'74 450SL Timing chain/guides replacement - Mercedes-Benz Forum

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post #1 of 3 (permalink) Old 09-06-2006, 10:59 AM Thread Starter
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'74 450SL Timing chain/guides replacement

Hello, folks,
Just thought I'd share some of my experience with my recently acquired '74, 450SL, equipped with D-Jetronic F/I system. When I purchased the car, the seller/owner claimed to have no knowledge of the timing chain and track guides ever having been replaced; an honest, if unwelcome, bit if news. He also added, he would have no qualms of driving the car long distances, with only a tune up! To which I said to myself, "right!" The car has 210K miles on it and was running very rough and rich at idle.

After reading everything I could get my hands on for replacing the timing chain and rails, I ordered the parts and planned the event. I am in the thick of it, as I write. This is a daunting task, to say the least, and I am a profesionally trained mechanic/machinist/ engineer with many years of working on engines of various types including, aircraft, and with access to a very complete, well equipped shop, though not specific to MB. It is a difficult task, not because of the technology involved, but because of poor design on the part of the manufacurer (YES, MB!), in the placing and mounting methods for the engine driven accessories, in addtition to poorly written manuals and thirty plus years of crud build-up in the surrounding work area. Placement of these accessories prevented proper cleaning of the surrounding work areas, prior to initiating the dismantling operation. Deciphering MB's budget-driven tech writing is only surpased by those of developing Asian manufacturer's, on how to assemble your child's swing set or a BBQ grill -- If you now what I mean! And, the Haynes manual -- well, what good can be said?

My findings, so far: don't waste your time making a slide hammer to attempt to remove the guide rail retaining pins. It, quite simply, does not work. Unless, the pins are fitting loosely in the head and the weight of the sliding mass is over five pounds; in which case the threaded 6mm bolt will fail and ruin your day by snapping off while embedded into the guide pin. Other methods will need to be employed if the pins are snug into the cylinder head(s), as mine were found to be. I will be glad to answer specific questions about this off-line, at Coresa@aol.com. No, I'm not selling anything!

A broken guide rail on the left cylinder bank, tells part of the story. The engine made no mechanical noises whatsoever and, aside from the uneven running, I was/am impressed with its quiet operation. This failure could have resulted in catastrophic engine failure, withing a few miles of driving. For once, I listened to my inner voice and did not procrastinate. It is a credit to the design of the engine that the chain and rails lasted 210 K miles! There is no evidence of this engine ever having been serviced internally! A solid record of timely oil changes and related filters and what not, are a testament to how important these preventive measures are: there is no evidence of build-up of any kind, and no perceptible wear on the camshafts. Really, a thing of beauty! I am also sure this prolonged the life of the chain and its related components. The cam sprockets show no wear whatsoever.

During my evaluation process of what would cause the engine to run so rough at idle, I used the common test equipement: timing light, dwell meter, DVOM, vacuum gage and a shop-built, cylinder leakdown tester. The vacuum gauge told most of the story: steady, low vacuum readings, at idle, under no load, at low throttle settings. The timing signal was jumping all over the place and suggested worn timing components and faulty distributor internals. The late valve timing being attributed to a severely worn timing chain and the broken slide rail (loss of chain tension and chain slap). Other factors included, a non-functioning ignition distributor, (advance mechanism frozen), and poor electrical connections.

The object of my post is not to critisize anyone or anything, but as an objective finding (as I see it, HA!) and to warn all of those who may be due for a timing chain replacement and are waiting to hear strange noises coming from the engine, broadcasting a warning of an impending need for this service. DON'T WAIT! This could cost you some serious bucks, and ruin your day, I assure you!

I hope this is of some benefit to someone.

Cheers, mates,
Norman C
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post #2 of 3 (permalink) Old 09-08-2006, 07:24 AM
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I feel your pain. Been there, done that. I would think with no wear on the camshaft sprockets that they must have been replaced at least once (especially at 210K). To get the pins out of Benzes, I simply use a long bolt with spacers that press against the head, screwing the bolt into the pin and pressing it out, adding more spacers as the pin comes out. Sometimes they stick pretty good, but once they start moving, they come out fairly easy. Cheap but effective.

At 210K, I am willing to bet that you are due for a valve job. The 560 I worked on at 150K had valve guides that were fairly loose. That model, which I assume is the same as the 450, used a brass/bronze valve guide which wears quicker than the old iron guides. You will be loosing your vacuum past the valve guides which will affect idle, not to mention burn oil and foul plugs. Wear can easily be seen by pushing the top of the valve stem with a screwdriver and looking for side to side movement. On the plus side the cylinders showed virtually no wear at 150K.
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post #3 of 3 (permalink) Old 09-08-2006, 10:03 AM Thread Starter
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Thanks for the sympathy and reassuring words. Prior to attacking the timing chain/track issue, I performed a cylinder leakdown test and the engine is showing a uniform loss of about 5%, in all cylinders. I believe this to be an acceptable range for the age/wear of the engine. I have wondered, however, about the condition of the valve guides and the valve stem seals. There is no evidence of excessive oil burning (blue smoke) from the exhaust. I have not driven the car more than a couple of miles since I bought it -- for which I'm only too glad! -- not knowing the condition it may be in. A valve job would not be out of the question in order to regain reliability and good running from the engine. It is daunting, however, to look at taking so much stuff off of the engine to get to the heads, and that alone would make me put it off to another day. I'm still having a bear of a time removing the front timing case. I posted this question separately to the board, as I did not have any idea what was waiting for me when I posted the first installment. Come to think of it, I may just end up having to take the heads off to get the timing chain case off! A horrible notion, indeed! A tribute to good ole "fahrtenfugen engineering," at its best!

Norman C
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