Possible bent valve stem causing engine to lock up

[lowact]

We could open a book on this, my bet is broke piston. Because there is plus minus half inch movement stopped by thunk at the rear of the engine

[George]

Hi Marek,
I have to say that I am totally impressed with your diagnosis and knowledge of the S3, it looks as though you are spot on with what I believe is not a common or an easily diagnosed fault.
I have now removed the sump and when looking up at the crankshaft everything looks fine, I can't see anything broken and the cylinders are all very smooth with no abrasions. However when I removed the baffles from the sump in the bottom of the sump I found several pieces of brass shims which presumably are pieces from the thrust washer that you diagnosed.
The pieces are a few thou thick (not measured them) about 10mm wide and look to be part of a semi circle.
I still can't see where they came from or where they are supposed to fit but at least I have some evidence of what the problem is. By the way the engine is still locked.
Once again Marek thank you very much, your help has been invaluable and probably saved the cost of a full engine rebuild.
George
P.S. any tips on removing the crank whilst being dripped with oil will be gratefully received.

[christopher storey]

Brass shim is definitely not a thrust washer . The thrust washers are indeed semi-circular, with the circles' inner diameter coinciding with the main journal diameter, and are steel , faced with lead-indium or similar bearing material, with an overall thickness of perhaps 50 or 60 thou , and ( I am talking here of 6 cylinder cars but I anticipate that the V12s are similar ) are held in position by the centre main bearing cap into which a recess is machined to accommodate - and hold in place - the thrust washer

I am troubled by the fact that the crank still will not move. Have you tried levering it gently to front and to rear to see if there is movement - something of the order of 4 thou minimum might be typical , and then if you can move it , see if it will also rotate. It might in any event be quicker to remove the centre main bearing cap and thrust washer ( if it is still there) and see if this allows free movement both axially and rotationally to be regained

[rswaffie]

The metal you describe as found in the sump, sounds like bearing shell material. Assuming it’s not the thrust washer. If there are still bits of the shell that have come adrift they may be what’s causing the crank to become jammed if they are lodged between bearing and crank?

[MarekH]

It's not obvious from your description whether this is a piece of disintegrated thrust bearing or one of the regular bearing shells. Thrust bearings are completely flat and semi circular like just the numbers on a protractor. Regular bearing shells are three dimensional and are like half of a low fat cylinder.

From under the car, with a long pointy screwdriver (i.e. one about 18" long for doing up VERY big screws), you should be able to wedge it into the ring and move the crank forward and backward a few degrees like you were previously. Only this time you might be able to see where it is sticking. Christopher is right that you want to pry the crank towards the back of the car, if only by a few thou and that should let things move more.

It is inevitable at this point that you will end up undoing all of the bearings to look for damage and to refloat the position of the crank, forward to back, relative to all of the things hanging on to the crank. There are (or were...) seven mains, twelve conrod bearings, plus the thrust washer (which is a bearing, not a washer), which is in line with the crank and half way down the crank. You'll only need to loosen stuff to free up the crank. At this stage, don't undo the rear main seal housing if you don't have to.

Keep everything in order as conrod halves and shell caps are not interchangeable with each other. If nothing is wrong with the bearings, then they may not even need replacing, but they are only available in complete sets. Look at the backs of the bearing shells and note any writing - if a crank has been ground or reground, then the bearing shells will have a number like "+010" on the back. It's utterly critical that replacements are also the same size and writing tells you this.

Typically, you roll the top halves out by pushing the bottom half of a shell back in and around the top a bit and then grabbing the top shell half and pulling it out. If everything is loosened but not taken off, it means there is enough wiggle room to do this without having to worry about dropping the crank out.

If all you have lost is a thrust bearing, then that's great news. With everything loose, pry the crank towards the back of the car, fit a new thrust washer and measure the gap forward to back. Consult the manual about spec.

You can then have some fun with Plastiguage to see what your main and conrod bearing wear is. Consult manual again.

Don't lose sight of the prize - you are supposed to be able to rotate the crank without stuff scraping or hitting and that includes after your left foot has been pushing the whole crank forwards every time you simulate a change of gear. You may move everything about more than than once to get it reliably lined up and missing by a few thou rather than touching by a few thou

Good luck.

kind regards
Marek

[George]

Many thanks to all of you for contributing to this discussion it is very much appreciated.
For the time being I am sticking to the theory that it is the thrust washers because the shim like pieces from the sump are curved and the same width as a thrust washer, it could well be the case that a thrust washer that started life at 1/8" thick has been worn down to its current shim like thickness and like Marek said when I fitted and operated the new clutch with the now excessive amount of play the crank has been pushed forward and jammed the engine.
Therefore hopefully tomorrow I will be removing the centre bearing carrier to inspect the state of the thrust washers and to try and remove them hopefully without taking out the crankshaft but if this is not successful then obviously I will have to remove the crankshaft.
I have to say that I was relieved to find the debris in the sump because at the very least it is something positive that I found as opposed to finding nothing on the top end.
I will report back to you all with my findings when I remove the centre bearing carrier.
Many thanks
George

