Help with repeat Piston Failue - Moose is stumped

[The Moosehunter]

Hi Guys

Sorry I've been a bit quiet recently, been a bit dejected after the Mighty Griff stopped being so Mighty ( again )

I am after helpful advice from serious "Piston" Headers to get to the bottom of yet another total engine failue

I have just extracted my engine at the weekend after No1 cylider lost all compression and bits of aluminium made it's way into the plenum ( not a good sign )

Upon removing the head I found this

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Now this is bad enough but this is the second time in 3 years I have had an almost identical failure in the same place on the same number piston

Last time it happened I took some precautions before re-assembling. Those were

1 - Replace piston ( fairly obvious this one, may have been metal fatigue)
2 - Replace Injector ( in case this lead to lean runing )
3 - Replace both valves ( in case faulty valve lead to problem or risk of damage as a result of bits of piston being squeezed with the mixture )
4 - Replace conrod ( just to cover all bases )
5 - Big ends / mains / gaskets / plugs changed as a matter of course


The only other works has been a stealth cam and double valve springs fitted by a reputable TVR specialist ( as i couldn't be arsed as my new boy had just been born 4 weeks earlier ) they also checked compressions / leak down and replaced / lapped in a few valves that weren't seating properly

Now for this same failure to have happened again has left me really stumped

The only thing I can think of is either just plain bad luck ( not convinced ) or there must be something wrong with either the new piston, injector or the ECU cusing it to run lean and cause failure

I plan to do much the same replacements again, except this time I plan to ask Mark Adams and / or other to run a full check and monitor primary temperatures to see if there are any signs of any ill - running balanced cylinders

Those who know me, know that I like to drive my car hard, but it is never to the detrement of any sustained or peak rev-limits. It's usually the clutch and tyres that suffer

Can anyone please give their thoughts as to what the causes might be and what other sort of diagnostics I could run to alleviate this issue, because if it's 3rd time, then it's third time and you're out and in goes an LS7

Come on guys .... make my day

Cheers in advance

Moose

[dbv8]

Detonation damage?

See my similar damage here and what ive done to avoid similar in the future http://www.pistonheads.com/gassing/topic.asp?h=0&a...

[blitzracing]

Just a though -I had a failure like this on a tuned fiat engine due to a badly worn main bearing that was enough to cause the piston to hit the head as the crank moved. I notice you have a light spot on the edge of the piston near the failure point, Id double check for any signs of corresponding contact on the head just to be sure.

Edited by blitzracing on Monday 10th January 12:32

[nrick]

Hi

I used to work for Wiseco pistons and could try to help you (fellow TVR owner). It is hard to tell if there is serious det damage, but it could be. The detonation undermines the ring groove until it fractures. If you PM we can have a chat.

[spend]

Is the ring broken, if not the gap looks very odd. Pics of the bore, heads + valves and piston skirts would help.

[yellowgriff]

Could it be the wrong spark plug running to hot.....Sorry but only a guess could be a bad one just trying to cover things not listed

[Bluebottle]

Sorry to hear of your bad luck Andy

When this happened the first time, did the affending piece of piston pepper the bowl of the cylinder head or bend the valves on its exit?
When I melted a piston (looked the same as your pic) it didn't bend the valve stem, but the valve seat was more recessed than the other pots, and the head was badly pitted from said peice of shrapnel...If your head had peppering and wasn't repaired this would make the cylinder run hotter leading to a second premature failure.

[dbv8]

Not as bad as mine but should be fixable...







Buy some of these...



My heads were welded up and repaired nicely...

[Aussie John]

Make sure the replacement piston is the same weight as the existing set. I was caught out in OZ when rebuilding a 4.4l carb Rover motor. It had a vibration which ended up being the replacement piston [ bought from the main dealer ] was heavier by some margin; had to dismantle and rebalance to cure it.

