Restoration Corner
Bruce Adams
 
Rebuilding a “Short Block”
 
After 100,000 miles of everyday driving, most engines develop an oil consumption problem and begin to experience signs of wear including loss of compression and power, increased emissions, lower oil pressure, and internal noise.
 
Article Image: Engine rebuilder Todd Prevatte testing a new short block rebuild.
 
The average life cycle of a Mercedes-Benz engine manufactured in the ’50s and ’60s is about 10 to 15 years under normal use. Carburetors are in part blame for the wear because rich fuel mixtures wash the lubricating oil off the cylinder walls and dilute the oil in the crankcase. These older engines were also built using wider tolerances than today’s engines, which create a “blow-by” situation, that is, oil bypassing the piston rings and entering the combustion chambers. When those conditions developed, the rings, bearings and valve guides all experienced accelerated wear.
 
The average service life of today’s engine is expected to be significant longer because of improvements in engine technology extending engine life and reducing the need for engine service. Fuel injection has all but eliminated the fuel wash-down problem, and much tighter tolerances have reduced blow-by and oil dilution in the crankcase. Fewer engines are being rebuilt today as a result.
 
In the last issue of The Star, (Nov/Dec 2011) pg. 66, compression and leakdown testing was discussed. Do not rebuild your engine without first doing due-diligence engine analysis. In our Workshop, we recently did such an analysis on a new restoration project. In this case the 190SL was drivable and during the test-drive the motor showed power and acceptable performance. Our next step was a compression and leakdown test to determine if a rebuild was absolutely necessary. The results of the test were:
 

Those results are “wet” figures (measured with a squirt of oil into each cylinder) to ensure accuracy).  The numbers indicate a major problem in the No. 2 cylinder. Our recommendation to this customer was to proceed with at least a short block rebuild, that is, a rebuild of the engine block without doing any work on the cyinder head valve train and valve seats. Once disassembled we would further evaluate the valve train and cylinder head.
 


The picture shows the No. 2 piston and confirms the test results, showing a ring stuck in the piston groove. The condition was probably caused by overheating and/or maintaining low oil levels.
 
Piston Rings and Cylinders
 
Low compression and oil burning are usually a sign of worn rings and/or cylinders. You will also note in the above reading the compression reading in cylinder No. 2 was very low. Replacing the piston rings can restore compression if the cylinders do not exceed service specifications. But if the cylinders are worn or damaged, boring the cylinders to oversize will be necessary to restore proper clearances and compression. Replacement rings come in various materials and sizes. Most compression rings are cast iron, though many import engines have steel rings. Rings may be plain-faced, chrome-plated, inlaid with molybdenum (“moly”), or nitrided (heat-cycle processed) for added durability. Replacement rings should generally be the same types as the original.

Refer to the engine data plate and corresponding M-B parts manual to determine the correct rings for the engine. Oversize rings and pistons of the corresponding size will obviously be needed if the cylinders need to be bored or honed to oversize.
 
If the cylinders are worn, they will have to be bored or honed to accept oversize pistons and rings. If not, you can run a glaze breaker down the bores. Worn guides can be reamed, replaced or relined in-house with a few special tools. But jobs such as head resurfacing, line boring, crank refinishing, and balancing will have to be done in a machine shop. Finding a local machine shop that has metric experience and Mercedes-Benz documentation and experience is a must.
 
Cylinders must always be cleaned before new rings and pistons are installed. This means scrubbing the bores with a mineral-based solvent such as naptha or Varsal to remove all traces of honing residue and metal.

Always use a ring expander to install new rings on pistons, and a ring compressor to install the piston assemblies in the block. Cylinder walls must also be lubed to protect the rings and pistons against scuffing when the engine is first started.
 
Crankshaft Bearings
 
These bearings are always required when rebuilding an engine. When you remove the old bearings, inspect them for unusual wear or damage such as scoring, wiping, dirt or debris embedded in the surface of the bearings. Anything other than normal wear may indicate an underlying problem that needs to be corrected before the new bearings are installed. Dirt contamination often causes premature bearing failure. The underlying cause may have been a missing air filter, air leaks into the crankcase, or not changing the oil and filter often enough.
 
