someone make after market pistons and step them? You will loose about 15 - 20 PSI of compression with those things right out of the box.

To compensate for an under spec deck or head thickness perhaps? Bring the compression back down?

Why would someone only replace 3 guides.

[114015]

Hello Colin,

Very interesting finding.
Engine looks like kind of "cleaned" before you received it. No dirt, no soot, no traces of burned oil, etc.
I once in the past had an issue (hairline crack) with the old cylinder head on my M180 and cylinder 3 looked like vapour cleaned.
Here, all cylinders look like that.

From the parts list I remember that "Kolbenschmidt" at least partly was the alternative supplier of "Kolben(s)" (= pistons) besides Mahle. Maybe the are from them ...

230 engines had three generations of pistons depending on the serial # of the engine: early, middle and late.
Please see here for an excellent explanation:

http://www.sls-hh-shop.de/en/230-280SL-W113/03-Engine-M127-M129-M130/03-b-Info-Pistons-230-280SL-R113/

The last generation of pistons was stepped (as are my M180 factory-original M180 pistons).
I think that's what the reason is for these pistons here as well: last generation.

What looks also funny to me is that each piston "appears to have" a "kind of hole" in the center...
But that's certainly a visual artefact I'd guess.

Still surprised,
Achim

[stickandrudderman]

I'm afraid my pictures are slightly misleading in that we have wiped the top of the Pistons with a rag. There was SOME evidence of combustion but it literally took a rag and some solvent to wipe them clean. SOME Pistons had wet, black deposits on them.........
I have heard that there is an Australian supplier of aftermarket Pistons but no-one I have spoken to recommends them. It's possible that these are those but unless I remove one to examine it I won't know and for that I need to consult with the customer. The engine was running fine and we don't want to open a Pandora's box!

[twistedtree]

Others have mentioned all this, but...

I think the valve guides are the most unexpected.  It looks like a few, but not all were replaced judging from their appearance.  At a minimum I'd expect to see the exhaust or intake guides replaced as full sets, and more typically all 12 would be replaced if any were sloppy.

And the piston's are curious, though not necessarily a problem.  I've never seen a ridge step like that, nor the dimple in the middle.  And .040 seems like a pretty large overbore, though I suppose serviceable.

[twistedtree]

Oh, and it looks like your water leak may originate to the top of the head?  That looks like coolant residue on top of and around the large plug bolt, running out to the side of the head.  I would suspect the plug bolt as the source of your coolant leak.  But it's hard to see how that area could be leaking coolant and not have it also get into the oil.  Was their coolant contamination in the oil?  You said the oil was leaking.  Was the source clearly identifiable, or could the the problems be related?

[wjsvb ✝︎]

As for the "dimple", many moons ago, pistons were available "semi-finished" and then turned to size to fit the block at hand.   The dimple was used to position the piston in the lathe.  I have a set of straight-eight Buick pistons with these.  Jon

[Tyler S]

The witness marks on the deck from the old head gasket look like its awfully close to the coolant passages and head bolt holes. Was the correct head gasket installed?

[Benz Dr.]

To compensate for an under spec deck or head thickness perhaps? Bring the compression back down?

Why would someone only replace 3 guides.

You might think so but a really good 230 engine only runs about 165 PSI when new. Stepped pistons will lower that amount by 10 to 15 PSI and if the head was milled to the point of compression problems it would be junk anyway. You actually want some compression in these engines so they'll make it up a hill.

[Tyler S]

There has to be a reason. Adding that step is one more process in the manufacture. The only other reasons I can think of is for valve interference issue or re-directing the flame front. Again from an under spec head. But the step down appears to be on the exhaust side so that doesn't make sense.

[Benz Dr.]

I was told they ran out of proper flat topped pistons and since they had a slew of low compression units sitting around collecting dust, they thought it was a good chance to unload them on those who have limited options .

 Since it makes zero sense to lower the compression ratio in an engine that really isn't very high to begin with, I kind of buy the story. All of the early 4 ring  pistons and the third version 3 ring pistons were all flat topped until this change was made and yet those early pistons worked just fine. In short, there is no good reason.

 That's what they have in stock, that's what you're gonna get, and if you don't like it, too bad.

 However, why make repro pistons that way too. That's just......... I don't know.......silly?

