280SE/SL engine rebuild - pistons

[wisnia07]

Hello,

I'm rebuilding a 280 engine, which has fake numbers stamped on. It's difficult to tell which exactly 130.xxx it is, and I'm having troubles in ordering oversize pistons.
I have mine out, and they are different from any piston for the M130 I found over the internet (ebay and specialized shops). I even spoke with Mahle factory in Poland, but they can't find anything similar. 

Correct me if I'm wrong - the M130 engines had two types of pistons, as on my attached table.
1. Flat, later USA version - Mahle 001 176 00, cast no. 86V16
2. Stepped, Euro version - Mahle 001 175 00, cast no. 86V13

My pistons have the sizes of the stepped Euro version, but their top is flat like in the US one. Cast no. is 86V13+
Had anyone heard about these pistons?

The engine head is 130 016 1101 with 9.5 comp ratio and it's marked 280SE.

[Shvegel]

There have been a few different pistons made that are no longer available.  I assume you may have some of those in your engine.  The most common obsolete version is the 4 piston ring version which was replaced by the 3 piston ring version with a 4th empty expansion groove.  86.5mm is standard bore so I assume you are trying to source 87mm pistons?  There are cheaper pistons in the 2nd oversize 87.5mm but the cylinder walls are very thin on that engine so I would stay with first oversize 87mm pistons.

One of the things to keep in mind that when an oversize piston is fitted the additional size of the larger bore needs to be accounted for otherwise the bigger piston would push more air up into the head increasing the cylinder pressure (Compression Ratio).  The two ways to do this on an oversize piston are to dish or step the top or to shrink the deck height which is the distance between the top of the piston and the top of the piston pin bore.

I guess the important thing to keep in mind is the that Mercedes changed compression in different markets by changing the head casting rather than the piston design and early and late 280's used the same pistons so if you can get pistons in the size you need they should be fine.
 

[Benz Dr.]

There have been a few different pistons made that are no longer available.  I assume you may have some of those in your engine.  The most common obsolete version is the 4 piston ring version which was replaced by the 3 piston ring version with a 4th empty expansion groove.  86.5mm is standard bore so I assume you are trying to source 87mm pistons?  There are cheaper pistons in the 2nd oversize 87.5mm but the cylinder walls are very thin on that engine so I would stay with first oversize 87mm pistons.

One of the things to keep in mind that when an oversize piston is fitted the additional size of the larger bore needs to be accounted for otherwise the bigger piston would push more air up into the head increasing the cylinder pressure (Compression Ratio).  The two ways to do this on an oversize piston are to dish or step the top or to shrink the deck height which is the distance between the top of the piston and the top of the piston pin bore.

I guess the important thing to keep in mind is the that Mercedes changed compression in different markets by changing the head casting rather than the piston design and early and late 280's used the same pistons so if you can get pistons in the size you need they should be fine.
[/quote

This isn't exactly what I've encountered by I agree mostly. Most oversize pistons I've used were slightly shorter and I ended up decking the block to get the proper piston recession. Planing the head will decrease the combustion chamber volume slightly while increasing the combustion ratio; so will decking the block. 175 - 180 PSI is optimal for a 280SL.  I would avoid using stepped pistons if you can find flat ones.

[wisnia07]

Thanks for your answers!

I will go for the first oversize pistons of course. The both previously mentioned Mahle pistons should be available in all sizes - they are in the 2017/18 catalogue. But I'm still not sure which should I choose.

- Stepped Mahle 001 75 01 - 1'st oversize - the deck height is 0,6mm shorter than the original (plus 1,3mm step on ca. 1/4 of the piston area).
or
- Flat Mahle 001 76 01 - 1'st oversize - deck height 1,3mm  shorter then original.

The closest in height and volume will be the first option, I believe. I will calculate it after I know how much the head planing took off.

Benz Dr. - Why would you avoid using stepped pistons?  I might order the flat option, but because of it's 1,3mm height difference I will probably end up decking the block. Wouldn't the valve timing be off after this operation?

Any suggestions?



 

[Benz Dr.]

There's more to engine building than boring out the block and fitting larger pistons.

 I avoid stepped pistons because they will lower compression by 10 to 15 PSI which relates to HP. Each pound of compression is roughly 1 HP on a 280SL so all of this counts. If you have to deck your block, the minimum you should be looking at is level with the top of the pistons. You can have your pistons higher than the top of the block by about .50 mm. I would look in the BBB to verify all of these specs. Regardless of all this info, you need .9 mm clearance between your intake valve and the piston crown with the crank shaft set to 5 degrees ATDC or you could run into trouble. You can have more clearance than that but I wouldn't go very much less. 