If the engine has a defective bearing, it is likely the bearings were starved for oil caused by a defective oil pump, an obstruction in the oil pump pickup screen, or too low an oil level in the crankcase caused by leaky gaskets or seals.
 
Excessive heat can be another cause of bearing failure. Bearings are primarily cooled by oil flow between the bearing and journal. Anything that disrupts or reduces the flow of oil not only raises bearing temperatures but also increases the risk of scoring or wiping the bearing. Conditions that can reduce oil flow and cause the bearings to run hot include a defective oil pump, restricted oil pickup screen, internal oil leaks, a low oil level in the crankcase, oil level too high, fuel-diluted oil from excessive blow by or coolant-contaminated oil from internal coolant leaks.
 
Misalignment is another condition that may indicate the need for additional work. If the center main bearings are worn more than the ones toward either end of the crankshaft, the crankshaft may be bent or the main bores may be out of alignment. Check the straightness of the crank by positioning a dial indicator on the center journal and watching the indicator as the crank is turned one complete revolution. If run-out exceeds limits, the crank must be  replaced.
 
Main bore alignment should be checked. Alignment is checked with a straight edge and feeler gauge. A deviation of more than .0380 mm in any bore calls for alignment boring, “line boring.” Line boring must also be done if a main cap is replaced. The concentricity of the main bores is also important, and should usually be within .0380 mm.  If not, boring will be necessary to install bearings with oversized outside diameters.

Connecting rods with elongated big-end bores can cause similar problems. If the rod bearings show a diagonal or uneven wear pattern, it usually means the rod is twisted. Rods with elongated crank journal bores or twist
must be  replaced.



Uneven bearing wear may also be seen if the crankshaft journals are not true. To check the roundness of the crank journals, measure each journal’s diameter at either bottom or top dead center and again at 90 degrees either way. Rod journals typically experience the most wear at top dead center. Comparing diameters at the two different positions should reveal any out-of-roundness if it exists.
 
To check for taper wear on the crankshaft journals (one end worn more than the other), barrel wear (ends worn more than the center) or hourglass wear (center worn more than the ends), measure the journal diameter at the center and both ends.
 
The journal diameter itself should be within .0254 mm of its original dimensions, or within .0254 mm of standard regrind dimensions for proper oil clearances with a replacement bearing. Using plastic gauge strips, available in SAE or metric, is the easiest way to do your measurements.
 
When you install new bearings, make sure you have the correct size (standard size for a standard crank, or oversized bearings for an undersize crank), that you have checked the installed bearing clearances, that the bearings are lubed to protect them against a dry start, that the oil holes and tangs on the bearings are all properly located, and that the rod and main cap bolts are torqued to specifications. Use lubricated threads to ensure proper torque readings by reducing mechanical friction and heat.
 
Another component that should be replaced along with the bearings is the oil pump. Oil pumps wear with age, and may cause a loss of oil pressure that can be very damaging to the bearings.

One additional note concerning crankshafts. Check the condition of the end of the crank where the rear main seal, (sometimes referred as the rope or cloth seal) is located. It should always be serrated so it retains lubrication. If the serrations are worn smooth, the crank will need to be replaced.
 
Priming A Newly Rebuilt Engine
 
After your engine has been assembled and installed on the test stand, prime the oil system before you first start it up. The assembly oil you use to lubricate the parts when you assembled the engine will provide some protection when the engine is first started. It may take 15 to 30 seconds for the engine to build oil pressure if the oil pump and oil galleries have not been primed. On engines with a pan-mounted oil pump driven by a distributor, you can use a priming tool to spin the pump and prime the engine.
 
Test stands to analyze an engine are worth the time it takes to build one. In our workshop we test all engines before rebuilding and afterward. The stand allows us to adjust the carburetors, run in the engine, and test all the associated components that go with this engine including the starter, generator, radiator, distributor, gauges, and carburetors.
 
Bruce L. Adams is a member of the Triangle Section and has owned and operated B.L. Adams, LLC,
specializing in restoration of the 190SL model Mercedes-Benz for 31 years.
For more information, visit www.bruceadams190SL.com