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Old 03-21-2002, 12:11 PM
tcane tcane is offline
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Join Date: Jul 2001
Location: San Antone
Posts: 408
Breaking-in any engine depends on several factors: like what tolerances/clearances were used for bearing surfaces, what type of piston rings, what piston type and what clearance to cylinder wall, etc., etc.?

An engine that is set up with minimum/tight clearances would require gentle/moderate treatment during the wearing-in process as bearing surfaces polish each other and take longer mileage-wise before the engine was fully ready for regular driving (in the old days the bearings were set so tight that often the car had to be towed to get it to start and then cold water was run through the engine to prevent it from heating up so the bearings could wear in). An engine that is set-up with maximum/loose clearances could be driven harder from the beginning, but would still require some consideration to driving it less hard to allow the bearing surfaces some time to wear in to each other. An engine set up in the middle would require a middle of the road type of wear-in process.

Personally, I like to baby an engine (after the correct ring seating process is done) for at least 1,000-1,500+ miles to allow good wearing-in of parts. This method has yielded engines that last a long time and consume minimal amounts of oil, if any.

Piston ring composition makes a difference as well. Any rebuilt engine should be run for about 20 minutes when it is first started at a fast idle of 1,000-1,500 RPM (diesels should be warmed a bit more before raising engine RPM's, but should still be run at slightly higher RPM's when first started when compared to the normal fast idle of a worn-in diesel engine). If the engine gets too hot during the initial start-up, then shut it down to cool off (and check for why it is hot, probably need to add a bit of coolant and/or oil after the engine cools). Cast iron or cast iron alloy rings need to be driven at varying RPM's for the first 1,000 miles or so (the rings are tensioned in such a way that they have constant pressure against the cylinder walls). Chrome faced rings need to be thrust against the cylinder walls so that they will wear/polish each other creating a good seal (these are low tension rings and depend on combustion pressure to force the ring against the cylinder wall for sealing, and also allow combustion gases into the crankcase more so than cast iron rings). Chrome rings because of the hardness of the chrome typically need to be forced to seat as soon as possible after the engine is first started and after the 20 minute initial run - the engine is accelerated from 30 mph to 50-55 mph at full throttle and then the throttle is closed to let the car slow to 30 mph or so in a gear so the engine slows the car (acceleration causes the rings to be thrust against the cylinder walls and then the vacuum of deceleration causes a lot of oil to coat the pistons, rings, and cylinder walls to aid in the wearing-in/polishing process and cool the components) - repeat 15-30 times. Molly faced rings are similar to chrome faced rings.

Another crucial factor is the finish on the cylinder walls. With the EPA requirements for emissions, today's cylinder walls are finished in such a way that the rings seat almost immediately and the rings are also finished to achieve a good seal (pre-lapped rings). Also, the composition of the rings requires a specified finish for good sealing and wear (cast iron 220 grit, chrome 280 grit, molly 280-320 grit - using carbide stones). The finish on the cylinder walls is done in a way that the rings do not do the final honing step as was done years ago - what is required is a finish that is close to a worn-in engine to minimize initial wear on the rings. The cylinder walls are bored to about 3-4 thousands of an inch (or the equivalent mm as required) undersized and then honed using several different grits of stone (from course to fine), then a soft honing process to finish and achieve the final bore dimension to match that particular piston. A very precise measuring tool called a profilometer is used to read the cylinder wall's surface roughness in Ra units (one millionth of an inch). If the finish on the cylinder walls is not correct than the pre-lapped rings may/will have their finish worn off causing sealing problems and oil consupmtion.

M-B diesel engines use tolerances/clearnaces that are much tighter than the typical Detroit gas engine. In fact, M-B worn out/max specs are close to the new engine specs/tolerances of many Detroit gas engines. To me, that means a wear-in period in miles that should be done easy/moderately and long enough to let the parts wear-in and polish each other to achieve the maximum life span these engines can attain (500,000 miles).

I just rebuilt a used 300D engine with some wear, but in very good condition tolerance wise. The cylinder walls were measured using a micrometer and did have some out-of-round and taper, but were within spec. However, the cylinder walls were worn such that using chrome faced rings (M-B uses them) would have probably worn out the liners soon (the liners on the early 300D's are known for wearing out because of the chrome rings). I used cast iron alloy rings (more flexible than regular cast iron rings) to re-ring the pistons. I soft honed the cylinder walls because there was a bit of rust and staining, but removed almost no material because of the type of hone (with cylinder walls in good condition, no honing is needed if using cast iron rings). Washed the cylinder walls with hot, soapy water, rinsed, and oiled them. After the initial 20 minute running, I ran the engine on the road at slow speeds (25-35 mph) for 80 miles and used about 1/2 quart of oil (Chevron Delo 15W-40). I changed the oil/filter and in the next almost 1,000 miles I have used a very small amount of oil (about 1/4 quart) that did not require any more oil to be added (speed 30-65 mph with the first 200 hundred miles or so all city driving with speeds not over 35 and then gradually increased speed, but varying the RPM's all the time). I am going to change the oil at 1,000 miles using Chevron 15W-40, and see what my oil consumption is. If the oil consumption stays were its at now and/or gets better, I will probably change to synthetic oil to reduce wear further so the engine will last as long as possible.

I'll consider the engine worn-in when there is no oil consumption and compression is as high as it will go. In any case, probably 2,500-3,000 miles. Once the engine has achieved this wear-in level, I would expect it to marginally get better for the next 100,000 miles or so, then plateau at this level for the next 200,000-300,000 miles, then begin to deteriorate until it uses lots of oil, compression goes down, performance suffers, and needs a rebuild.

My $0.02 worth!!
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1977 300D: 300,000+ miles

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Last edited by tcane; 03-21-2002 at 12:20 PM.
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