Cylinder Head Problem

[68_white]

Hi You all (that's what we say here in Texas)

I got upset with my mechanic today. A few months ago he completely overhauled the engine on my 68 SL. After about 3,000 break in miles and some additional adjustments on the head and valves, I don't fell the car runs as strong as before. After I complained, he ran a compression test. He measured between 140 and 150 PSI.  From what I have read in MB manuals compression should be at least above 160 on a newly overhauled engine. He indicated to me that the cylinder head on my engine was the low compression one, because (he claimed) early 280SLs had a different head that the later ones.  I checked this out, because I took pictures when he was overhauling, and it is true. The shape of the area where the valves sit is different that a 71 model i found. The head on my engine says "E 9.5". So, wouldn't this be a high compression head also??

Please help.

Harry

68-white :evil:

[ja17]

Hello Harry,
Yes the combustion chambers in the early and late 280SL heads are different shapes, however the compression on the early and late heads  is exactly the same, 9.5 to 1.  

Take a look at our "Cylinder Head Chart"


Download Attachment: Cylinder head chart.jpg
24.31 KB

Make sure the engine is warm when the compression test is taken. I presume that the valve timing is set correct. A new engine may tend to be tighter and will loosen up with a little more use.

My biggest concern is that some other factor may be robbing power. Make sure the distributor and timing are correct. The vacuume advance, and centrifical advance along with the emission relays must work correctly for maximum power. The camshaft valve timing must be set right to have max power.  Also make sure the linkages are set correctly or your injection may be out of sync.

There is a slim chance that lower compression pistons were used in the rebuild. Were pistons replaced? How does the engine run otherwise, smooth?



Joe Alexander
Blacklick, Ohio

[Benz Dr.]

I had the same problem with the Rocket engine. The pistons were all slightly below the parting surface so I had the block decked about .030'' and the compression came up 30 PSI to around 175 - 180. I left the pistons in the block while doing this and it worked out OK.
I think  max oversize pistons are shorter just in case any extra machining is needed. Better to have lower pistons than ones that stick out of the block too far.

Dan Caron's
 SL Barn
benzbarn@ebtech.net
 slbarn.mbz.org
  1 877 661 6061

[68_white]

Joe, Dan, Thank you very much for your valuable feedback.  Joe, I like the chart you attached. It certainly clarifies the compression issue.

The engine was overhauled for the first time, and according to the mechanic who works only on classic MBs, the pistons were MAHLE "first oversize". How can I check if the pistons are the correct ones?? The engine timing according to the machanic is set correctly.  The engine runs fairly smoothly,but not as smooth as my friend's 71 model. Here is the deal on the power. Taking off from a stop is very good, but going from say 50mph to 80mph the car does not show the will to go, as it was the case before the overhaul. I think it is time to change mechanics.

[ja17]

Hello Harry,
With engine off, have someone sit in the car adn depress the accelerator pedal all the way  to the floor and hold it there. Now check the intake linkage at the intake venturi, make sure that the venturi is all the way open or you may not be getting full throttle!



Joe Alexander
Blacklick, Ohio

[68_white]

I will check the intake linkage and venturi. Then I will let you know. Thanks again.

Harry

HARRY

68_WHITE

[68_white]

I checked the venturi linkages today. The flap in the venturi control unit is wide open when the accelerator is depress all the way to the floor. There is no play in the linkages. Everything is nice and snug. Any other clues as to what is happening with the poor performance of the engine?? Thank you for your help.

HARRY

68_WHITE

[Benz Dr.]

Your problem with power loss is beacuse the engine is down on compression. The top of the piston has to be at least level with the top of the block. They can stick out a small amount but anything below the parting surface will mean a compression loss.
You can figure about 1 lb per .001'' below the parting surface. It makes that much difference.....

Dan Caron's
 SL Barn
benzbarn@ebtech.net
 slbarn.mbz.org
  1 877 661 6061

[68_white]

Thank you for the feedback Dan.  This is exactly what I was suspecting. Is it possible to check if the top of the piston comes up to the block surface without taking the head off??

Thanks again for your help.

HARRY

68_WHITE

[Benz Dr.]

Probably not.  You might be able to see inside of the cylinder but not right at the top.
My engine is about 175- 180 PSI. The small variation is from slightly different rod lengths. With the head off I could see that some of the pistons were at the parting surface and some were a bit below or above. None of them were very much but it still makes a difference.

Dan Caron's
 SL Barn
benzbarn@ebtech.net
 slbarn.mbz.org
  1 877 661 6061

[68_white]

I appreciate your kind response.



