Technical Foreign Object Damage

[ronstox]

Recently my 110 engine in my 71 Fiat 500F topped stopped running. Amoung other things, I tested the compression and found close to 0 in the #2 (forward) cylinder. I squirted some oil in and tested again. No change. OK, I must have valve trouble, says I. I pulled the engine, (my first time) and removed the head (also my first time). What I found looks like some foreign object was in pinging around inside both cylinders and did some damage to the pistons & head. There appears to be a chunk missing from #2 piston on the bottom, right hand side of the picture. The valves actually look OK.
What do you think, is the head salvageable? I think I should replace #2 piston, but #1 as well?

 

[Toshi 975]

The pictures are not very clear but I would suspect you will get away with the head after a strip down and clean up. Pistons look to be shot so definitely new pistons and if you have bore damage or excessive wear a rebore will be required. You need to establish if this was caused by a foreign body getting in there or the piston rings letting go. Bottom end will also need a complete flush and clean in case any particles have got in there.

 

[Goldnrust]

It’s not always easy to see from photos, whilst it could well be foreign object damage, it could also be a particularly nasty case of pre-ignition or detonation.

The head look perfectly salvageable to me, I would want to pull the valves and inspect them and the valve seats. If it is FOD then whatever went through it needs to have got in and out of the cylinders via the valves.

I would remove the barrels and pistons and inspect both. Personally I would replace both pistons unless you’re on a really tight budget. Check the barrels for signs of scoring or metal transfer. If it is FOD then whatever bits have been going round inside the engine may well have scored the bores. If pre-ignition has raised cylinder temps high enough to melt the top of the piston, you might find aluminium smeared up the side of the bore. This should come off with some wd40 and scotchbrite or very fine wire wool.

From there I would inspect the big end bearings, either scenario would shock load them quite heavily, especially the upper bearing shell. Hopefully they are fine but doesn’t hurt to look.

Then I would carefully inspect the oil pan and filter for signs of debris, and do a full deep clean off the whole engine including all the oil Galliers if you find any. That piece of piston (not to mention the potential object) had to go somewhere. When a friend of mine had detonation damage on his engine, the melted bits of aluminium from the piston got everywhere, and I mean everywhere, inside the engine.

Finally, don’t forget to solve the root cause of the problem when you put the now repaired engine back together. If it’s a foreign object, then how did it get in? Presuming you’re running an air filter (and to avoid this kind of damage is why we do!) then carefully inspect the full intake track looking for any more debris and try to see how something could get in past the filter, is it seated correctly? Or torn?

If you decided it might be pre-ignition or detonation then you need to carefully check that the timing isn’t over advanced, that the carb isn’t running lean and that there is not other reason for the engine to get hot, such as a damaged cooling fan or stuck thermostat.

Lots to think about! As I wrote in my own recent engine autopsy post, it’s always interesting trying to follow the logical train of events that lead to the failure, but not fun when it’s your own engine/money that is sitting there looking worse for wear! Hope you can get it sorted soon and get back out on the road

 

[Toshi 975]

Have you got any back up spare parts with the car? I would guess that you are in the same sort of position as the guys in the US with regard to spares.

 

[Kiltedboarder]

My job is investigating large industrial gas engines that fail. But none the less an engine is an engine is an engine

Does not look like a ring failure as would be odd for both cylinders to be affected

Since both cylinders are affected I would rule out FOD

pre or post combustion would seem to be the most obvious root cause

Looks like you have a big job to rebuild that so sorry but it’s also a great project to tackle

 

[Toshi 975]

My job is investigating large industrial gas engines that fail. But none the less an engine is an engine is an engine

Does not look like a ring failure as would be odd for both cylinders to be affected

Since both cylinders are affected I would rule out FOD

pre or post combustion would seem to be the most obvious root cause

Looks like you have a big job to rebuild that so sorry but it’s also a great project to tackle

That job must be quite challenging but most interesting.
Actually I have experienced the foreign object scenario myself on a tuned 650 engine. All my own stupid fault, I was running a Weber 30DIC twin choke downdraught carb and I had secured the rocker cover vent pipe to the air filter wit a circlip inside the filter. Whilst changing the jets I saw that the circlip was missing and there was only one way it could go, sucked into the carb. I had heard a kind of screeching noise when the engine had been running so it meant another strip down on a virtually new engine. Of course although the circlip was a relatively small object it is made of hardened spring steel so not the best thing to have floating about in there. The findings were that the circlip had entered one of the bores and been broken into bits, there was minor damage to the piston crown and the combustion chamber but fortunately the bores and piston rings were intact. Any fragments of the circlip had been blown out with the exhaust but the thing that surprised me was that some fragments had obviously blown back past the inlet valve then into the other bore causing similar but more minor damage.

 

[Bounding Bambino]

After looking at your photos, it definitely looks like foreign matter ingress. Assuming you have stock intake, carb, air filter assembly. The only real source of a potential metal part is potentially the throttle plate screws. I cant remember if they are brass or steel, but the item that the engine sucked in was definitely a harder steel, and there was more then 1 pc.

