Engine Block.

[tel76]

Another question for the Benz Dr ,JA17 and anyone else,
The cylinder block on the 1968 had excessive wear so i have had it re bored,the pistons that were fitted are Maule and they were stepped, before i took it to be re bored i calculated that the pistons were located 010 thou: down below the block face.
I also measured the cyl: block and this confirmed that the block had not been surface ground.
I have got the block back to gather with the oversize pistons,( the new pistons are JE manufactured in Australia) i refitted the crankshaft using the original bearings(i will be fitting new ones on final assy:).
I fitted the new pistons on the con: rods 1 and 6 and fitted them into the block (no rings fitted).
The reason for doing this was to ascertain how much metal i could remove from the block to get the pistons flush with the block.
I was surprised to find that the new pistons protrude 015 thou above the block surface.
As i want to have the block surface ground so that i can start the rebuild with a surface that is flat, i have returned the block to the machinist with the instructions to remove 005 thou:.
As i will then have to have the pistons topped what is your recommendation,should i have the pistons flush with the top of the block or go to 010 thou: below as per the Maule pistons?
PS i do not want to have to use 5* fuel

[Shvegel]

It is unusual to see oversize pistons actually higher in the block. Usually they will be lower or there will be some sort of dishing in the crown to account for the larger bore size which would normally increase compression.

I just went in the cellar to deep storage and unwrapped one of my Mahle 86.97mm diameter pistons(1st oversize 87mm bore) that I stored away for when I need them.  I measured the distance from the top of the piston pin bore to the crown at 1.3940".

If you are trying to keep the compression the same as before you will have to keep in mind that you increased the bore size which will increase the compression ratio, you dropped the block deck down and if you shaved the head as well that all needs to be taken into account.

If anything your pistons should end up further down the bore than before.

I just did some rough calculations and according to my math the stock cylinder(86.5mm or 3.405" X 78.8mm or 3.102" stroke) is 28.22 cubic inches and the .5mm oversize(87.0mm or 3.425" bore X 78.8mm or 3.102" stroke) is 28.55 cubic inches. Which means that without taking into account any machine work on the head(resurfacing and valves being deeper in the seats) the pistons should be .036" lower than before.  

If the pistons were .010 down before they should be .046 down from the finished block surface now.



I can't get a compression ratio with out the volume of the cylinder head combustion chamber and the installed head gasket thickness but for comparisons sake if the chamber and gasket are 2.7 cubic inches the stock bore compression would be 10.45 to 1(28.22 cubic inches of swept volume divided by 2.7 cubic inches in the combustion chamber.)  with the larger bore it would be 10.57 to 1 so I wouldn't sweat it too much providing piston installed height were the same.

Here is where it gets fun. Because the piston top is higher and therefore protruding into the combustion chamber .025" higher than before we lose with the .005 cut to the block included .276 cubic inches which leaves us 2.42 cubic inches rather than our 2.7 that we had with the stock piston. When we divide our swept volume of 28.55 cubic inches by the new combustion chamber volume  we get 11.7 which is 11.7 to 1 compression which is race engine territory.

Also since the Valves are now .030" closer to the pistons without adding any material removed from the head. You will need to verify piston to valve clearance.

Please check the math as it is not my strong suit.

[pj-tigger]

I had a similar issue with a 250 block i was rebuilding. The engine had snapped a conrod  & has to have extensive machining to recover it. As a backup i'd obtained a 250 engine from a 108 saloon. The 108 engine had Mahle pistons and these had a slot/groove (about 25mm wide) machined across the piston. The high part of the piston was about 10 thou above the deck height and the slot level with the deck. The reason for the slot was a bit of a mystery & when we went through all the Mahle books the machine shop had we could not find it listed in any of the specs for the 250 pistons.

[Naj ✝︎]

pj,

Here's some info on M129 pistons.

Stepped pistons are shown on the M 130 engines but not this one.

http://www.motorenteile.mahle.com/eLIZA/mahle/query/engine/byParams?cocoon-action-0c6907340c8c2c5c27526c712d8d7b7c693c4c24:EngineQueryResult:queryProductDetails&%2Fresult%2Fselector%2FnodePath=%2F7073%2FrelatedProducts%2F83424#spa-1499384432


naj

[tel76]

Hello Naj,
Do you have the compression height of the +040 pistons fitted to the 130 engine?
Is there a difference between MB and Mahle pistons? IE compression heights.