HARRY

68_WHITE

[68_white]

I have done some investigative work by reading and searching this website and others.

Also, I have gotten some information on the type of pistons my mechanic installed when he overhauled the engine.

The pistons are Mahle first oversize (87.0 mm). The MB part number on the box is 130 030 9017. Also, on the top of the piston it shows two numbers (86 97) and "USA"  stamped.

In one of the German supplier's websites I found the attached table that shows two different types of pistons for the 280SL.

The question that comes in my mind is: Does the type of piston have anything to do with the compression, and if yes, are the new pistons the correct ones :?:  :?:

My car has the original engine block and head, constructed in early 1968.



Download Attachment: PISTONS_3.JPG
75.44 KB

HARRY

68_WHITE

[68_white]

Well, I finally got some answers to my own questions. For those that would be overhauling your engines in the future, here is the scoop.

When overhauling my 1968 280sl engine, my mechanic installed Mahle 1st oversize pistons MB part no. 130-030-9017.  While these pistons are correct for later engines (eng no. 12-007230 and after on auto transm.), they are not correct for the early 280sl. The correct MB part no. for first oversize pistons on the early 280sl is MB 130-030-9917. When I contacted SLS in Germany they confirmed that and that the replacement pistons are made my NURAL not Mahle.

Here are two photos showing the difference between the two types of pistons. The early type has a little step, while the later type is flat on top.



Download Attachment: EARLY_280SL_PISTON.JPG
60.06 KB

Download Attachment: LATER_280SL_PISTON.jpg
23.48 KB

Now I got to go back to the drawing board and spend more money to replace the pistons.

HARRY

68_WHITE

[Benz Dr.]

That's completely bogus information. From what I've seen ALL standard pistons are flat on top. The pistons with the stepped area are low compression pistons.
I did an engine a few years ago where we used stepped pistons and the compression was 20 - 30 PSI lower than I wanted. I also have 2 early blocks here with fitted pistons, one is a 280SE and the other a 280SL ( 68/69) and they both have flat top pistons.
Stepped pistons are NOT standard issue but may be what you get, or all you can get.
 86.97 is the size of the piston at the bottom of the skirt. MB pistons always have the same .03 mm piston to wall clearance regardless of piston size. Even a 102mm 6.3 piston still has the same clearance.


Dan Caron's
 SL Barn
benzbarn@ebtech.net
 slbarn.mbz.org
  1 877 661 6061

[ja17]

Hello Guys,

Just to add to the confusion, I have been told the stepped pistons are  just a different manufacturer's design. There may be stepped high compression and stepped low compression pistons? The stepped pistons are three rings and the flat pistons are four rings. The distance of the first ring down from the top of the piston will also effect compression. I have seen stepped pistons made by Mahle also. USA delivery 280SLs did not have lower compression.

 I am not taking a stand on the issue. I think some more research would be needed to confirm these facts.

Measuring the displacement of each piston type may be the only way to the facts?
 
   
 



Joe Alexander
Blacklick, Ohio

[Benz Dr.]

These were Mahle pistons used in a 230SL - max oversize I think. Everything after mid 230SL pistons are all 3 ring pistons from what I've seen. I think the stepped pistons are not standard at all and they produce a lot lower compression ratios.
I'm still going to say that the piston height has a lot to do with how much compression you will get. I check the piston height to block surface before I take them out and compare the new piston to the old from the piston pin hole to the top of the piston. This should be the same but may not be.
When the conecting rod big ends are re sized this will also lower the pistons slightly. You would have to find the short rod and machine all the others to match to get them the same. A light cut on the top of the block would make up for this loss and I pretty much have all blocks machined now. After 35 + years they're not very flat any more.

Dan Caron's
 SL Barn
benzbarn@ebtech.net
 slbarn.mbz.org
  1 877 661 6061

[68_white]

I would not disagree with the Benz Dr., I simply want to state the facts. MB facts that is. I had contacted MB classics and they confirmed that 1st oversize replacement pistons for my early 68 280sl are 130-030-9917 (stepped) made by NURAL. They measure as follows:

dia = 86.975 mm
wrist pin to top of piston = 48.60 mm

The standard pistons taken out of my car (see photo below) measured as follows:

dia = 86.50 mm (std)
wrist pin to top of piston = 49.20 mm

Here is a close up photo of the engine block without the head. You may see that the original pistons had the step.