   

[stickandrudderman]

OK.
I have, over the years, seen many stepped pistons but since until recently I would only ever buy parts direct from Mercedes and not question whatever they sent me (I'm a slow learner......) I have never bothered to research the reason for the step.
However, I have never before seen pistons with what I can confirm is a detent, presumably for centring!
You all readily spotted the 3 new valve guides which is just the most bizarre thing (that three are new, not that you spotted it!) but want you can't really deduce from my poor photos is that not all of the rest of the guides are the same height! Some are 3mm taller than the others and have resulted in the spring cup hitting the valve seal and damaging it. Naturally this causes seal failure and the subsequent wet blackness on some of the pistons. Additionally, all of the exhaust valve guides have excessive wear.
There were signs of coolant leaks from not only the head gasket but one of the plugs in the head and one of the plates on the side of the block.
Lastly, I suspect we will have to be stripping pistons out because I think the block needs decking but I need to speak to the owner to get authority to proceed and that can't happen until Wednesday.

[drmb]

My understanding of the step in oversize pistons is to keep the compression ratio the same as when the cyl. bores were std.when the cylinders are rebored the swept volume is increased so the step is put in place to compensate,i have fitted stepped oversize pistons for years and have never seen a drop in compression pressure when checked at a later date.

[Benz Dr.]

Hummmm......... maybe compression gauges work like vortexes down under.

  I don't believe that at all. You will get a small increase in compression with max over bore but it's not so much that you would need to decompress the engine. All I've ever seen is a decrease in overall compression numbers on every one I've looked at. 190SL's have flat top pistons and so do most of the other cars from the time period. Some run at lower ratios and others are about the same as a 113 so I just don't see it. Compression is HP and knocking 15 PSI off the top end doesn't make a lot of sense. My source, for what ever that's worth, told me they were low compression 230 sedan pistons and since they had lots of them, and they fit this engine, why not clear them out. They work, you can use regular 87 gas and most owners wouldn't notice the difference unless they lived in the mountains.

Some may believe this was a modification with some intended purpose. I don't think so. The math isn't there.

[66andBlue]

Hmmm .... Dan, you have become soft, or should I say diplomatic?
11 year ago when we had this discussion you wrote in response to posted information "That's completely bogus information."   
http://www.sl113.org/forums/index.php?topic=4022
So after all this time we still don't know why these stepped pistons were appropriate for a 280SL??

[Benz Dr.]

Hmmm .... Dan, you have become soft, or should I say diplomatic?
11 year ago when we had this discussion you wrote in response to posted information "That's completely bogus information."   
http://www.sl113.org/forums/index.php?topic=4022
So after all this time we still don't know why these stepped pistons were appropriate for a 280SL??

My views haven't changed at all - MB is still selling us a bill of goods. I was in very poor health back then and I think that everything bothered me a lot more because of it. I've mellowed a little bit since then.......... 

That's the problem with the internet. Someone will be able to go back 100 years from now and read this to my great grandchildren so I'm being nicer now, OK? 

[stickandrudderman]

Turns out this car was about to self destruct due to one rod end cap being installed the wrong way around. The bearing was well on its way to failing.
You can also see the detail difference between aftermarket and original pistons here.

[Shvegel]

My understanding of the step would be the same as Drmb in that it is in compensation for the additional swept volume due to overbore.  If one were willing to do the math I would guess that the material missing would closely approximate  the original bore in mm X 3.14 X stroke in mm X 1.015mm(.040" oversize in mm) divided by the design compression ratio. If anyone has flat top Mahle pistons in standard bore and first oversize I would bet the pin bore to crown measurement would be less on the oversize piston. I have seen dished pistons that had visibly deeper dishing on the oversize as well as flat top pistons that had deeper valve pockets in the oversize dimensions. 

Cutting a step is a weird way to do it which is probably why we don't see it very often.  Not only do you have to probably move the pistons to a milling machine but that hard edge is a great place to start a little detonation hot spot.

Just looked up Nural pistons to see if the markings matched the stepped piston and it looks like they do.  That being said how do the Jag guys get so lucky with the price?  http://empirebritishcarparts.com/JAGUAR-4-2-NURAL-ENGINE-PISTON-SET-9-1-020-p2318.html

[Shvegel]

I did the math and I would guess the step is pretty close to 1.5mm.  That is assuming the area of the step is 1/4 of the area of the piston top.  given the overbore you would need to remove 2046 cubic millimeters from the combustion chamber to keep the same compression ratio.