Timing Belt Snapped

[spabax]

2005.5 / 210.000 km / 2.0 TFSI
The timing belt snapped while I was driving (speed under 30km/h). What damage can I expect? Will I need to replace the entire engine or just the head? Any tips overall?

edit: During gear change from 2nd to 3rd. Low RPMs

[pezgoon]

2005.5 / 210.000 km
The timing belt snapped while I was driving (speed under 30km/h). What damage can I expect? Will I need to replace the entire engine or just the head? Any tips overall?

At that low of a speed it should almost guaranteed be just the head but without pulling it off there’s no way to be sure. You may have some minor damage to the piston heads but it will may be easily fixable without having to send them out or anything.

Only thing suggested is you could try looking through the spark plug holes to verify what happened but the only real tip is get the head off and see what happened

Out of random curiosity when was the timing belt last changed?

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[Theiceman]

yup head damage ( valves) ... there is an earlier thread of a guy doing this as we speak. Speed under 30km/hr is incredibly fast for an engine. By the time you roll to stop those valves have been hammered hundreds of times.

If you are doing it yourself definitely read that thread.

http://www.audizine.com/forum/showth...or-even-fire-(

If you are having it done you have to weigh up the costs of mechanic, machine shop work, engine and engine replacement costs possibly as they might convince you its cheaper to go that route , and balance against worth of the car.

if you want to limit your investment just to quantify damage, you can throw a timing belt on and time it. Then do a compression check to see if there are any bent valves. That would tell you immediately what you are into . I think we are convinced valves will be bent but it will give you peace of mind and certainty.

[Brillo]

Theiceman is right, unfortunately you probably have a number of bent valves. Look at it this way - if you were doing 30km/hr at or close to 2000 rpm, that would equate to about 33 revolution per second (of the crankshaft) and 16 revolutions of the cams before they stop turning due to their rotational inertia. It's very likely that during that time there was piston-to-valve contact which would at the very least bend the valves on one or more cylinders. This will require removing the head to fix which will not be cheap. Since you asked the question I suspect you are not one to fix it yourself. Then you need to make the decision that Theiceman mentions.

[GTA_G20]

Road speed is irrelevant here. Engine speed is your issue. 30km an hour in 4th gear at 800rpm is vastly different than 1st feat at 7000rpm.

Context here ppl. Auto vs manual transmission would also be a factor if you don’t remember your tach reading


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[Ashtonts]

Theiceman is right, unfortunately you probably have a number of bent valves. Look at it this way - if you were doing 30km/hr at or close to 2000 rpm, that would equate to about 33 revolution per second (of the crankshaft) and 16 revolutions of the cams before they stop turning due to their rotational inertia. It's very likely that during that time there was piston-to-valve contact which would at the very least bend the valves on one or more cylinders. This will require removing the head to fix which will not be cheap. Since you asked the question I suspect you are not one to fix it yourself. Then you need to make the decision that Theiceman mentions.

Curious how you came up with 16 revolutions of the cams before they stop... wouldn’t you need to know the time it took them to go from ω0=33rev/s to ω=0rev/s? If you knew time you can definitely use a simple kinematic equation to solve that (neglecting friction, impact of valves throwing off angular velocity, etc), but without it I’m confused as to how you got that number.

Genuinely curious; I’ve been doing similar problems in my Engineering Dynamics class so it’s kinda neat to see a real-life application of that.

[Brillo]

Ashtonts Your point is well taken but I wasn't trying to do a rigorous calculations since there are too many unknowns. So I made a gross assumption which was that the the inertial mass allowed the cams to spin for one second after the belt snapped - maybe more maybe less, who knows. They would have been spinning at 16.5 rps at the time of belt breakage (assuming an engine rpm of 2000 rpm) and zero after one second. That's likely close because think about how long it takes your engine to shut down after you turn the key off, and that's at idle speed (about 800 rpm). Assuming linear deceleration that would amount to about 8.25 complete revolutions (I said 16 but the point is the same). The crankshaft probably would have spun longer than the cams on account of having much more rotating mass and not having to drive the cams on account of the belt breakage. Again, without being too rigorous, that is plenty of crank and cam rotations for the pistons to come into contact with the valves on one or more cylinders and bend them. My limited experience is that on interference engines (obviously) when a timing belt breaks valves get bent. Based on your experiences or your dynamics class let us know if a more accurate assessment can be made.

