Major Engine Failure Pt.2 - The Aftermath.

A few months ago I posted about a piston that let go in my engine.



http://www.jalopyjournal.com/forum/showthread.php?t=184838

I've finally got round to pulling it apart in preparation for a rebuild and here's what I found.

These are the pics of the no.1 bore that suffered the most damage. The bore is smeared in melted aluminium but there doesn't seem to be any damage to the bore itself.
No.2 cylinder has the same thing directly over the narrowest gap of the two bores. A symptom of excess heat from no.1?





A detail shot of the piston that went bang.





It can't be seen in this picture but the skirt of the piston is covered in hairline cracks.



The gudgeon pin boss has broken.



The little end of the rod is dis-coloured. Seems to go from light drown to black. I thinking this is more discoloration from combustion gases rather than excess heat to the rod



Damage to no.2 piston.



The peppered exhaust valve from no.1 cylinder.



Now things get a little confusing. With 3 of the pistons, 1,2 and 4 the top ring was completed jammed in the ring groove. The rings are almost completely flush with the piston, locked solid with a total loss of the ring gap. Ignoring pistons 1 and 2 which are damaged. No.4 piston shows no signs of damage or contact at all yet is suffering from the same top ring problem.

Whats going here? A cause of the failure with No.1?

I've had damaged pistons and locked rings from racing 2-strokes. When the piston gets to hot and strikes the bore and burs the piston over the rings. Never seen it in a 4-stroke.

I've tried to show a picture of the top ring on no.4 piston. You can just see that the ring is totally closed.

Rings were gaped according to manufacturers specs.



I think I'm lucky the piston held together long enough for me to shut it down. I'm hopeing the rod and crank are still straight.

Any opinions on what you've seen?

 

Detonation, lean cylinder? You were probably making some good power just before the failure eh? I beat my A silly but I must not be making enough ponies yet to break parts. I am jealous.

 

Did you file fit the rings? Too little end gap? Custom pistons, Top ring too close to business end of piston? Just shooting from the hip. Too big of nitrous load?

 

I can't tell from the pics if the pistons have pits in 'em or not.If the piston tops are pitted it's a detonation issue.
If they are not pitted then the bore is prolly too tight.

 

Looks like detonation to me , but thats based on 2 stroke experience ...

 

Well, it looks to me that the pin on #1 got hot enough to cause the rod end to get hot enough to coke the oil (the discoloration) might have even been red hot and the oil was quenching it. The oil spashing around is supposed to cool the piston, but if there is a a whole lot of extra heat being caused by a tight pin, I could see the whole piston getting hot enough to do that damage.
And if it ran for a long enough time with this heat buildup, it could get #2 cylinder hot, too. the whiping of the piston material onto the cylinder wall is the piston expanding to an interferance fit and just draging itself to death, creating even more heat.

I've seen rings get stuck in the grooves from high heat. And then they loose the tension and no more gap.

Didn't you feel this thing tightening up and loosing power? It will be interesting to know what went on in the last 10 minutes.

Frank

 

Thats the thing it was running normal then just gave up.

It was a fresh engine rebuild with about 1500 trouble free miles on it. I had driven half way across Europe, trouble free. It had been drag raced, trouble free.

Then I was doing 55-60 mph, taking it easy and its goes "bang"!

 

Anybody got anything to add?

 

What else was in the last tank of fuel ?

 

I'm with the 'lack of lubrication' theory.

It looks like everything got really hot in hole #1. Hot enough to melt the piston into the wall, but not hurt the bore...that's why I think it was lubrication versus something mechanical. Nothing is really 'broken' as much as things are 'heat damaged' - and the main thing keeping heat off cylinder walls is oil.

I'd bet you lost an oil pump or whatever means this engine has to lubricate cylinder walls/piston pins.

Detonation would have similar damage, but without the excess heat apparent in these photos. I've seen nitrous damage that didn't melt the piston into the bore like that. This engine didn't see lube and got hot until it 'almost' locked up, but then let go instead.

For whatever reason, you couldn't hear it until it was too late...but it seems obvious to me you lost oil pressure to the bottom end, or whatever would normally lube the cylinder walls and piston pin.