Download Attachment: BLOCK2.JPG
63.41 KB

Now the question still remains what to do?? Well, one thing I will do for sure, is I will not order any parts until everything is taken apart and measured properly and then I will make the decision what type of pistons to put back in. Dan, I certainly like your idea of resurfacing the block, but I do not want a domino effect, eg. something that would require more cutting.  Besides, I do not want my car to be anything other than what MB intended it to be.

I have more detailed photos, so if anyone want to see details, just let me know.




HARRY

68_WHITE

[TheEngineer]

Harry: You say, that yours is an early '68. Mine was built in January '68 and my s/n is 007537. My engine was rebored to first oversize sometime before 1991. I just had the head off and measured: The pistons are flat on top and marked: 86.97 and SP (Spiel) 0,03 meaning, there is 0,03 clearance between skirt and bore designed into the piston. No manufacturer's name or logo, except something that may look like an "m",but stylized, like three vertical lines and a horizontal top line. These pistons come to within 0.3 m/m of the top of the block parting surface as measured by feelergage. I measured compression yesterday and get about 185 psi except in the last two cylinders, but that is another subject. I also measured the bore on all cylinders: Top 3.4261, bottom 3.4245 inches. That is cylinder #6, which is the most worn. That tranlates to 0,038 m/m wear. Wear limit is 0,1 m/m. On the head, the valves were ground and I measure 2.2 m/m of intake valve travel with the crankshaft 5º BTDC pushing the valve down until it contacts the piston. That measurement should be a minimum of 0,9 m/m. Hope this helps. I have plenty of power, more than before.

'69 280SL,Signal Red,007537,tired engineer, West-Seattle, WA

[Benz Dr.]

Don't get confused here. The pistons that came out of your block are real but they're still low compression. MB is trying to hose us by telling us these are stock pistons and they're not. Flat top pistons are stock so someone might have already changed them in your car. Just because you find something in your engine doesn't mean it's right or done right. I see this all the time....

Like I said earlier, this is what MB has available these days. Several things have happened since the ''CLassic '' centre idea came along:
a)dramaticly higher prices on some items
b)WD's and re sellers without anything to sell to you.

I would only use stepped pistons as a last resort. You can use regular or low octaine fuel so that might be a consideration but the car will be clearly slower and probably run rich. Remember, the injection pump is set up to run at full compression......

Dan Caron's
 SL Barn
benzbarn@ebtech.net
 slbarn.mbz.org
  1 877 661 6061

[Raymond]

For my overhaul, I bought a block that was original bore and had it bored out to first oversize.  I had the deck honed just enough to true it.  I put in brand new Mahle pistons and they are flat topped, come up to the top of the block.  My cylinder head has been overhauled at least twice and is shaved to minimums.  The engine runs just fine now.  I don't think you want the stepped pistons.

Ray
'68 280SL 4-spd Coupe

[68_white]

Dan:

I agree that MB parts have gotten very expensive. I like your idea about trimming the block surface. The blue book (p01-0/2) shows that top of piston may project above parting surface by 0.2-0.7 mm

Assuming that I replace the pistons in my engine now (flat head - pin to top = 47.9mm) with those MB specifies (stepped head - pin to top = 48.6mm), what kind of compression increase should I get??

If I also trim the block surface by say 0.5mm, how much additional compression should I get??

Peter:

I appreciate the detailed measuments, that can only come from an engineer. Engineering is my passion also. According to the data base in this web site, my car was built in February 1968, and I tend to believe that the standard pistons (Mahle stepped) taken out for the overhaul were original. If not, why would someone change pistons with no reason and do nothing else to the head or not rebore the cylinders. With these pistons I was getting 155-160 PSI before the overhall (with 130,000 mi on the engine).

Raymond:

What is the height of your new pistons??  DO you have the MB or Mahle part numbers??

Any idea what compression you are getting??

Thank you all for your kind response.


HARRY

68_WHITE

[Benz Dr.]

What size are the pistons that you took out? Why did you take it apart if it had good compression since it was really only a bit lower than new?

 All I've even seen in just about everything I've taken apart is flat top pistons. You have to be carefull decking the block so that you don't remove too much. The piston has a maximum it can stick out of the block and I think I'd stay at least .010 - .020 above that amount. You can always go back and cut the block again some day but if you do it all now then that's all there is. Figure on at least level with the top of the block on the highest part of the piston if they're stepped - maybe another .010'' but not more.

Dan Caron's
 SL Barn
benzbarn@ebtech.net
 slbarn.mbz.org
  1 877 661 6061