[pezgoon]

Or shattered lol


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[spabax]

Out of random curiosity when was the timing belt last changed?

Audizine

As far as I know it was never changed. Stupid me didn't check the service books, previous owner said it had been changed 'recently' but there is no record of it.

I will try to fix this myself. Never done anything like it before but thought it could be a fun learning experience. New engine would cost me 1-2000 euros, not too bad. I have access to a car lift, engine lift and all required tools, so why not give it a try.

I might start this weekend so I'll update this thread with pictures etc.

[Ashtonts]

Ashtonts Your point is well taken but I wasn't trying to do a rigorous calculations since there are too many unknowns. So I made a gross assumption which was that the the inertial mass allowed the cams to spin for one second after the belt snapped - maybe more maybe less, who knows. They would have been spinning at 16.5 rps at the time of belt breakage (assuming an engine rpm of 2000 rpm) and zero after one second. That's likely close because think about how long it takes your engine to shut down after you turn the key off, and that's at idle speed (about 800 rpm). Assuming linear deceleration that would amount to about 8.25 complete revolutions (I said 16 but the point is the same). The crankshaft probably would have spun longer than the cams on account of having much more rotating mass and not having to drive the cams on account of the belt breakage. Again, without being too rigorous, that is plenty of crank and cam rotations for the pistons to come into contact with the valves on one or more cylinders and bend them. My limited experience is that on interference engines (obviously) when a timing belt breaks valves get bent. Based on your experiences or your dynamics class let us know if a more accurate assessment can be made.

I may sound like I’m being pedantic but I feel the distinction is important:
α(average) = dω/dt. In other words, average angular acceleration (deceleration in this case) is equal to the change in angular velocity over the change in time. We aren’t explicitly assuming linear angular deceleration, we’re modeling the average deceleration as constant. This is important because we don’t necessarily care about exactly how the graph of dω/dt looks (it could be parabolic, etc—the path of the velocity of you will), we only care that over the time (1s in this case) the angular velocity changes from 2000 rpm to 0 rpm.

So since we know the start and end points of the velocity and assume the start and end points of the time, we can ignore the path the velocity takes to get there because it won’t matter in our particular analysis, and instead just model it as an average. This is important because if we wanted to use this same equation to determine the position at .5s or .75s, our answer could potentially be very far off depending on what the actual path was.

Pedantry aside, I agree completely with the way you modeled it. Plugging it into my kinematics equations, I got the same output of 8 revolutions of the camshafts.

I will add, however, that assuming timing is lost immediately, and that the timing never coincides again as the cam and crank both decelerate to a stop (may or may not be a good assumption—I have no idea honestly), and that both stop at the same point (t=1s), the crank will spin 2x the distance (16 revolutions). Since the movement of the pistons is tied to the crank, and with the valves being out of time, there is the chance for damage to the valves on all cylinders for all 16 of those revolutions, caused by the pistons.

So all this effort to say what we already know: OPs valves are fucked.

If I knew a few more of the unknowns here, it might even be possible to determine how much force is being exerted on the valves when they crash against the pistons, compare it to the yeild point of the valves, and get a definitive answer to whether or not the valves (and pistons for that matter) are smashed... buuuut I’m a little lazy for that.

[Theiceman]

Exactly Brillo. We all know RPMs matter more than engine speed but we are trying to give orders of magnitude not finite detail so subjects of auto vs manual or 7000 vs 2000 irrelevant when you are trying to state the valves took multiple hits.
They will be bent regardless


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[Theiceman]

Road speed is irrelevant here. Engine speed is your issue. 30km an hour in 4th gear at 800rpm is vastly different than 1st feat at 7000rpm.

Context here ppl. Auto vs manual transmission would also be a factor if you don’t remember your tach reading


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Context here imo is Not relevant at all. Valves will be bent. Car would stall at 800 rpm in fourth at 30km/hr. And its impossible to do 30km/hr in first at 7000rpm. Thats why you stay away from minutia when discussing orders of magnitude. Just the car was moving at 30km/hr when the belt snapped so you know it was more than once that the valves got hit.