[tel76]

Hello Shvegel,
Just to add another dimension, the block has been bored to 2nd oversize +040 and the head has been skimmed(by the previous repairer).
I have read(using the search function)all the posts regarding pistons and i do not think anyone has answered the question why Mahle introduced the stepped piston(i stand to be corrected if someone has).
Is it possible that the stepped piston was introduced to lower the comp: ratio to overcome this very problem together with the valve/piston timing issue?
In the next few days i will find my Burette and ascertain the combustion space volume.
The Mahle pistons that you have in stock do they have a step or are they flat?
Hello Pj-Tigger.
Is it possible that your machined pistons could be for the same reason ie lower the comp: ratio?

[Naj ✝︎]

Hello Naj,
Do you have the compression height of the +040 pistons fitted to the 130 engine?
Is there a difference between MB and Mahle pistons? IE compression heights.

Hi, Eric,

Some dimensions here that may help:

http://www.motorenteile.mahle.com/eLIZA/mahle/query/engine/byParams?cocoon-action-34775c7959863d57011c1f657b37483e08651478:EngineQueryResult:queryProductDetails&%2Fresult%2Fselector%2FnodePath=%2F11083%2FrelatedProducts%2F83328#spa1083001323

[Shvegel]

Hi,
The Mahle first oversize pistons I have are not stepped. I assumed Mahle was just adjusting the installed height to compensate for the increased bore.  At second oversize I would expect a small dish in the middle say half the diameter of the piston about .030" deep.

If you are digging out a Burette don't forget to add the installed thickness of the head gasket to your equation as well as subtracting the piston protrusion above the block which will decrease the size of the combustion chamber.

There were I recall also low compression versions of our engine that were used in developing countries that should also use a dished piston. If memory serves there was also a suspension that had greater ground clearance for rough roads. Sort of a Rhino packaged SL.

[ja17]

Hello,

Like Pat, I always figured that the step pistons were to compensate for the increase in volume of the enlarged oversize bores.  Even the larger oversizes are stepped.

Often times manufacturers make improved design changes.?

Pj, at some point so many variables will be changed that a volume check of the combustion chamber and cylinders will be the only way to make sure what your compression will be.

Normally variations in piston sizes, bore oversizes, and stock piston designs will not make more  than a 5% difference from new compression.  Radical changes in piston design, cylinder head cut and block cut, bore and/or stroke may make a more significant change in compression.

Following the "permissable limits" for machining and wear as listed in the "Technical Data Booklets" will normally keep you out of trouble.

Certain areas of the head can be carefully carved out to increase the volume of the combustion chambers to even up the compression in all cylinders or to lower the compression overall  in all cylinders if needed.

Latest design pistons did have that large groove between some of the rings, almost like another ring groove left empty.
Extreme care must be taken during installation. This gap weakens the ring groove bellow enough that if the lower ring catches during piston installation the pison ring groove land will break.

[Benz Dr.]

There some info here, but which is right?

Over bore will not increase CR very much, if at all. New pistons are often a bit shorter            ( about .010 '' ) to compensate for decking the block or milling the head - that IS a design feature.
 Those stepped pistons are crap and I loath to use them . However, for some engines that's all you can get. I believe this has nothing to do with design change and it's more likely MB off loading a pile of low compression stuff sitting on their shelves.

A well built 230SL can produce up to 165 PSI. Each point of compression is worth about 1 HP so every one of them counts. The low compression pistons have a hard time knocking out 150 and high compression is where you make power in a small bore engine. The only positive spin you can get from this is the low compression pistons will allow you to run on regular 87 octaine fuel.
  Valve duration plays a large roll in what kind of compression figures you will get. The more duration the lowerr the compression numbers at starter speed but there's also the benifit of lowered risk to pinging. These engines have medium duration which why they have such good mid range torque.

