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300 Tdi cam belt - how not to do it


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Finished work at five o clock Thurs, thought "I know, I'll just pop a timing belt on the 110" 300 Tdi. I'm a bit nervy about timing belts and like to change them frequently - never had one break. So, drove it in, open the bonnet etc etc. I don't have a fan or shroud so plenty of space without taking the rad out. Old one off, new one on, new tensioner felt a bit odd but never mind. Used new Snap On digital torque wrench to set the tension. Turned it over by hand for 2 revolutions to check all OK, turn key, absolutely no sign of life. Sounds normal so assuming valve timing ok but not firing. After some very tired and useless fretting decided to go home and look at the next morning (supposed to be leaving yard 6am to fix other machines).

Too excited/worried to sleep well so in w/shop 0530. Having thought about it decided pump timing must be out. So, front off again, line up marks and yes the pump timing is out. Turned out the 9.5mm drill bit had a burr on the end and hadn't gone into the hole properly. Correct it, retension, (tensioner still felt odd - had to slacken idler bolt to move it but too tired and stupid to realise significance of this), reassemble, turn key and it started. Chuck tools in, set off to fix machine listening to what I thought was the serpentine belt and tensioner squealing. anyway got 2.9 miles and bang rattle stop. By chance I was changing gear at the time, I knew straight away what it was so kept my foot on the clutch and coasted onto the verge. Ring up mate for tow back to yard (he's got a Hilux Invincible - how embarrassing was that?!), front off - yes the belt has snapped - rocker cover off - yes there are 4 broken rockers and bent pushrods. B*gg*r!!!!! Ordered rockers, pushrods, new cam belt. Had to use a 420 hp V8 Scania tractor unit as the service truck for the rest of the day!

Next morning, rocker shaft out, change the rockers, fish all broken bits and valve caps out and reassemble. Rad out this time, time everything up and fit belt. Hold breath and cross fingers, turn the engine over by hand and watch to see if the valves open and close. Hooray! Looks ok. Set tappets, cross fingers and start it. Runs fine and drives as before, so I hope I have done no further damage.

When I had a look at the bits I realised that I had fited the idler/tensioner assembly incorrectly. As you will be aware there is a spigot on the bottom of the idler that fits through the tensioner plate allowing the tensioner to pivot. I had fitted this wrongly so that the spigot was jammed across the hole, the tensioner couldn't pivot without slackening the idler bolt and the idler was touching the timing cover. The idler had seized, the back of the belt ran against it, got hot, shredded and snapped.

How can I have been so stupid? I got the pump timing wrong first time. I put the idler assembly together wrong. I had to slacken the idler bolt to adjust the tensioner - I know it should't be like this. I heard it squealing and ignored it.

The lessons I have learned are 1) don't do things at the end of the day when you are tired and want to go home. 2) TAKE THE RADIATOR OUT! Then you can look at everything square on. 3) Concentrate; if you think something is wrong it probably is.

I realise many people here will have changed dozens/hundreds of cam belts and will leave the rad in, but I only do it every 40 000 miles or so and I need all the help I can get.

I hope you can either laugh at my incompetence or learn from it.


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Its very easy to look back and say 'I shouldn't have done that' but when you're tired and/or rushing its so easy to make a mistake. I've done several classics like forgetting to put the nuts on propshaft bolts (was a trailered vehicle so it came off at about 2 mph), forgotten to refit drainplugs and only realised when I start to wonder why I'm being covered in oil, left spanners on top of battery box only to knock them off, set the timing 180 degrees out on an engine only to wonder why flames were coming out of the carb, I could go on...... Hindsight is a wonderful thing!

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Sorry you had to learn that way - but it is generally the best way!

I learned that machining a lump of metal you've been working on for best part of a week while drunk is not a good idea - the hard way.

Turned handle on mill wrong way, lump of metal finds new home in bin, spend next week machining from scratch while sober!


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