DISASSEMBLY All Models
See Figures 1 through 19
This procedure is general and intended to apply to all Mercedes-Benz engines. It is suggested however, that you be entirely familiar with Mercedes-Benz engines and be equipped with the numerous special tools before attempting an engine rebuild. If at all in doubt concerning any procedure, refer the job to a qualified dealer. While this may be more expensive, it will probably produce better results in the end.
- Remove the engine and support it on an engine stand or other suitable support.
- Set the engine at TDC and matchmark the timing chain and timing gear(s). Be sure the timing marks align. Remove the cylinder head(s) and gasket(s).
- Remove the oil pan bolts, the pan, and, on most models, the lower crankcase section.
- Remove the oil pump.
- Matchmark the connecting rod bearing caps to identify the proper cylinder for reassembly. Matchmark the sides of the connecting rod and the side of the bearing cap for proper alignment. Pistons should bear an arrow indicating the front. If not, mark the front of the piston with an arrow with magic marker. Also identify pistons as to cylinder, so they may be replaced in their original location.
- Remove the connecting rod nuts, bearing caps, and lower bearing shells.
- Place small pieces of plastic tubing on rod bolts to prevent crankshaft damage.
- Inspect the crankshaft journals for nicks and roughness and measure diameters.
- Turn the engine over and ream the ridge from the top of the cylinders to remove all carbon deposits.
- Using a hammer handle or other piece of hardwood, gently tap the pistons and rods out from the bottom.
- The cylinder bores can be inspected at this time for taper and general wear.
- Check the pistons for proper size and inspect the ring grooves. If any rings are cracked, it is almost certain that the grooves are no longer true, because broken rings work up and down. It is best to replace any such worn pistons.
- The pistons, wrist pins, and connecting rods may be marked with a color dot assembly code. If a color code is present, only parts having the same color may be used together.
- If the cylinders are bored, make sure the machinist has the pistons beforehandcylinder bore sizes are nominal and the pistons must be individually fitted to the block. Maximum piston weight deviation in any one engine is 4 grams.
- The flywheel and crankshaft are balanced together as a unit. Matchmark the location of the flywheel relative to the crankshaft and remove the flywheel. Stretch bolts are used on some newer flywheels and can be identified by their "hourglass" shape. Once used, they should be discarded and replaced at assembly.
- Remove the water pump, alternator, and fuel pump, if not done previously.
- Unbolt and remove the vibration damper and crankshaft pulley. On certain models, it is necessary to clamp the vibration damper with C-clamps before removing the bolts. Otherwise, the vibration damper may come apart.
- Remove the timing chain tensioner and chain cover.
- Matchmark the position of the timing chain on the timing gear of the crankshaft.
- Matchmark the main bearing caps for number and position in the block. It is important that they are installed in their original positions. Most bearing caps are numbered for position. Remove the bearing caps.
- Lift the crankshaft out of the block in a forward direction.
- With the block completely disassembled, inspect the water passages and bearing webs for cracks. If the water passages are plugged with rust, they can be cleaned out by boiling the block at a radiator shop.
- Measure piston ring end-gap by sliding a new ring into the bore and measuring. Measure the gap at the top, bottom, and midpoint of piston travel and correct by filing or grinding the ring ends.
- To check bearing clearances, use Plastigagereg; inserted between the bearing and the crankshaft journal. Blow out all crankshaft oil passages before measuring; torque the bolts to specification. Plastigagereg; is a thin plastic strip that is crushed by the bearing cap and spreads out an amount in proportion to clearance. After torquing the bearing cap, remove the cap and compare the width of the Plastigagereg; with the scale.
Do not rotate the crankshaft. Bearing shells of various thicknesses are available and should be used to correct clearance; it may be necessary to machine the crankshaft journals undersize to obtain the proper oil clearance.
- Check crankshaft end-play using a feeler gauge.
- When installing new piston rings, ring grooves must be cleaned out, preferably using a special groove cleaner, although a broken ring will work as well. After installing the rings, check ring side clearance.
See Figures 20, 21, 22 and 23
- Assemble the engine using all new gaskets and seals, and make sure all parts are properly lubricated. Bearing shells and cylinder walls must be lubricated with engine oil before assembly. Make sure no metal chips remain in the cylinder bores or the crankcase.
- To install the piston and rod, turn the engine right side up and insert the rod into the cylinder. Clamp the rings to the piston, with their gaps equally spaced around the circumference, using a piston ring compressor. Gently tap the piston into the bore, using a hammer handle or similar hard wood, making sure the rings clear the edge.
Pistons on the 3.8 and 5.0L V-8's are installed with the arrow facing in the driving direction.
- Torque the rod and main caps to specification and try to turn the crankshaft by hand. It should turn with moderate resistance, not spin freely or be locked up. Stretch bolts are used for the connecting rods. These bolts are tightened by angle of rotation rather than by use of a torque wrench. Make sure the stretch section diameter is greater than 0.35 inch (-0.003 inch). Remove the bolt from the rod and measure the diameter at the point normally covered by the rod; it should be at least 0.31 in. For reasons of standardization, the angle of rotation for all the screw connections tightened according to angle of rotation has been set to 90- + 10-. Initially, the bolts should be torqued to 22-25 ft. lbs., then 90- past that point.
The bearing shells on the 3rd main bearing of the 190E series engines are fitted with thrust washers. The thrust washers in the bearing cap have two locating tabs to keep them from rotating. During assembly the grooves in the washers should face the crankcase thrust surfaces.
- Disassemble the oil pump and check the gear backlash. Place a straightedge on the cover and check for warpage. Deep scoring on the cover usually indicates that metal or dirt particles have been circulating through the oil system. Covers can be machined, but it is best to replace them if damaged.
- Install the oil pump.
- Install the oil pan and lower crankcase and tighten the bolts evenly all around, then turn the engine right side up and install the cylinder head gasket and head. Make sure the gasket surfaces are clean before installationa small dirt particle could cause gasket failure. Tighten the cylinder heat bolts in sequence to insure against distortion. Don't forget the small bolts at the front of the head.
- Install the engine into the vehicle.
It is a good practice to use a good break-in oil after an engine overhaul. Be sure that all fluids have been replaced and perform a general tune-up. Check the valve timing.