Timing questions on a twin cam W201

[Will]

Hi guys,

A quick question regarding the timing on a 2.5-16:

With the engine at TDC on cylinder 1, crank pulley exactly at 0/T marking, and the cams in the correct postion, how low should the dots on the cam sprockets be? What I mean is, from all the pics/diagrams I've seen, they look like they are opposite each other, and slightly raised (literally a couple of mm) from the head/valve cover mating surface. Is this correct or should they be literally invisible and absolutely level on the actual mating surface?

With a used timing chain compared to new, should there be even the slightest difference regarding the alignment of 0T on the crank pulley and the two dots being perfectly opposite? I've heard of chain wear/stretch causing timing to be a degree or so out? Should/Would this show up when timing the engine and turning over slowly manually?

I'm trying to establish how much difference could be made by the timing being one tooth up or down somewhere along the line, surely if the crank pulley was one tooth either way, it would cause a difference of several degrees advance/lag compared to the cam aligment?

If when timing an engine up both cams were, for example, a mm or two too high with regards to the dots, if you turned the exhaust cam ever so slightly clockwise, and the inlet ever so slightly anti-clockwise, would there be enough movement to allow for chain to be 1/2 tooth further round on each - if that makes any sense!

Ie, by turning both cams round, it would put the chain one tooth overall further round, allowing them to line up with the dots being 100% exactly level with the head/valve cover mounting surface?

Is this something that is very easy to get wrong, and very hard to get perfectly set up?

Many thanks,

Will

 

[Will]

I'm having more and more thoughts on this now.

Right, here's my logic. When setting the timing up, as in, before the tensioner is installed, there is obviously some slack in the chain. If you set it up with the cam markers being opposite each other and the crank pulley at TDC on the 0T mark, once the tensioner is intalled and taught, it moves the two cam sprockets ever so slightly once it's all up to tension.

Due to the way the tension is taken up, it causes the the exhaust cam sprocket to move slightly anti-clockwise, causing the timing mark to rise ever so slightly, and the inlet cam sprocket moves anti-clockwise as well, but this will cause the timing mark to drop, as it's 180 degrees round (opposing) the exhaust.

My thoughts are whether it is worth accounting for this slightly when looking at the timing in the beginning (before fitting the tensioner), erring on the side of exhaust being slightly lower and the inlet slightly higher (if at all needed), so that when the slack is taken up it will pull everything together in the desired direction?

Does this sound at all plausable? I'm probably clutching at straws but I'm stuck at work and trying to clear it up in my head (excuse the pun)!

Any advice most welcomed

Will

 

[bolide]

Logically the tensioner won't be in the drive path between the drive and the cam - it'll be in the return path to keep tension in the "loose" part of the chain - like the lower bit of the chain on a bike

On that basis you should be able to work out if the tensioner will affect the timing. I'd say definitely not

But stretch would affect it. If stretch is bad enough to be noticeable by eye I'd say the chain needs to be replaced

Nick Froome
www.w124.co.uk

 

[bolide]

If you've got a PC then a WIS & EPC subscription might be the answer:

http://www.service-and-parts.net/dcagportal/DCAGPortal


Nick Froome
www.w124.co.uk

 

[Will]

Hi Nick,

The car in question isn't my regular 2.5-16 btw, this is a 2nd car that is currently undergoing a bit of TLC

Anyway, the previous owner had the chain and tensioner replaced less than 10k miles ago by a 'MB specialist'.

I agree with you about the tensioner, once the timing is 'set' although what I am querying is whether I need to account for the chain being pulled taught on the exhaust/tensioner side once the tensioner is installed. Ie, timing looks spot on before tensioner is installed, and once the slack is taken up it appears to be a smidgen off?

What I think I'll try later is to remove the tensioner, unbolt the cams from the sprockets and re-align the gears allowing for the slight movement that I am anticipating from once the slack is taken up, ie a fraction of a mm in the opposite direction of travel. Then re-attach the cams to the sprockets and check the timing again.

If anyone has any other suggestions/advice I'd be most grateful

I've had a look on the MB WIS documents but it's not really any more detailed than what I've already looked at.

Cheers,

Will

 

[grober]

Forgive me if I state the obvious but having lined everything up did you turn the engine over a couple of times to see what happens? Also do you think the-- position/ state of --of the chain guide between the cam sprockets might cause the effect you describe?? theres an article here http://mb190.tripod.com/ on changing the timing chain where they mention getting the correct guide for example? they also seem to think everything should deffo line up as described so might be worth trying the compensatory adjustments you suggested? Theres another post with pics here.http://www.mercedes190.co.uk/ under technical-engine and drivetrain -engine-changing timing chain and valve clearances but no further info really.

 

[Will]

Cheers Grober,

I've slowly rotated the engine over a couple of times and that's where I've left it currently, cam markers seem to be opposite each other but I'm not 100% convinced that they're absolutely level now that the tensioner has installed, and appear to be a degree or so 'out' with regard to the crank pulley 0T TDC marker.

The top rail isn't actually fitted just yet, I'm waiting till I'm happy with the basic timing before I put that on.

I'm going to go with the plan in my previous post and see how that works out.

I'll post up later with how I get one

Finger's crossed

Will

 

[Will]

Right,

Just been out and had another look, I think I might be worrying over nothing

It was getting late last night and I was a bit paranoid and cautious to get this right, I've just rotated it over by hand a few times and it seems pretty much ok?

If I stop the crank pulley at exactly the point where the two timing marks line up, it looks like there might be a hair's width gap between the 0T (TDC) mark and the pin? But as soon as you release the force that you apply to the pulley and let it rest, it seems to be pointing at the pin.

I'm fairly confident that it can't be out, one tooth on any sprocket is never going to be as close as this?

The photos might be a better indication.

Cheers,

Will

 

[grober]

Don't think it gets much better than that. I would be inclined to try it and see how it runs. Remember to reset the tensioner when you re-install it but I guess you know this already

 

[Will]

I think I'd better get back to bolting everything up then! I doubt I'll have it done today (daylight) but hopefully tomorrow or soon enough I should have the proof of the pudding

Thanks for the advice guys

Will

 

[Will]

Dark again

Anyway, making good progress, most of the awkward bits bolted back up, top chain guide rail and bracket, torqued the camshaft caps up, oil and ATF dipstick brackets, coolant hoses near the bulkhead, inlet manifold nuts and bolts etc. Should just be a couple of bits from underneath and a few bits from round the front, then I'll see how she runs

More updates when I have them

Will

 

[Will]

A few quick pics:

Ports cleaned

Valves cleaned, reground/reseated and head faced

Head ready to fit, valve clearances re-set and new MB gaskets

That all important branding!

Will

 

[Mercedes cozy]

Excellent pictures. A Good job. Well Done.

 

[Will]

Cheers Virdi,

Thanks for all your help and advice, very much appreciated.

Very knowledgeable guy, most helpful

Will

 

[Will]

Woohoo!!

She's all back together now and I'm over the moon.

Changed the oil and filter, filled coolant, cranked over with coil disconnected - full 3bar oil pressure

Re-connected the distributor, engine started within less than a second, rock solid idle and sounds as sweet as a nut :bannana:

Many many thanks to Virdi (Mercedes Cozy) for his invaluable advice and assistance, I'm sure he'll add to the events

Another Mercedes fighting fit for the future

Will