Is this a chain tensioner failure?

[tim69s]

How do I confirm that the tensioner has failed and this is the cause of the broken rocker arm?
What would cause the swarf seen in the chain case?






Will probably do the hydraulic tensioner upgrade anyway, but want to be sure what has happened before I rebuild.
Thanks for any advice


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[911hillclimber]

Swarf from a slack chained striking the case somewhere, could be caused by ramp wear and failure.
The noise of the failure when the engine is running is very gut-wrenching...
Conversion a good idea.
I doubt the failed rocker was caused by the tensioner failing.
I've had 2 broken rockers in my 28 years with 4 911 engines, both were due to over revving of the engine.

[fetuhoe]

Broken rockers can be a sign of an engine having cam timing issues and slack chains can, in some circumstances jump a tooth and create this problem.

Sadly the investment cast rockers do also suffer from occasional fatigue failures and just break.

However, swarf in the timing case is never a good sign.

The tensioner you show in the photograph seems to have been converted to be a 'fixed' unit which would need manual adjustment and if not periodically adjusted will allow slack as the chain wears.

An 'urban' myth is that chains 'stretch' with age. They do increase in length but due to wear of the pins and the holes in the side plates. As the chain elongates the cam timing of the engine will also change and an increase in length will retard the cam which will tend to bring the exhaust valve closer to the piston.

A standard oil damped tensioner should have the appearance shown below:



It is unlikely that the modified tensioner can be rebuilt back to a standard configuration so it will need replacing.

I am not a great fan of hydraulic tensioners as they do have some issues and can still fail but may people do believe they are indestructible.

We generally rebuild the standard tensioner and then fit a safety collar, also known as a 'chain saver' so if the internal spring does break the tensioner doesn't collapse and the chain doesn't jump a tooth.

This approach is generally cost effective as rebuild kits are around £40 each.



While you are looking at rockers it may also be worth checking to see which rocker arms are fitted. From 1967 to 1969 the Investment Cast Rockers were used without the Glyco Bushes fitted to the early forged rockers and the later Cast Rockers.

This means the rocker runs a steel/steel interface and with the relatively poor lubrication conditions the rocker/shaft interface can gall quite badly and in the limit could make the rocker stick on the shaft.

If you have these types of rockers I would suggest changing them for later spec parts with bushes and change the shafts at the same time.

It is possible that they have already been changed but worth a look.

[tim69s]

Thanks both, the broken rocker might be from an over rev, as I was flooring it on a track when it went.

Thanks for the info on the tensioner, seems likely the chain has 'stretched' or been incorrectly adjusted in the first place.

With the 'chain saver', how would you know the tensioner has failed. Would the engine simply run badly out of tune?


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[fetuhoe]

You hear loose chains on the over run as when the engine is accelerating they will always be tight due to the tension.

When you lift and the engine slows they will slacken and if there is insufficient tension they will clatter quite badly.

If you drive the car with a failed tensioner that has a safety collar carefully and gently it should be reasonably safe enough to get home unless you are a the other end of the country.

I like mechanical tensioners on race engines as when they rattle it is time to adjust the tension and to keep the valve timing optimised.

If you look at the change in timing that occurs with a chain from new to the wear limit the timing change is approaching 7 degrees and with a tensioner you will never know.

This is only a real issue on very highly tuned race engines where you want to have everything bang on.

[tim69s]

Spent a coupe of hours tearing down the left side of the engine this afternoon.
Evidence of piston 3 impacting the exhaust valve and likely bent valve.
Not really surprised as that fits a chain jumping scenario on that side. Although the broken rocker is on cylinder 2 not 3.

It was only when I pulled off cylinder 1 that things went from bad to a lot worse.
Swarf everywhere in the crank case. Oh..

Rotate pulley to get piston 1 in the right place to remove pin. What the ........

Only piston 1 moved.

Rotate flywheel, only piston 2 and 3 move.

F**k. Broken crankshaft.




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[Gary71]

Wow. That’s really unusual.
What happened when you were driving?

[Gary71]

Also meant to say. shite that’s bad.

[tim69s]

What happened when you were driving?

Not a lot really, just felt instant loss of power. No big bangs or anything.
Coasted into pits (was at goodwood) and tried turning it over. Sounded like loss of compression as the engine spun over very quickly.
But actually I was just spinning the half of the engine still connected to the flywheel.



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[hot66]

Ouch !!!

[Gary71]

I know your car history was a bit limited, but do you know if it had been rebuilt (and maybe crank reground) at some point?

[bjmullan]

F**k. Broken crankshaft.

[tim69s]

Has definitely been rebuilt at some point in the past, but I was thinking the bottom end might not because the chains have split links in them. See what else I find as we split the case.

Stripping the rest down now, get ready for some gory photos!!




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[tim69s]

So the case is ready to split, have left it for the night as getting late and there was oil coming out everywhere so let it drain out overnight.

Cylinder 5 is where the failure has happened. I can't move the piston, don't want to force it and risk damaging the case. The cylinder is smashed and I can see a big mess through the gap in the other side of the case.
If the case is damaged then this engine is dead.

Swarf..


Cylinder 5...


Piston stuck


Doesn't look nice in here





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[Gary71]

Not good.
Hopefully the case can be saved but I’m guessing not much else can be due to the swarf pumped around the engine and the unknown stress on various components.