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tcane · #6 01-21-2002, 02:19 PM

The method I wrote about for checking timing chain stretch came form the official M-B engine service manual. Both my print M-B engine manual and my M-B CD-ROM engine service manual state that for both the non-turbo and turbo engines that the timing marks on the camshaft washer and cam bearing tower are used to assemble the engine. M-B goes on to say that in "special cases, e.g. complaints about poor performance, it is neccessary to check and adjust as follows" and the manual goes into detail about the method I covered in my first reply (using the dial gauge, etc.). You can read other, older threads written by members with a lot of professional experience with M-B diesel engines saying to use the dial gauge method.

Checking the crankshaft to camshaft timing using only the marks on the harmonic balancer and camshaft washer/cam bearing does not provide an accurate measurement of one the most critical relationships/adjustments in a diesel engine. If you do not get enough air into the cylinders, then there will not be enough air compressed/heated on the compression stroke to provide complete combustion for the incoming diesel fuel when it is injected into the cylinders. This critical relationship/adjustment is probably higher with a turbo diesel since the turbo is forcing more air into the cylinders coupled with more diesel fuel from the IP through the injection nozzles, and a new camshaft design (beginning in 1980) that has longer duration (opening time) for the valves to take full advantage of the turbo.

I know when I was looking at my timing chain marks the position of where and how I looked at the marks made a difference. Looking at the harmonic balancer and the timing pointer you must have a perpendicular, 90 degree angle to have correct alignment - the vacuum pump housing is almost in the way on my engine. Also, my harmonic balancer and others that I have seen are marked in 5-degree increments - so unless the harmonic balancer is measured and marked for increments of less than 5-degrees any reading is going to be a guess. The cam washer and cam bearing marks must be looked at using a position along the center line of the camshaft using either the parting surface of the block and head or the contact surface of the head and valve cover as a reference point to make sure you are not too high or low from the center line of the camshaft and then looking back into the recessed area where the marks are located. Even using a strong light and correct positioning it is hard to see the cam marks accurately and those marks are large and not precise. A small deviation in alignment when looking at any of these marks will have significant effect on the crankshaft to camshaft positioning in the degrees that are being read/measured.

M-B does not provide a spec saying that if the timing chain is stretched X degrees, then the chain needs to be replaced. What M-B discusses is the use of offset woodruff keys to correct the crankshaft and camshaft tiiming because of timing chain stretch. The offset woodruff keys go up to 10 degrees of camshaft timing correction, which is a lot - a one tooth difference on the camshaft sprocket equals 18 degrees, so a 10 degree offset key is over 1/2 tooth correction. The M-B engine manual goes on stating that if the timing chain needs adjustment to correct the timing relationship between the piston position for cylinder #1 and #1 intake valve (zereo clearance at 2mm opening) then use the offset woodruff key to correct the timing, unless the timing chain is excessively elongated (not defined by M-B), then the chain needs to be replaced. I recall a thread where one member who worked on M-B diesels professionally for over 20 years stated that if the timing chain had not worn grooves and/or was slapping around in the valve cover and the tensioning rail did not have deep grooves in it that the timing is corrected by using offset woodruff keys.

When I replaced the timing chain on my 300D, with over 300,000 miles on it, I did so because the tensioning rail had some pretty deep grooves in it (about half way through the rubber). I measured the new and old chains and found that the new chain was 50 15/16 inches long and the old chain was 51 1/32 inch long, for a total stretch of 3/32 inch (using the dial gauge method, my chain had 17 1/2 to 18 degrees of stretch, 13 1/2 degrees is the M-B spec for a used chain). I cleaned and inspected my old chain and did not find any cracks in the side plates or cracked rollers (I did use a magnifying glass - 10X - to inspect parts of the chain, but not the entire chain). The old chain looked like it was in good condition and still serviceable.

It was written that you can use the cam washer mark and cam bearing mark to determine the timing relationship of the crankshaft and camshaft instead of the dial gauge method recommended by M-B - and then going on to say how good the engine runs, etc., etc. and that the timing marks are now off by only 2 degrees with the new timing chain instead of 10 degrees with the old timing chain. From this info this engine's timing is off and that when the new timing chain stretches (as they all will) the timing will be off even more - meaning this engine will require a 4 degree offset woodruff key (perhaps another woodruff key) to correct the timing and enable this engine to run as well as it can. Compared to the way this engine ran with the old timing chain to the way it runs now with the new timing chain is probably like night and day - and that's great. However, how much better will this engine run if the crankshaft/camshaft timing is properly adjusted using the M-B dial gauge method? But, hey the engine timing is almost correct and it does run better - so, if the owner is satisfied with an engine that is almost timed correctly then I am almost satisfied that his engine almost runs as well as it can. The point of this forum (as I undersatnd it) is to exchange relevant and accurate information to help each other with our respective problems and questions. All I am doing is passing on the the M-B info for timing chains: how to correctly time the crankshaft to the camshaft - a most critical relationship/adjustment in a diesel engine - and timing chain stretch correction using offset woodruff keys.

I looked at my CD-ROM engine manual for the turbo 300D engines for the timing chain and crankshaft/camshaft relationship. The specs I wrote in my first reply for the relationship of #1 piston and #1 intake valve are good for 300D turbo diesels up to the 1979 model year with engine 917.950 and camshaft code 00 or 08.

The 1980 model year and newer have a redesigned camshaft and the timing specs are different. These specs apply to the 917.950 engine with cam code 05 (cam made of chilled cast iron) and also the 917.951 and 917.952 engines. Using the M-B dial gauge method (#1 intake at zero clearance and 2mm open on #1 cylinder) the intake valve should be at 9 degrees ATDC (after top dead center) read on the harmonic balancer and a used chain with 20,000+ km has a spec of 11 degrees ATDC.

Good Luck!
Tom