The intake valve on the head (rear most cyl), looks to have something stuck in between the valve and seat, and the valve is not fully seated. See if there are remaining bits which might help identify.

Your best course of action (if its a stock 499cc), is upgrade the engine with new cyl and pistons to a 540cc kit. that should be bolt in and go (no machining). As for the head. A bit of work with a die grinder to help smooth down the rough dimpling on the head will help reduce pre-ignition. And a slight skim of the head should be all it needs. Obviously the head should be fully torn down to check where the metal fragments came from, and if the seats/valves are still in serviceable condition.

Personally i dont have many stock parts here in Canada, but have a few resources if you get into a pinch.

 

[ronstox]

Thanks for all the input. It looks like I have a bigger job ahead of me than I originally planned. Perhaps I've found myself a new winter project.
First I'll answer the questions:
1. The engine is a stock 499cc with the 26IMB10 carb and a 123 distributor and have timed it statically, in accordance with instructions. .
2. I have no spare parts. Like Bounding Bambino, I have to order mine from EU/UK/USA. The 540cc kit sounds interesting, but I'll put off that decision until I've had a chance to strip down, clean and inspect the rest of the engine.
3. It seemed to run well enough, but I did notice tiny black bits (I assumed was carbon) coming out of the exhaust and what I felt was excessive carbon on the plugs. I assumed I was running too rich, so I leaned out the carb as much as I could while maintaining RPM.
4. My thermostat is stuck in a partially open position. Maybe 1.5 cm. I intent to sort that out during this rebuild.
5. There is a filter properly in place, but in the past I have lost one of the clips to the lid of the airbox and found the filter rolling around in there. It was fixed after only a few miles.

I will follow Goldrust's plan as closely as I can with Mr Haynes and this forum as a guide.
I'll keep you posted.

 

[bugsymike]

Some photos of severe detonation can look similar. What were the any indications prior to failure?

 

[the hobbler]

Some photos of severe detonation can look similar. What were the any indications prior to failure?

Not meant as a silly question---what fuel are you using? Ethanol contains water, and if a 'high-ethanol-content' fuel is being used and no 'anti-ethanol' gunge being put in with the fuel, the outcome CAN look like water has gone through the head. I was using what I had been assured was a 'low-ethano fuel that would not need additive' in my engine (before the rebore and the new head fitted), so I didn't put an 'ethanol-protection' additive in and the combustion chambers had hundreds of little 'dimples'in them, as if I HAD put water through with the fuel. Although the fuel companies are stinging us at the moment, I ONLY use the "Esso Super +99 octane" fuel in my car and bugger the cost. The 500/126 engine might be a relatively simple engine, but modern 'high-ethanol-content' fuel wreaks havoc with them. ALWAYS use the very best fuel that you can lay your hands on, and if in ANY doubt, also use one of the propiatry additives.

 

[Bleeding Knuckles]

Hi Ron, I'm afraid that I will have to come down on the "Pre ignition and Ethanol" answers. If this was a case of FOD you would have evidence of impact damage all over the interior of the head not located in one place. Bad pre ignition starts by making small 'nibbles' into the head around the periphery of the bore edge, the longer this goes on the worse it gets and the actual pre ignition detonation effects the squish area in the head. If left the end result I believe is what you are looking at, the observation of debris holding open the number two intake valve is valid, but looking closer there is a gap between the valve seat and the head material - this points to poor machining and fitting of the seat. Personally I would bite the bullet and have the head fully cleaned prior to measuring to see if machining can save it, otherwise can it and replace.
Ian.

 

[Kiltedboarder]

Agree with the last comment FOD would make a lot more mess and damage throughout. So pre ignition is still my best guess

 

[Goldnrust]

3. It seemed to run well enough, but I did notice tiny black bits (I assumed was carbon) coming out of the exhaust and what I felt was excessive carbon on the plugs. I assumed I was running too rich, so I leaned out the carb as much as I could while maintaining RPM.

This is interesting, and could be a potential smoking gun. Certainly there is a fair amount of of carbon in your engine, it's possible it had been run rich or been burning oil with a previous owner allowing the carbon to build up.

If the carbon becomes loose or flakey then these carbon flakes inside the engine can get red hot in the heat of combustion (carbon stuck securely to the top of the piston will be cooled by the piston, but small free floating flakes or bits standing up production of the piston top wont be cooled in the same way), and if they're still glowing when the the next combustion cycle starts they can set fire to the fresh fuel and air long before the spark plug would. This is pre-ignition and can do the sort of damage you're seeing there.

In this case leaning the carb out may not have helped matters (hindsight is 20;20 here, it sounded like a reasonable choice to make) as a lean mixture burns hotter and so may have been the straw that broke the camels back. Have you done many miles on those spark plugs, the insulators do look rather pale/lean?