Valve timing will be affected slightly by planing the head or block but you can get offset keys to bring it back into spec. I would also use .015'' cam bearing shims to raise the cam shaft so that your timing chain stays tight.

[Shvegel]

Dan,
This is one of the areas we disagree.  When you bore a cylinder larger and retain the same piston deck height you are moving more air than stock which means you are going to have a higher cylinder pressure (Higher compression ratio) unless you either remove some material from the top of the piston (Stepped piston) or reduce the deck height of the piston slightly to bring the cylinder pressure back in line with stock numbers.  By removing material from the deck of the block (decking) you are actually raising the compression beyond stock.

[Benz Dr.]

Yes, if you have the same piston height as the original ones. .50 mm over size won't make a significant difference in compression increase and I would NEVER cut anything off the top of a piston - new or old. New pistons are shorter so you can deck the top of the block which is generally required anyway. Compression is HP and stepped pistons won't get you there.

[wisnia07]

Thanks for your advice!

I will follow it and order flat pistons.

I went to pick up my block from the mechanical workshop, to do some more measurements. My old pistons at top position are 0.4mm above the block. I took it as a starting point - I will order 001 76 01 flat pistons and deck the block to achieve the same cylinder volume.
The new pistons are 1,3mm shorter, so if I minus the oversize compensation and the head planning thickness I should be somewhere close.
1,3 - 0,095 - 0,15 = 1,055mm to take off the block. The new pistons will stick out by about 0,155mm.
Does it seem right to you?

[Benz Dr.]

According to what you have, your old pistons were .015'' above the parting surface of the block. This sounds like a stock configuration to me so you can consider that it will work OK because it did before.
You might want to test fit a piston before you deck the block just to see where it sits at the top of the bore. At any rate, you should try to get the top of your piston at least level with the top of the block. I usually leave a bit on the block in case I ever have to plane it again. So if you get the pistons .005'' above the block that should be OK.

 Keep in mind that every piston will sit at a slightly different height due to the connecting rods also being different lengths from center to center. This happens when the big end of the rod gets re-sized. This can be corrected through careful measuring/grinding but most of the time the differences are small and not of any real concern. I mention this because I've run into cases where number one connecting rod was shorter than all the other rods but it was what I used to base my measurements from. This is also the same reason why I don't take a maximum cut from the top of the block keeping in mind that all of the rods can be slightly different lengths.   

[wisnia07]

Thanks Benz Dr.  for all your help and advises! I will keep them in mind while working on this engine.

Unfortunately yesterday I have found out some cracks around one of the head bolts in the block. I somehow missed it before (as the machine shop did, telling me that the block is ready for 1st oversize boring..)

I spoke to a company being specialized in block repairs, but I still believe that the block should be made in one piece. Their technique is to cut the damaged part away, cast new material over it, and machine it as it was before.

Does any of you have some experience with that kind of repairs? I'm treating this as a last option and first I will try to source some replacement block,  but I wonder what is your opinion. Does it make any sense to you?

[mdsalemi]

...I have found out some cracks around one of the head bolts in the block...Does any of you have some experience with that kind of repairs? I'm treating this as a last option and first I will try to source some replacement block...

Have a read of this article: http://www.enginebuildermag.com/2015/04/engine-block-and-head-repair/

[ja17]

Latest versions of the M130 engine had removable threaded inserts in  the block for the head bolts. Maybe you can use these factory inserts to repair your cracked block.

[Shvegel]

There is a welder here in Cleveland Ohio who I use for Cast Iron welding.  His shop is littered with engine blocks that he is repairing.  He has been doing it for years.  I think any professional repair should be fine.  Another thing to keep in mind is Mercedes uses "Good Cast Iron" according to my welder.  He says it is much easier to repair than the cheaper grades.

[Benz Dr.]

I've seen cracks like this before. All of yours go towards the cylinder head screw hole which means coolant could leak into the threads and then slowly migrate between the screw and the hole through the cylinder head. Since you have two dissimilar metals, steel in the screw and the aluminium cylinder head, you can expect all lot of galvanic reaction to happen once coolant enters. I would  clean the hole with a tap and then use thread sealer in the hole to prevent coolant from entering. Repairing that area effectively by welding might be very difficult to manage.