[George]

I have now removed the bearing carrier that holds the thrust washers and as soon as I took it off thankfully the engine turned freely.
When I removed it there was a lot of what is best described as gold flake (brass) around the carrier and crankshaft which was obviously from the thrust washers. I then removed the thrust washers luckily without having to take the crankshaft out to find one of them with all the brass missing and badly scored whilst the other was in good condition.
I have now ordered a new pair, plus gaskets, oil etc from SNG and hope to very shortly have my maiden trip in my newly converted manual V12.
Full marks and thanks to Marek you got it spot on and again thank you to all who contributed to this discussion.
At the risk of stating the obvious this is an excellent forum.
Kind regards
George

[DWW]

Glad you found the culprit, with the expert help of Marek, but what was the cause of this near catastrophic failure? Just to make sure it won't happen again with the new part.

[MarekH]

The cause is the auto to manual conversion.

The thrust washer was already worn, but it never mattered when it was an automatic, because the crank was never moving backwards and forwards. Once the clutch was pushed in one too many times, the crank was pushed forward too far. It can only be pushed forward by ~4thou before all of the conrods are non vertical and so they ended up touching and immediately stopped the engine.

The forward-backward bearing clearance at the worn thrust washer exceeded how much the crank could safely move forwards before metal to metal contact occurred.

Unless your engine has low miles, drop the sump and check amount of freeplay forward-backward at the thrust washer if doing an auto to manual conversion.

kind regards
Marek

[George]

I am certainly not going to disagree with Marek but I will add that the engine has done 34k miles and it was the actual thrust washer that jammed the engine rather than the con rods touching in this case.
Just for the benefit of anyone referencing this post in the future what may have also contributed to moving the crankshaft forward a few thou may have been tapping in the spigot bush, this was not hammered in but with the worn thrust washer it could have been enough to cause the problem and then the 2 minutes of running the engine did the rest.
Kind regards
George

[abowie]

with all the brass missing and badly scored whilst the other was in good condition.

Before you reassemble it have a good look at the thrust face on the crankshaft where the damaged thrust washer was and ensure that it's in good condition.

[Tommd]

George, in your first post, you state the engine ran two minutes then froze, you never said you pushed in the clutch pedal. Could there be another cause, such as the transmission input shaft being too long, jamming into the flywheel, jamming the crankshaft against the thrust washer causing the failure?
Tom

[George]

Hi Andrew,
I have already checked the mating faces to the thrust washers and they are fine. The thrust washers are a softer material than the crankshaft faces so you could argue that they did their job in protecting the crankshaft.
Hi Tom,
If the input shaft was too long then I would not be able to get the bellhousing face to snug up to the block which is not the case because I have no trouble locating and fixing the bellhousing and gearbox to the engine.
Many thanks to both of you for your input.
Kind regards
George

[mgcjag]

Hi George...Tom.s point is valid.. were only talking a few thou ...the o/p shaft could be pressing on the crank as you finally tighten the bell housing.....at least check it

[George]

Hi Steve,
The bellhousing goes onto the dowels and the 2 faces meet together before any bolts go in, but its a fair point and I will check the clearance from the end of the input shaft to the back of the spigot bush.
Thanks
George

[abowie]

Hi Andrew,
I have already checked the mating faces to the thrust washers and they are fine.

Ah that's a relief!

[AussieEtype]

Even with worn and broken thrust bearings I am not sure I understand what caused the engine to lock up while running. Even if the conrods or crank could catch on something the forces of a running engine would push past that - might hear a bang or a thump but the engine would keep running. Same as deteriorating thrust bearings, the engine running would overpower them and just spit them out.

Obviously new thrust bearings are needed but I would be examining the bottom end carefully looking for further damage and trying to determine the actual cause of the engine lock up. Also check the gearbox carefully as I lockup there could have caused the engine to lockup and the failing thrust washers are just a side issue.

Garry

[Allrand]

I would theorise that if a thrust washer was excessively worn or too thin it could dislodge and become wedged in the area above the bearing cap, being drawn in by the crankshaft only to a point where the locating tab resist further movement, hence the small amount of movement before it locks.

[MarekH]

... the forces of a running engine would push past that -

Oh no they won't!

An idling engine is only producing enough useful work to power the alternator, power steering and water pumps - a couple of horsepower only. Even putting an automatic into "drive" drops the revs. Witness how easy it is to stall a manual transmission car without upping the revs when you pull away. That's how little power you've got.

Don't get fooled by modern fuel injected cars. They'll sense the rpm and the negative rate of change of rpm and probably bump up the amount of ignition advance and open an idle valve to admit more air - none of that happens here.

kind regards
Marek

[lowact]

Twas mine, while I was there I'd be dropping the caps off the big ends, just to verify these weren't on the way out also.