[Pupp]

Looks knock/ignition related to me... what spark timing are you running under load conditions? Dizzy or coilpack and if the former, what condition is the advance/retard unit in?

One avenue to eliminate is valve guides and seals... oil finding its way into the chambers via these can lower the effective octane of the fuel charge such that knock can occur that might not be noticeable but enough to fatigue the piston crown.

Ideas on a mechanical front to check would include the piston/bore clearance (just to make sure the pistons aren't rocking excessively) and that the ring to groove clearance and ring gaps are right

[plasticman]

The plugs will be batch fired so it will not be the ecu , You could look for some damage in the loom operating that injector .

[The Moosehunter]

Hi Guys

Firstly thank you all so much for your input to date

I will try to answer as best I can:

Firstly photos of damaged units

All 4



No 1



No 2



No 3



No 4



Bore 1a



Bore 1b



Previous failure



Unit failed whilst on a track day at Bedford. Was going like a train all day until ....

Previous unit seemed to fail over time. No sudden dramas ( although a lot of smoke on sudden deceleration )

DBV8 - Does sound like the most probable diagnosis. Have not spent enough time in the hands of people who know enough about tuning. Nice pistons who did your head repair and if you don't mind ... how much ( pm if it's scary figures )

Always run on Optimax where possible if not then 97 minimum

Blitzracing - New main bearings fitted on previous rebuild no signs of any clouting this or other time. New valves fitted as a precaution

Nrick - YHM

Spendy - No the rings are all in one piece. Photos above. piston not out yet but have photo'd bore as best i can

Yellowgriff - Worth a shout. New plugs fitted last time so some pretty strange co-incidence if so

Bluebottle - Cheer Hamish, could really have done without it I must admit. I did plan on replacing the spigot bearing this winter but this is a drastic way about making it happen
No the head and bore escaped any signs of any noticeable damage. Never did find the bits Nothig like the damaged DBV8 has suffered. It only drew my attention to it becuse the electrode of the plug got bent double resulting is a mild mis/ no fire

Quinny - I have to say sounds like the most probable outcome. Bad luck / bad set up last time combined with new mis-matched piston this time. Entirely probable. Looks like I may have to get some balancing done to erradicate before next time

Aussie John - Sounds like this will going in my pile of spares to be balanced / matched

Pupp - Not too sure of the exact timing, set up by TVR specialist. Running Dizzy at present. Condition believed to be OK all part moving freely but will need to form part of new RR set up session

Plasticman - Would you think that this should just be a continuity test or should I look for something else ( sparky bits not my speciality - Not that metally bits are )

I really do appreciate your input guys and will provide any further infor and more pics opnce the unit comes apart soon

Is it worth replacing to a set of red Bosch injectors whilst I'm at it

List of things to do ( add as we go )

Parts

Piston
Piston Rings
Valves
Plugs
Dizzy cap
Rotor
Red Injectors ?

Services

Weld / polish head
Balance Pistons
Balance compression ratios

Rolling Road

Check Dizzy operation
Check Advance / retard
New MA chip and set up
Check primary temperatures


Cheers

Andy

Edited by The Moosehunter on Wednesday 12th January 15:15

[dbv8]

When you remove your sump drain off the oil through a seive. You may be suprised. If what i found in mine was gold it would have paid for my rebuild!

I cleaned up my heads then took them to a local friendly engineering firm. They tig welded up the damage.
I gave them to Mike at Boosted along with the the short engine. He had them machined, 3 angle seats cut, ports improved and bronze valve guides fitted and machined and valves lapped in... came to £527.

Pistons cost a small fortune!

[Boosted LS1]

Look at the squish pad in the heads. You can see the detonation effect, pin holes etc. Also catterpiller tracks on the piston crowns is sign of mini explosions burning off carbon. That banks detonating imo. You'll probably find pin holes on the upper ring lands and small vertical scratches.

What's happening in the other engine bank, if normal then it's a fault common to one bank.