~Scotch~

 

sure looks like detonation to me. you check your distributor to see if your timing swung way out of wack? only thing I can think of that would cause an otherwise good running motor to suddenly go boom.

 

Hey mate, that sure is weird. I m trying to nut out why it would let go just like that after som much street/track driving?

What did ther plug look like in that cylinder? Did it look lean, peppered with specks of aluminium?, was the ground strap wasted? If so that would indicate a leanout at hi rpm. Excess cyl temp would do that - either excessive timing or heat at WOT, **** fuel or a combo of both would do that to a piston. IMO that much heat would kill the grouind dtrap on the plug first ot at least trash the gead hasket first.

Maybe damage was incurred at the track and it progressively got worse with street mileage until it let go? On that freeway run before it let go, it was running real well?

Have you ever measure EGT's at WOT to see if its leaning out?

With my blown flathead, I ve gotten into the habit of sussing out the plugs from time to time, esp if Ive been leaning on it a bit. Just my way of keeping an eye on it.

Do you have any of retarding timing at WOT?

Im stumped....

 

Your telling me!

The situation at the moment is as follows.

The piston was attacked from the top by excessive heat. This was detonation from either a momentary ignition failure or a change in fuel/air ratio.

The mag is still in service and working faultlessly on another engine. Could have been a plug that gave up? I'll try and get the mag on a tester to see if there is anything unusual going on with it just for piece of mind.

The plugs were always a nice chocolate brown colour but checking the plugs after the incident there was a distinct change in colour to the lean side. Possibly a manifold leak developed and leaned out the mixture?

It seems like all the pistons were suffering from excess heat it just happens that no.1 gave up first.

The destruction of the the top of the no.1 piston seems to be misleading in that it was a result of the piston getting too hot.

A interesting discovery was made after no.1 piston was removed from the rod. The gudgeon pin boss has been pounded out of round which has lead to the boss cracking in half (No pictures yet).
It seems like this has nothing to do with the detonation and the piston was breaking up long before it went bang.

There generally seems to be a poor quality issue going on with these pistons.
I was unhappy when I discovered they were Mexican specials and even the machine shop tells me they had issues with them but didn't mention it as they are what I supplied. Thanks!

 

Its not an oil issue.

If I'd lost the pump I'd be dealing with a seized mess. The bearings aren't even scuffed thats how fresh it is inside.

 

Which mag are you running?

 

Pistons...better ones all discontinued. There is someone modifying good quality 283 pistons--will seek out.
Now--is all the violence on the sides primary, the result of piston running out of clearance (seems unlikely to me since engine had some history) or did disintegration of top dump aluminum down there till space was full and cause the overheating as a result??
Also, look at plugs 1&2 carefully with illuminated magnifier...if original type, you could remove electrode/insulator.
Any metal specks, little black dots, oddities??

 

Scintilla Vertex.

 

So from this clue, could it be that the pin boss being loose allowed the piston to **** slightly in the bore and heat up, to the point where catastrophic failure occurred? Perhaps the pin boss broke or started to get sloppy after one of your hard runs. A slight ****ing of the piston (very slight) will absolutely cause your oil film to be s****ed away and lead to something like this. However, I would think you would have heard some knocking for quite a while if you did have a boss that was getting loose. Just my 2 pence worth....

 

The electrobe had melted ali on it and the gap had been closed shut.

 

What's the chance of your front carb suddenly leaning out or a jet blocking? Sure looks like 1 and 2 ran a bit lean compared to 3 and 4.

 

Front carb, gasket at front of manifold...

 

Detonation can cause the pin boss to break in extreme conditions. Piston quality was prolly not a factor in this deal.

 

Those 2 cylinders got waaay lean. The piston looks like a cast piston after a couple nitrous runs...

 

I'm with Bruce Lancaster on this one....definetely shows signs of bad lean-out caused by air being introduced into #1 hole. Could be air leak at carb or even at the intake gasket, pinhole in intake casting etc. Prolly hard to detect problem leak resulting in lumpy idle if the engine had a big cam.