timing chain 107

[enodia2]

I recently purchased a 450SL with 200K KM, as a policy I like to know chain and tensioner have been replaced at around 100K KM. I understand this is not offical MB "policy", however the catastrophic consequence of such a failure make it worthwhile in my mind. Comments????

Still learning my way round the forum. This post should be under mechanicals not modifications. Enodia

 

[hoddo]

timing chain

My 450 slc had 190,000 km on it when I bought it. It had full log books and I rang the dealer to confirm if a timing chain had ever been done, it had not been. As you say the result of a failure (usually the guide rails not the chain) is catastrophic, so I had chain, tensioner, top guides and cam oilers (little plastic things that can become blocked or worn) all replaced by a local indie mech. Cost around $1000, chain had a bit of strech but was in good condition but the guides were showing signs of wear and had become discoloured. For the peace of mind I think its a job well worth doing at 200k unless you can verify it has been done by the last owner.

 

[WGB]

The problems are three in number and they are separate but the same.

1) The chain stretches and this is measured by placing the right side Cam at the TDC mark and measuring where the timing mark on the Pulley is - the limit is 11 degrees after TDC. The result of chain stretch is largely poorer performance as the timing changes.

2) The plastic "Sliders" that the chain runs against become old brown and brittle and particularly the plastic insert on the surface of the Chain tensioner slipper breaks up and large pieces of plastic can float around and cause the chain to jump in relation to the cogs.

3) The hydraulic tensioner in the M116/117 motor has no ratchet mechanism (Unlike the 6.9 tensioner) and when they age they allow the chain to loosen when the engine is shut down. Starting the engine allows the chain to slap around for a few seconds until oil pressure causes the tensioner to expand and take up the slack.

The problem is a sloppy chain and old hard and brittle plastic sliders will lead to a catastrophic jumping of the chain which then allows valves and pistons to meet and the cam towers appear through the rocker covers - usually the Left hand bank.

It might cost some monety but is very necessary preventive maintenance.

Bill

 

[B13]

Hi Bill, how important is it to replace the hydraulic chain tensioner (the thing which sticks out the side of the engine)?

How does it wear if its had oil running thru it all its life? Do you have any pictures of a new one and one that has done a few hundred thousand km and is considered to be worn out?

As you know I build japanese wankel engines, and have never once replaced an oil pump, even on a race engine.... its always got oil running through it, so although I check them each time I tear an engine down, they are always within Mazda's allowable wear limits... (The same goes for the oil metering pump on these engines which injects small amounts of oil into the carburetor - due to its job, they never wear out.)

Just a thought.

Since I have to do this job soon to the Team116 car, I want to make sure there are valid reasons for doing things... the chain I can understand, the plastic sliders I can also understand, the hydraulic chain tensioner I don't get it...

 

[WGB]

I assume the bore of the tensioner wears in some way but I have a used one sitting in a box and the plunger was very easy to push in and out whereas a new one isn't.

There are two different types that I know of.
A straight Euro one and an Aussie ?USA one with a large ?oil feed through the plate.

The tensioner is not hugely expensive but is the most important part of the system as I believe this is the part that allows the freedom for the chain to thrash around and why these chains normally give trouble at start up or at a slow idle.

If I was to do anything at all I would replace the tensioner and then do everything else later.

Bill

 

[Styria]

Well summarized, Bill. I also think that it is a 'must' to replace the tensioner. The dire consequences that you describe are absolutely spot-on, and most of the damage occurs on the left hand cylinder head bank. The cost to rectify the problem would be very close to the $3.5k. mark - not a cheap fix.

The trouble is that, invariably, you get no warning. The damage is usually caused on start-up and all of a sudden the car won't start or even run very roughly. As far as location of this thread is concerned, we will transfer it to the Mechanical Section. Regards, Styria