M100 6.9 Timing Chain Replacement

Here is a synopsis of replacing a 6.9 timing chain as I performed yesterday. It does not include any discussion of the tensioner which is peculiar to the 6.9 and which I will photograph and display in another post in the future. I also did not change any sliders as in the 6.9 unlike the M116/7 the ratcheting tensioner design precludes chain slop so the plastic sliders are in less danger. Other than that the method of chain replacement is the same for all three V8 engines.

I do not possess an M-B valve spring compressor so I was not able to remove the Right Hand rocker arms as recommended in the manual. This means you must be especially careful not to let the chain jump a cog on the right hand sprocket.

Chain stretch can be checked by placing the Right Cam on it's TDC timing mark

[img]http://i257.photobucket.com/albums/hh219/WGB_album/RHTimingMark.jpg[/img]

The LH Cam Timing mark is at this point slightly past where it should be as shown here

[img]http://i257.photobucket.com/albums/hh219/WGB_album/LHtimingMark.jpg[/img]

The degree of stretch is then measured off the crankshaft damper thus.

[img]http://i257.photobucket.com/albums/hh219/WGB_album/16degreesofchainstretch.jpg[/img]

I do not know what the maximum allowable stretch of a 6.9 chain is but as an M117 maximum stretch is 11 degrees I am sure the 16 degrees shown is way past anything allowable. It is only 18 degrees per sprocket tooth so the timing was nearly a whole tooth out.

Here are the new chain and tensioner rod as received from The Classic Centre.

[img]http://i257.photobucket.com/albums/hh219/WGB_album/NewChainandtensioner.jpg[/img]

Parts to be removed - Battery and tray, Aircleaner, Ignition leads and distributor cap, Spark plugs (To enable engine to be turned easily), fuel lines to CIS unit (Careful these are fragile) over left cam cover and battery cable support bracket that runs over right Camcover.

[img]http://i257.photobucket.com/albums/hh219/WGB_album/Strippedenginebay.jpg[/img]

I had already removed the air-con compressor and pipework last week for other repairs and when (On a suggestion from S-class) I also removed the alternator and it's attendant bracket I wished I had removed this before I tackled the compressor.

Here is the bracket held on by a number of 13mm headed bolts

[img]http://i257.photobucket.com/albums/hh219/WGB_album/AlternatorBracket.jpg[/img]

Here is the space with it removed.

[img]http://i257.photobucket.com/albums/hh219/WGB_album/AlternatorandbracketandA-Ccompresso.jpg[/img]

The cam covers are then removed and extreme care should be used to remove all the little spring washers that are supposed to live under the domed nuts as it would be easy to miss one and drop it into the engine - I removed mine with a small extension magnet before I lifted the cam covers. The RH cam cover comes off easly but the left is sandwiched between some pipework and the heater valve and needs some gentle twisting and turning to extricate the firewall end.

The timing chain tensioner is then removed in the reverse order to normal installation ( I will photograph the steps when I put it back and make a separate post).

Here is the distributor at TDC No 1 cylinder. There is a small scribe mark on the side face of the distributor which should be central to the rotor at this position and the distributor will need to be adjusted when the new chain is in to give static timing before the engine is started as it will have changed.

[img]http://i257.photobucket.com/albums/hh219/WGB_album/PositionofdistributorrotoratTDCposi.jpg[/img]

The chain is then immobilised by three Vice-grips of a design that gives a good grip. The endless chain on my motor must then be divided by grinding off the end of a link with a Dremel or using a more correct chain tool - if you have one. Many will have had a prior chain replacement so there may be a link - if you can find it. Whatever you do seal up every possible opening to protect the innards from grinding dust and to stop any piece of tensioner link from dropping down.....

[img]http://i257.photobucket.com/albums/hh219/WGB_album/Linkendscutwithadremel.jpg[/img]

The new chain is dispensed from it's original box and connected to the cut end of the old chain while a link of soft wire is hooked throught the last link just in case grip is lost and the end of the chain dives down into the casing (This would require a complete front casing tear-down to retrieve - hence the insurance)

[img]http://i257.photobucket.com/albums/hh219/WGB_album/Linkremovedandendofoldchaingivensom.jpg[/img]

Here is the new chain linked with the old and starting it's journey

[img]http://i257.photobucket.com/albums/hh219/WGB_album/Newchainlinkedandcommenced.jpg[/img]

The engine is now turned clockwise by using a 22 mm open ended spanner on the end of the power steering pulley with correctly tensioned belts. I do have the necessary 50mm socket for the 6.9 crankshaft pulley but it's a tight fit down there and really the fan would have to be removed at the very least.

As the pulley turns the vice-grips are transferred around with the best two used for the new chain as this is the area where slippage will cause trouble. The alternative is to use cable ties and this is what I used when I did my M117 chain but I found I needed two ties through each sprocket hole to give enough tension to prevent the chain from jumping a tooth.

The vice-grips and particularly the two with the design shown in the photo were very secure but each time the camshaft started down the back of a lobe it would lurch forward and slacken the chain between the two camshaft sprockets and scare the living S@#T out of me.

Slowly the chain comes through

[img]http://i257.photobucket.com/albums/hh219/WGB_album/NewchaincomingontoLHsprocket.jpg[/img]

When it is right through and secure this is what you should have if there has been no slippage. Every extra visible tooth means one chain sprocket of slippage (If the motor hasn't jammed up in the process).

It took me about 1 1/2 hours of slowly turning and moving vice-grips to get the chain through.

[img]http://i257.photobucket.com/albums/hh219/WGB_album/Joiningthetwoendsup.jpg[/img]

The new master link is then applied and if using this one-piece type of clip the tails should face away from the direction of rotation. I have ordered another one from AutohausAZ and if it has the two small c-clips I will replace it with that type as it is potentially a little less likely to come apart.

[img]http://i257.photobucket.com/albums/hh219/WGB_album/Linkwithclipincorrectdirection.jpg[/img]

The engine is then rotated a couple of times to seat everything and to make sure there is no internal interference of valves with pistons.

Here is the final measure - 2 degrees ATDC which will be due to sprocket wear.

[img]http://i257.photobucket.com/albums/hh219/WGB_album/2degreesretarded-1.jpg[/img]

I have ordered a selection of offset timing keys from AutohausAZ and I am hoping that a 2 degree one fitted under the RH camshaft sprocket will make it all line up - although probably one degree under each would be more correct and may be too difficult. The correct method of measuring the timing is replacing a hydraulic lifter with a standardised solid version and measuring the angle of 2 mm of camshaft lift but with my car it will be a matter of the timing marks lining up and then back together again.

Bill