Chain Tensioner

[Graham]

I just bought a chain tensioner for our '72 350SL 4.5. C$55.00 from Quebec Worldpac dealer. Early in week, I saw them for $49.95 from a Toronto dealer, but they have bumped price up to $65.

These low prices are only for the units without smog connection.

I had shop put in new chain, but when I picked car up, he had left my old tensioner in place - I think he may have been sent the wrong one. Anyway, now I must do it.

He tells me it is hard to compress the unit. May have to use longer bolts at first.

Any guidance?

[Mike D]

1. "Pre-soak" the tensioner in a container of engine oil. 4 hours minimum and preferably overnight.

2. If you do have to use a longer bolt/bolts (some of the MB engines use a stud on the lower mounting) tighten the bolts evenly and SLOWLY! A quarter turn at a time. Hearing the crack of cast metal breaking can be a most unpleasant sound.

3. Use the longer bolts only until you can get the correct length bolt started for three or four COMPLETE threads. Use the original length bolts to pull the tensioner to it's final torque.

[Graham]

Originally Posted by Mike D

1. "Pre-soak" the tensioner in a container of engine oil. 4 hours minimum and preferably overnight.

2. If you do have to use a longer bolt/bolts (some of the MB engines use a stud on the lower mounting) tighten the bolts evenly and SLOWLY! A quarter turn at a time. Hearing the crack of cast metal breaking can be a most unpleasant sound.

3. Use the longer bolts only until you can get the correct length bolt started for three or four COMPLETE threads. Use the original length bolts to pull the tensioner to it's final torque.

Thanks for the info.

Regarding the presoak - I read that in the engine manual, so asked shop how they would fill tensioner. He said that the oil would just find it's way in there. He also said that what is written by the design engineers and what mechanics actually do are sometimes quite different!

I will follow the MB instructions!

[Mike D]

The oil will find it's way in there but why would you want to crank over a new chain with the extra slop until the air bubble bleeds out of the tensioner?

Before anyone tells you,"It's the same as when the car sits overnight" let me tell you, it is not! Even when the tensioner is worn and doesn't hold full pressure there is still enough oil in it to create a "dampening" effect.

Go by the book and you'll be fine.

[Graham]

Originally Posted by Mike D

The oil will find it's way in there but why would you want to crank over a new chain with the extra slop until the air bubble bleeds out of the tensioner?

Before anyone tells you,"It's the same as when the car sits overnight" let me tell you, it is not! Even when the tensioner is worn and doesn't hold full pressure there is still enough oil in it to create a "dampening" effect.

Go by the book and you'll be fine.

Mike, I will go with you and MB rather than the mechanics quick method! Makes sense to me to ensure chain is as tight as possible before you start engine.

[Tomguy]

Also, many mechanics are used to non-interference engines. A 4.5 is not one of those. In other words, if there's slop in the chain and you shut it off or stop cranking before it starts up, you may jump time and the resulting internal crash can cost a pretty penny to repair.

[Graham]

Originally Posted by Mike D

1. "Pre-soak" the tensioner in a container of engine oil. 4 hours minimum and preferably overnight.

2. If you do have to use a longer bolt/bolts (some of the MB engines use a stud on the lower mounting) tighten the bolts evenly and SLOWLY! A quarter turn at a time. Hearing the crack of cast metal breaking can be a most unpleasant sound.

3. Use the longer bolts only until you can get the correct length bolt started for three or four COMPLETE threads. Use the original length bolts to pull the tensioner to it's final torque.

The MB manual says to depress the rod that pushes against the guide while the unit is totally immersed in oil, flange up. This apparently allows oil to enter and air to escape.

I did this and bubbles escaped from the vent hole in the flange. But I was surprised to also see bubbles coming up from some place on the outside of the unit. Only source I can see, would be from the large nut which seems to have an internal seal. Would not expect this to leak.

I removed unit from oil and again depressed rod. No apparent leakage. But of course, I have no oil pressure.

Not sure if the unit is sound, but will install it and watch for leaks!

Question - With OLD tensioner in place and NEW chain, I rotated engine until timing marks lined up on passenger side. At this point pointer on flywheel indicated 7 deg ATDC. Shouldn't this be much lower with new chain - maybe +/- 2deg from TDC? If tensioner is tighter, will this likely improve situation so pointer is closer to TDC?

[Mike D]

Check timing marks using the crankshaft pulley. Don't try to find TDC using the cam marks. Rotate the engine twice or more to be sure you are on the compression cycle. Rotate the engine in the correct rotation. If you go backwards then you will see a discrepency due to the "slack" being taken up.

The tensioner is just a tensioner. It doesn't advance the timing chain. If you are 7 degrees off at TDC then you are off a tooth.

Bring the engine to TDC (compression stroke) and set your cams accordingly.

They make "offset" keys to compensate for gear sprocket wear. You might need to go that route.

[Tomguy]

The tensioner will not affect cam timing, unless you're turning the cam or turning the engine backwards. You need to turn the engine itself by the crank. And 7° is not too bad, and NOT a tooth off. 1 tooth off is 20° at the crank. You either have a slightly worn chain, or worn rails. (If the rails are not solid plastic but rather aluminum-backed, do NOT replace them!)

[Graham]

Originally Posted by Tomguy

The tensioner will not affect cam timing, unless you're turning the cam or turning the engine backwards. You need to turn the engine itself by the crank. And 7° is not too bad, and NOT a tooth off. 1 tooth off is 20° at the crank. You either have a slightly worn chain, or worn rails. (If the rails are not solid plastic but rather aluminum-backed, do NOT replace them!)

Thanks for input guys.

Manual says one tooth is 18deg, so I had ruled that out.

I had checked timing marks with old chain and now with new. There has been no change - still 7 deg.



The rails don't appear to be worn and they are the original metal backed ones

Some more pictures taken before chain replacement - note wear marks where chain hit casing! Also spacer on backwards (changed with new chain) and metal backed rail.

[Mike D]

Thanks, Tomguy! I didn't think 7 degrees was enough off to be a complete tooth but I figured he ought to check and be sure.

[Tomguy]

Originally Posted by Graham

Also spacer on backwards (changed with new chain) and metal backed rail.

Wow, what previous mechanic did that? The washer is on the wrong side of the spacer! You did turn the crank to where the cam was TDC then snapped the pic, versus turning the cam correct? And you never went backwards? It does seem high for a new chain, maybe the cam sprocket wore irregularly due to the spacing and geometry being off?

The reason I ask about the method is because it looks like there is slack in that chain.

[Graham]

Originally Posted by Tomguy

Wow, what previous mechanic did that? The washer is on the wrong side of the spacer! You did turn the crank to where the cam was TDC then snapped the pic, versus turning the cam correct? And you never went backwards? It does seem high for a new chain, maybe the cam sprocket wore irregularly due to the spacing and geometry being off?

The reason I ask about the method is because it looks like there is slack in that chain.

Those pictures were as the car was before changing the timing chain.

I always rotated harmonic balancer (using PS pulley nut in proper CW direction) until the cam timing marks lined up (even if they are backwards!)

A week or so after the new chain (but not tensioner) was installed by shop, I heard some alarming noises on start up , so opened it up and checked - Chain seemed loose, but got almost same results - 7deg or so.

Today I installed new tensioner. I set balancer to TDC and checked timing marks. Now they lined up! I know this should not have made a change, but it did. See pictures in this thread: timing marks on 1975 450sl