Edited by Boosted LS1 on Wednesday 12th January 16:12

[chris52]

I agree with boosted it certainley looks like detonation. I also noticed that the bore's don't look like they were honed when you replaced the rings last time I would expect to still see some of the criss cross marks on the bore if only done 3k ago but that may be just the picture.

Chris

[The Moosehunter]

Boosted LS1 said:

What's happening in the other engine bank, if normal then it's a fault common to one bank.

Hi Guys

Other bank appear to be the same as the "healthy" cylinders of the LH bank

chris52 said:

I would expect to still see some of the criss cross marks on the bore if only done 3k ago but that may be just the picture.

Chris

Hi Chris that was 3 years ago not 3K miles and it was honed out. Whether it was done properly of course .....

[chris52]

Ok. Just as a tester though, it may be worth checking that the ring gap was set correctly when it was fitted. I know Dave has noted this already, as the engine heats up the ring will expand and start to close the gap If it closes totaly and cant expand any more it will either break the ring or the piston (normally the ring but possible to break the piston), your piston has broken diectly above the ring gap and also at the weakest point of the piston. You also say the you were on a track day so no dought you were giving it some beans and as a result will have had a hotter engine especially in the combustion chamber.

Its very easy check just remove the ring from the damaged piston and fit it into the bore as level as you can then using some feeler gauges measure the gap. Not sure whet the gap is on the 500 but a call to tvr power should help that.

edit to add a link about ring gaps have a look at the pictures of broken pistons due to the wrong gap look familier http://kb-silvolite.com/article.php?action=read&am...

Edited by chris52 on Thursday 13th January 14:07

[v8 racing]

Definetly detonation that has caused that, to be honest it is not un common on the 500 engine at all so dont feel you are the minority, you do need to find the cure though or you will be posting pics of a broken crank next!, to be fair too much advance is normally the case, but as mentioned oil can cause these symptoms, ps dont forgot an engine will be detting before the human ear can hear it!, certainly no more than 28deg advance is needed

[The Moosehunter]

chris52 said:

have a look at the pictures of broken pistons due to the wrong gap look familier

You know what Chris .... I can't help but feel you/Pupp hit the nail on the head ( or the piston )

That photo looks mighty familiar

Whilst there is clearly some massive room for improvement in the set up / avoidance of detonation that can happen during and once rebuilt, you're suggestion of ring gap clearance issues certinly gives me a plausible reason for the failure of that one cylinder over any of the others

You are quite right the car was being given some beans on the day and it was August Bank Holiday so not the coldest day of the year

No signs of overheating as a whole but of there is some unequality of the combustion volumes on the replacement piston leading to localised overheating on No 1 cylinder after a good hard session that caused the ring to expand lock up and blow the piston crown, then with my middling mechanical understanding / ability levels this feels like a very plausible chain of events. Especially as the ring gap is directly under the blown section of piston crown ( assuming if it was going to distort this would be the easiest point of deflection )

I'm happy ( relatively speaking ) to accept this as the cause and will be certainly checking for all of the gaps on the new set when they go back in. As well as sack the guy who re-built it last time

Anyone know a sensible compression ring clearance gap for a hard used 5 ltr ?

I'm gonna make sure also that when it goes back together that all top end rings are fitted with the gaps as far away from the valve cut outs as possible. Do they have a tendancy to rotate around the piston once fitted ?

Thanks for all your help on this one guys and if there's any other ideas to add into the mix to keep Moose on the road / track then these will be equally welcomed

Good on ya !

Moose

Now ... where did I put my credit card ....

Edited by The Moosehunter on Saturday 15th January 07:06

[spend]

Rings don't stay in one place, they should spin round. Its not just ring fit in the bore, they are like mini pistons within the grooves as well as wings getting blown around. Thats why you look for grooves worn in the bores.

Anally cleaning piston grooves and hand finishing rings takes much more patience than just gapping the rings IME.