PISTONS SPECIFIC

[Skitzerow]

Does anyone know to which side of the engine the arrows on the Mahle pistons are suppost to point.  My old pistons do not have arrows.  These are little arrows on the top surface of the piston.

There is a size number etched on the top of piston also. I would assume it is the exact size of the piston, not the bore size for the piston. Right?  I think I am right.  I just ask to confirm my convictions.

Thanks

SDK

[JimVillers]

SDK .... The arrows point forwards.  Yes the number stamped on the piston is the piston size.  The Technical Data Book has the information about the proper size for the cylinder machining to match the pistons.

Jim Villers
190SL, 230SL 5-Speed, 190E 2.3-16 Kompressor

[Benz Dr.]

Further to that, when you install the conecting rods the small notches in the big end where the bearing shells fit go to the distributor side of the engine. Make sure you oil the piston pin hole before you try and install the pin. It should slide freely as these are full floating piston pins. If you're using old pistons you may have to hone the holes a bit with a brake cylinder honer until the pins slide through the piston pin holes. Failure to do this could bite you.
 
The wire clips are hard to install unless you know how. I put a rag over the hole when I remove them because they can pop out and hit you in the eye. ( don' ask ) There's a small recess at the piston pin hole where you need to use a small screw driver to pry the clip in or out. The gap in the clip should be at about the 4 o'clock position. Start one end into the hole at about the 3 o'clock position and use a screw driver to carefully move the clip into place. What you want is to have the gap in the clip about 3 or 4mm above the notch. This will aid in removal some day. You will likely have to press the clip down a bit until it seats in a groove. Install the piston pin in the hole while you do this but keep it back away from the clip. What you don't want is to have the clip move too far into the hole and damage the finely machined surface.
Push the pin back out to make room for the connecting rod and then push it through the conecting rod and press it up against the clip you just installed. Make sure you install the other clip. The arrow on the top of the piston points towards the front of the engine and the notches in the conecting rod point to the left or distributor side.

The piston rings should be oiled well before you install. I like to spin them around in their grooves a few times to work the oil in well. Put the gap of the top ring directly over the centre of the piston skirt pointing towards the manifold side. The gap of the second ring should be set 180 degrees opposite to the top ring and the 3rd ring gap ( oil control ring ) should point towards the back of the engine. If there was a 4th ring the gap would point forwards.

Use a standard piston ring compressor and oil the bore in the block. Make sure the compressor is seated properly by taping it until it sits level with the hole in the block. I use a hammer handle to knock the piston into place. Going too slowly while doing this usually makes the rings catch on the top of the block. If it seems like it's getting stuck back it out and try again before you dammage something. I put the bearing shell into the big end of the conecting rod before I fit the pistons.
Once the compressor falls away and the piston is in the block stop and make sure the rod is going in straight. If not, turn it a bit so it will fit into the cranck shaft. I use a product called cam lube which is a very thick red oil. It's quite sticky and will stay where you put it. I'm sure there are several good products out there and even motor oil is OK if you plan on starting the engine as soon as the build is done.
 Put some oil on the threads before you install the nuts for the bearing caps. The nut goes on with the hex side up and the round side down. 280SL uses angle of rotation after the inital tighening. Use a bar not a ratchet for this job and turn it 90 degrees from your starting positon and then stop. Make sure you actually have it tourqued down to the right amount before you do the angle of rotation. These are stretch bolts which require this method. Don't worry it's easy to do.
 
Some people prime the engine by turning the oil pump with a rod on an electric drill. I almost broke my arm doing this when the oil pressure spiked up so I use the starter now and put lots of oil in the cylinders and around as many moving parts as I can. This is unlike a dry start for the most part because you will have no compression without the spark plugs installed. Spin the starter until you get oil pressure. The engine will slow down a bit when the oil pressure builds and then start to free up a bit once everything has oil running through.

See? Nothing to it.

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

[ja17]

Hello Scott,

Also, send a piston to the machine shop with the block fot fitting during the boring and honing process.

As Jim has mentioned the one set of numbers on the pistons is the piston diameter. The other "sp .003" number is the piston spacing or clearance number. Depending on which oversize piston you are using, the pistons may have a size of 87.48 mm stamped on the top along with  "sp .02 mm" which adds up to the bore size of 87.5 mm.

The M130 engine has a standard cylinder bore size of 86.5 so the stamped piston diameters may be 86.48 which gives the "sp" (spacing) of .02 mm.

First oversize of the cylinders is 87.00 mm so the pistons are stamped 86.98 mm diameter, giving a "sp" (spacing) again of .02 mm.

The second oversize of the cylinder bore is 87.50 mm so the pistons are stamped 87.48 mm diameter, giving a "sp" (spacing) again of .02 mm.



Joe Alexander
Blacklick, Ohio