S3 Big problems

**acmo** · 06-10-2016,08:11 PM

Hi all
So my S3 with 180k on the clock was burning a little oil so decided to spend a bit of time and money on it and get the head serviced.
Sure enough it needed new guides and was treated to a seat grind too.
While the head was off I also decided to replace the rings and big end shells.
I got the head back and started rebuilding slowly and thoroughly, I was in no rush as i'm using my banger for work.
I have never been so cautious when timing up an engine and so got everything spot on, marks aligned, 16 rollers on the cam sprockets etc.
Turned engine over god knows how many times by hand to make sure all was good.
All was good and time to fire her up. removed the fuel pump fuse and turned over to get some oil pressure, popped the fuse back in and hey presto sweet as a nut!
Running at high idle for a minute, then started to settle down, keeping my eye on water etc sat there idling for about 3-4 mins and suddenly a clatter and it stopped.

I have just ripped the head off to find 1 exhaust valve on cylinder 4 has snapped off from the spring cap leaving part of the stem in the cap with collets attached, the rest of the stem in the bore along with the head of the valve. This has in turn struck two inlet valves.
I just can understand A. why it's happened at all, and B. why only one exhaust valve has been hit?
If iv'e done something wrong I will take all the **** i deserve but I just don't see what I could have done. Should I be questioning my local engine reman' shop for not installing the valves correctly?

**zollaf** · 06-10-2016,08:17 PM

what make of valves were fitted, presuming new ones were fitted ?

**acmo** · 06-10-2016,08:22 PM

Hi Zollaf, The old valves were used.

**zollaf** · 06-10-2016,08:27 PM

ok, very unusual for a valve to snap . even if a piston kisses it then it will bend and not just snap. can you post some pics of it at all. i suppose the top of the piston is a mess now so you cant identify any tell tale marks from where it hit ? also check the height of the piston at tdc, i.e. is it level with the rest, can it lift up any more. what we are checking is that a big end shell hasnt collapsed allowing the piston to travel too high and smack a valve. other option is that the valve got stuck in the new guide. like i said, some pics would help to tell whats happened. a lot can be learned from the way its broken.

**Rob69** · 06-10-2016,08:35 PM

Possibly that old valve was too tight in the new guide or stem was already slightly bent and failed to close fully or fast enough once it heated up after a few minutes running, and got clipped, which then snapped the stem where it comes out the bottom of the guide ? Don't think your timing was out or it would have happened straight away.
Total b#ll ache after you've spent that time and money.

**acmo** · 06-10-2016,08:42 PM

It has made a fantastic job of the top of the piston, however there is no dent suggesting the piston has struck the valve while the valve has been in its correct position if that makes sense.
Here are some pics.
Piston
Valve

**zollaf** · 06-10-2016,08:47 PM

ouch, never seen a valve snap in 2 places like that. my guess is what rob suggested, valve has got stuck in the guide and stayed open for the piston to slam into it. quite why its snapped the top off is anyone guess, theres not really any stress point there. can you see the vw and part number engraved on the valve stem anywhere. thats the sort of thing you expect to see with an fai valve. i also wonder what make the guides were. was the head work done by someone reputable ? i would certainly be speaking to them about it anyway.

**acmo** · 06-10-2016,08:53 PM

I will have a look at valve brand tomorrow, I would hazard a guess that they are aftermarket as I noticed all 4 pistons have 2 exhaust valve marks where something has happened before.
I will have a chat with the shop tomorrow. They are reputable, pretty old school, it's an old workshop with Nissan Navara engines scattered all over the place.

**zollaf** · 06-10-2016,08:57 PM

yeah, good engines those navaras. not

be interesting to know what make it is... bet its not a genuine one now you have said that.. at least, looking on the bright side, the bore looks ok so all migh not be lost and replacement heads can be found cheap enough. i did one on an a4 recently, it had a gas conversion which had cooked the valve seats, got a used head for 80 notes and fitted that... most of the 20v heads are all identical castings and valves.

**Rob69** · 06-10-2016,09:05 PM

So are you saying that all 4 pistons have clipped the exhaust valves sometime in the past ? If so, the stem was probably slightly bent to begin with, and seized in the new tighter guide. I think the valve has seized fully open, giving the piston maximum leverage on the stem and stem guide, rather than just kissing the valve head.
If you take the head back to the shop and get them to remove and check run out of the remaining valves in a lathe or valve grinding machine, i think you might find some others are also bent. Did they regrind them or just lap them in to the recut seats?