This is why i suggested a compression check


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[vce1232000]

Here's the plain & simple truth 99% of the time when your timing belt snap some or all your valves will contact the piston. 1% chance you have 0 damage. If that the case. Go play the lottery. Because you just hit the jackpot with your engine . If your doing yourself. All you need to do is replace just the best valves and relap those valves Stock valves arent that expensive

[aluthman]

Here's the plain & simple truth 99% of the time when your timing belt snap some or all your valves will contact the piston. 1% chance you have 0 damage. If that the case. Go play the lottery. Because you just hit the jackpot with your engine . If your doing yourself. All you need to do is replace just the best valves and relap those valves Stock valves arent that expensive

Used stock valves are even cheaper... I've got piles of them.

[Solarsuplex]

Im glad that all that physics gave us the answer we were looking for.

Valves = fucked.

Nice

[Brillo]

Adam - So you're going to Ohio, have you been to Sweden??

[dalmation53]

Damn all you guys went to physics and science... The point is that he has bent valves.. Lol jesus christ..

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[Ashtonts]

Damn all you guys went to physics and science... The point is that he has bent valves.. Lol jesus christ..

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But science is fun!!

[dalmation53]

But science is fun!!

I do agree in that.

[Brillo]

...and there might have been two or three other members who enjoyed the analysis!

[Theiceman]

But science is fun!!

My point exactly lol.


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[Theiceman]

Here's the plain & simple truth 99% of the time when your timing belt snap some or all your valves will contact the piston. 1% chance you have 0 damage. If that the case. Go play the lottery. Because you just hit the jackpot with your engine . If your doing yourself. All you need to do is replace just the best valves and relap those valves Stock valves arent that expensive

I don’t even think you sir would replace the best ones !! But maybe the bent ones


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[tloft4]

If I knew a few more of the unknowns here, it might even be possible to determine how much force is being exerted on the valves when they crash against the pistons, compare it to the yeild point of the valves, and get a definitive answer to whether or not the valves (and pistons for that matter) are smashed... buuuut I’m a little lazy for that.

you mean the 12 or 100 unknowns to get an exact answer?

[Ashtonts]

you mean the 12 or 100 unknowns to get an exact answer?

This is so true. Probably no way to get a really good answer without a computer model and FEA software.

[Theiceman]

Yeah there is ... a borescope with a mirror attachment !!


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[vce1232000]

I don’t even think you sir would replace the best ones !! But maybe the bent ones

Key stroke error .... Kill me why dont you

- - - Updated - - -

Used stock valves are even cheaper... I've got piles of them.

Agree . Why do new when used will do .................

[pezgoon]

...and there might have been two or three other members who enjoyed the analysis!

I did


So all this and my first response was still the right one???


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[Theiceman]

I did


So all this and my first response was still the right one???


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yup .. interested in which way OP goes with this ..

[spabax]

Any tips on removing the head? I have knowledgeable friends who will help me but would be nice to see an old thread or something :)

[pezgoon]

Any tips on removing the head? I have knowledgeable friends who will help me but would be nice to see an old thread or something :)

Baggies and mark all the bolts! Makes it easier for reassembly past that I don’t know enough to suggest more.

I do remember a year or so ago someone who was a complete noob at wrenching bought an A4 and the belt broke like a week later and there was like a 32 page thread that people walked him through doing it and he was successful in the end

Maybe one of the other contributors may remember it as well as it was massive for awhile, if you could find that it would be good. Aluthman helped a lot on it maybe he’ll remember it I just don’t remember the title or who it was


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[tloft4]

thatd be dalmation

[Solarsuplex]

It was like AZ builds a motor through this guy that had flat head screw drivers and $50 bucks.

[pezgoon]

thatd be dalmation

Thank you

It was like AZ builds a motor through this guy that had flat head screw drivers and $50 bucks.

Hahahaha yes, yes it was

g

Here you go op https://www.audizine.com/forum/showt...have-Questions

Long read but it tells you how to do it for someone who has never done it (or anything like it) and as said above, has a couple flat head screw drivers and 50$


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[spabax]

Thanks for the link, now I have something to do this afternoon.

[pezgoon]

Thanks for the link, now I have something to do this afternoon.

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[Theiceman]

isn't there a guy doing this as we speak ??? with another thread running ?

[tloft4]

chuckvmusic

[pezgoon]

chuckvmusic

Dude.

Stop it.




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[A4B7Avant]

Creeper! (Quickly covers up webcam with tape)

[tloft4]

Might as well peruse every thread. And tape your webcams