[ja17]

Hello,

Does anyone have some hard facts on these stepped pistons?  Some actual documentation from MB or the manufacturer would dispell a lot of speculation.

I am tempted to go out and do a volume measurement with one of each piston on one of the engines laying around here.

[tel76]

Hello Shvegel,
Following your comments ,when i returned the block for re-surface i asked them to remove the minimum,as a result only 0.0025thou was removed.
I found the Burette and today came up with the following-
1- The combustion volume is 46.7cc s
2-The inlet valve is 0.015thou below the head face.
3-Old cylinder head gasket is 0.080thou thick.
Your observations and calculations would be appreciated.
Hello Benz Dr.
As stated the JP Pistons are 0.015 ABOVE the block surface (flat top) and as i do not want to use 5* fuel the pistons will have to be machined,but the question is by how much?

[Benz Dr.]

It's not speculation Joe, they're definately a lot lower compression.

( to the guy who asked me this question - and because you asked  )
OK, you are going in the wrong direction. Never think about cutting a piston up - that's a bad idea. Your intake valves are sticking out of the head too far. They should be least .5 mm recessed into the head and never sticking out. This would indicate that the valves are fairly new and the head has been cut down quite a bit. You will need to have the intake valve seats cut deeper. This will make the valve stems stick out more which affects rocker height. I would consider placing a . 020 valve shim under the spring to retain spring tension.

To test the valve to piston clearance you need to install the head with the timing chain. Turn the engine over to number one piston, 5 degrees ATDC, which is where the piston and the intake valve will be very close to each other. Move the intake valve down and measure with a dial gage to see how far it moves. The intake valve will need to move down .9 mm or more before it touches the piston to be in spec.
With a 1 mm head gasket and .5 mm valve depth there should never be a problem
 with valves hitting pistons. The head gasket will squeeze down a bit once the screws are tight so minimum average will be around .050'' or more.

Your fears about the compression being too high are unfounded I think. These cars will easily handle 170 PSI.  My own engine is in the 175 - 180 range and it never pings.

If you end up cutting the valve seats deeper watch to make sure that the rockers sit level and that the ball stud adjusters don't bottom out. You can quickly ruin a whole set of new valve guides by the time the engine is warmed up. It will smoke and it will always smoke. The only cure is to install new guides or bore the holes out and install sleeves in the installed valve guides.
I use a .015'' shim under each cam bearing pretty much as a standard proceedure these days. This allows proper rocker geometry and plenty of adjustment on the ball studs. Most of those are replaced as standard practice as well.

[Shvegel]

Here is what I came up with:
Cylinder swept volume: 28.87 cubic inches.  (87.5mm bore to inches is 3.444 divide by 2 to get radius of 1.72. Pi X radius squared = 9.310 square inches X 3.102 stroke is 28.87 cubic inches)

Combustion chamber is 3.44 cubic inches.  (46.7 cc to cubic inches is 2.84 cubic inches, add the head gasket(.080 X 9.310 square inch cylinder or .744 ci and subtract the piston protrusion in the chamber(.015 X 9.310 or .139ci)

So 28.87ci / 3.44 is 8.39 to 1 which seems really low. could you re-check the combustion chamber volume?

[Travis71280]

I believe when you calculate compression ratio, you take the sweep volume plus the total chamber volume then you divide by the total chamber volume. Other than that i think all your calculations are correct.

[Travis71280]

Came out to 9.39 to 1 compression, is that about right?

[Shvegel]

Travis you are absolutely right. My mistake. Actually 9.39 to 1 isn't horrible. I might be tempted to flush the pistons with the top of the block but not much more.

[tel76]

Hello All,
Sorry about not keeping up with developments, but i have just arrived in Cyprus for the holiday period,will get back in due course

[Benz Dr.]

I talked to my machinist the other day and asked him about some of this.
 His answer was, '' Before you start machinig pistons you better know what you're doing and have a damn good reason for doing it.''

At 9.3 :1 you are well within the specs of the standard build. I'm not familair with number 5 fuel. How does that compare to the octaine rating system we employ in NA. I know the fuels have changed a lot since these cars were built so most of the old specs don't mean as much.