Classic Audi Technical Mech/Tech Engine » If you have a timing belt change due on a 20vT you must read this...

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Old 23-06-10, 07:50 AM   #1
TheCrispyNoodle
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Exclamation If you have a timing belt change due on a 20vT you must read this...

2 weeks ago I suffered a 'catastrophic' engine failure on an AAN. The timing belt which according to the cars previous service history was changed 15k/2 years ok was first believed to have slipped causing the usual valve damage etc. Yesterday I discovered the root cause was a sheared key on the lower crank timing pulley that mates with the crank pulley bolt. This has disintegrated causing the timing pulley to spin on the crankshaft. Ouch.

Some of the various guides to doing a 20vT timing belt change do refer to inspecting this pulley/key for wear and replacing if necessary. I cannot stress how important this would appear to be, the lower timing pulley is still a readily available part and well worth the effort of changing it given the risk and cost of correcting its failure.

The previous timing belt change was performed by 'joes garage', and it seems that changing the water pump, tensioner and temperature sensor were also considered too much effort. The crank pulley bolt was insufficiently torqued/loctited and this seems to have eventually worked lose causing the increased stress on the pulley key. Grim.

Lessons learned.

1. Don't let anyone near these engines who doesn't know what they are doing.
2. Assume this part is a must when doing a timing belt change and at the very least inspect it if not spend the extra 50 on the part and change it regardless. EFI and 034 motorsport do hardened versions if you are really paranoid, for obviously more than 50.

The only thing I can be grateful for is it not happening until the day after I returned from a family holiday covering 950 miles to, around and back from Devon.

I've now seen my engine disassembled into more pieces than I ever want to again....
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Old 23-06-10, 09:29 AM   #2
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Really sorry to hear that...

This advice applies to all I5 engines (turbo and non turbo), unless the crank bolt is done up to the correct torque, there is a definate posibility that something will break in the crank pulley region and wreck the engine.
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Old 23-06-10, 09:33 AM   #3
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Really sorry to hear that...

This advice applies to all I5 engines (turbo and non turbo), unless the crank bolt is done up to the correct torque, there is a definate posibility that something will break in the crank pulley region and wreck the engine.
And loctited! Cannot be too careful. Changed the belt on the 3B before running it for the first time 15K ago and the bolt only needed 50Lb/ft to remove it, very lucky. Needless to say I used my special locking tool and a 3/4" drive +loctite on refitting.
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Old 23-06-10, 11:12 AM   #4
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Yesterday I discovered the root cause was a sheared key on the lower crank timing pulley that mates with the crank pulley bolt. This has disintegrated causing the timing pulley to spin on the crankshaft. Ouch.
This is a VERY common problem.

I ran into this for the first time about 20 years ago on my 1983 100 Avant. I didn't have time to do a t-belt change myself, so I put it in to a local specialist garage (that I don't use any more). Everything was fine until about a year later when the engine lost all power. It would start and run, but wouldn't rev under any kind of load - could just about get 5 mph out of it!

I discovered that the timing was out and presumed that the t-belt had jumped a tooth. I retimed it, retensioned the belt and after a day or two, it lost power again. I realised that something was seriously wrong and took it all to bits. I was quite surprised to find that I could undo the crank bolt with my standard ratchet with virtually no effort, as I normally need a 6-foot extension to break it loose. I was even more surprised to find that the locating key (it's not a separate key - it's part of the casting) on the crank gear was nowhere to be seen - just a little bit of metallic powder.

The garage had either not torqued it properly, or had not used loctite, or both. That's the last time I used a garage for this kind of work.

As I've had six I5 engines over the years, I now keep a spare crank gear (got it for GBP30 about 10 years ago), and always inspect the removed one for any wear or cracking near the key.

There is some confusion over the torque required for the crank gear bolt. The manual says 350 Nm. But that's using the 2079 special tool, which has a torque multiplying effect. Using a standard torque wrench (with no multiplier), it works out to be about 450Nm. Without a torque wrench, that seems ridiculously large which is why I reckon many garages (whose mechanics often do things by feel), consistently under-torque it. My method in the past used to involve me wedging a huge screwdriver in the teeth of the flywheel, while a calibrated wife would stand exactly 2 feet along the handle of the breaker bar. She's a bit out of calibration at the moment, and refuses to be recalibrated, so I've bought a big torque wrench!

Scott Mockry's website (essential reading for any 10VT/20VT owner) has a detailed page on t-belt changing:
http://www.sjmautotechnik.com/troubl...g/S4timing.htm

Keep us informed on the engine rebuild. As you can see from my signature, I've got one of these to do - haven't got around to starting it yet. I'm optimistic that it's just a few bent inlet valves, so it might just be a couple of hundred to fix it.

Paul
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Old 23-06-10, 11:16 AM   #5
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Basically the powder you found was what was left of the woodruff key (that is a vital piece you need to check when doing a cambelt)
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Old 23-06-10, 11:42 AM   #6
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Basically the powder you found was what was left of the woodruff key (that is a vital piece you need to check when doing a cambelt)
I think I've still got the keyless gear on a shelf in the garage somewhere. I know this group likes photos, so I'll take a photo if I can find it tonight.

Can I be pedantic and suggest it's not a woodruff key, just a keyed gear on a slotted shaft? No? Sorry! I'll get me coat ...
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Old 23-06-10, 11:53 AM   #7
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You really should not use a screwdriver in the flywheel to remove the crank bolt as this puts enormous strain on the bearings, flywheel ring gear and crank (which although rated at 750Lb/ft it might take more than this to loosen the bolt originally tightened to 450).
I made a simple tool that slips into the damper and works a treat and cost nothing as used parts out of the scrap bin at work...
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Old 23-06-10, 12:02 PM   #8
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You really should not use a screwdriver in the flywheel to remove the crank bolt as this puts enormous strain on the bearings, flywheel ring gear and crank (which although rated at 750Lb/ft it might take more than this to loosen the bolt originally tightened to 450).
I made a simple tool that slips into the damper and works a treat and cost nothing as used parts out of the scrap bin at work...
I've heard various methods suggested for locking the engine - some of them quite scary (like feeding a load of nylon rope through a spark plug hole). After my experience with the screwdriver in the flywheel teeth, I bought the proper locking tool - the next t-belt change was MUCH easier.

Wish I had your welding skills - your 2084 looks excellent!
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Old 23-06-10, 12:08 PM   #9
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I think I've still got the keyless gear on a shelf in the garage somewhere. I know this group likes photos, so I'll take a photo if I can find it tonight.

Can I be pedantic and suggest it's not a woodruff key, just a keyed gear on a slotted shaft? No? Sorry! I'll get me coat ...
Don't let Phil Payne hear you say that......

And its definately one of those on the 10v I5

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Old 23-06-10, 12:10 PM   #10
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Cambelt change on quasi, thanks for reminding me!- One thing to do next before i even fire that engine!

Thanks for the heads up.
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