 

[Bounding Bambino]

Run a rare earth magnet over the damaged areas (head & piston) to see if there are any metal bits still embedded/left. Assuming that they are steel/magnetic. Based on the sharpness of the damage, aluminum on aluminum impact wouldn't be as sharp. Would also be worth it to pull the cylinders and see if by chance any rings are broken, and magically made their way up into the cylinder cavity. Any bad scoring on the cyl walls to suggest ring damage?

Im still in my belief this is ingress damage. The fact that the damage is mainly contained in the cylinder squish area, but if you look closely you can see impact spots all over in the head chamber around the valves also. Something larger then the squish area was in there, but not big enough to cause valve interference with the head. Damage looks identical to damage ive seen in 2strokes when they decide to call it quits, and clearances are tight.

Pre-ignition can be a concern, however these engines are not very high compression to begin with. Yes being air cooled, and seeing you potentially have a cooling issue might have potentially caused it to happen with elevated engine temps. I would assume you were most likely tuning/adjusting the engine without load (parked, working on from the engine bay), which makes it very unlikely scenario for pre-ignition, which typically happens under heavy load.

Anyways, looking forward to what you might find during teardown.

 

[g8rpi]

+1 for pre-ignition "knocking" . FOD would not be all in the same area on both cylinders. When you put it back together make sure the timing is correct. I would however suspect the fuel. The "regular" grade fuel 87 AKI octane is only 91 RON (UK/EU) octane. You should relaly be running 95 RON so north american premium 91 AKI. However all Canadian fuel contains ethanol so I'd go for the lowest grade at 90 or higher that is E5 (5% ethanol).

 

[AndyRKett]

There is quite a big chunk of one of those pistons missing which may account for some of the damage. The "FOD" coming from within and causing the damage rather than something external being sucked in.

The damage is going to concentrate where the dome of the cylinder head is lowest, I,e less clearance between the top of the piston and the bottom of the cylinder head. in those areas it will pinch what ever is in there. where there is more clearance then the damage won't appear the same, but there are still signs of little marks and dents in the higher portions of the cylinder head?

 

[ronstox]

My own feelings on the FOD vs Pre-ignition debate: For all the reasons previously stated, I agree that there is significant pre-ignition detonation going on in both cylinders. But there is also a large chunk of the piston in #2 that is missing. So, my current theory is that I ran the engine too lean on low octane fuel and a faulty thermostat which lead over-heating which lead to pre-ignition damage, and I just kept driving it (My philosophy about this car is that if it starts, then it's drive-able.) The damage built up until there was enough to loosen a chunk of piston. This dropped my #2 compression down to almost zero, and that was the end. There was little FOD damage from that chunk because I was not able to start the whole engine after that. I was able to start the engine on #1 cylinder alone with the plug out of #2. (What a great little engine!) But I wasn't able to find the chunk, so maybe it did blow out of the exhaust? maybe?
Are the pistons Aluminium? Would the chunk be picked up by a magnet?

Ok, more answers to questions:
Toshi & g8rpi: I'm using regular unleaded from Irving Oil (a local petrol station). I called the company and they assure me that there's no ethanol in any of their unleaded gasoline. The octane levels here are: Regular = 87; Plus = 89; Supreme = 91; AKI, I believe. Once I get this engine back on the road, I'll switch to the higher octane gasoline.
Bleeding Knuckles: My original plan was to replace/re-seat the valves and clean up the head. I'll still do that, and now, the pistons/cylinders as well.
Goldenrust: There are some carbon flakes laying on the piston crown. There's an especially big one on #1 (Left-hand side). I've scraped a bit on the piston crown and the carbon is soft and thick and comes off easily. This would support your comments, I think. As part of my initial trouble shooting of the engine, I changed the plugs and I'm not sure if I kept the old ones. If they show up, I'll post a photo.
Bounding Bambino: You are correct, all my tuning/adjusting of the engine was done without load while parked in my driveway.
Toshi: I'll PM you to see what parts you might have in your cupboard that I might use.

 

[ronstox]

News Flash!
I found the FOD!
Following Bounding Bambino's suggestion, I ran a magnet around the head & pistons, and the stuff I'd cleaned up already and voila! It looks like a piece of piston ring.
Also, I found the old plugs. not sure which came from which cylinder though.

 

[AndyRKett]

News Flash!
I found the FOD!
Following Bounding Bambino's suggestion, I ran a magnet around the head & pistons, and the stuff I'd cleaned up already and voila! It looks like a piece of piston ring.
Also, I found the old plugs. not sure which came from which cylinder though.

looks like its been beaten nice and smooth by its time rattling around in the cylinder.

What's your plan. Are bits for these easy to come by where you are ?
Perfect opportunity to make some "improvement" to give a little boost in performance.

 

[g8rpi]

I think you are correct, the bit of piston ring is effect, not root cause. I thought Ethnol was mandated in most if not all of Canada.

https://www.canada.ca/en/environment-climate-change/services/managing-pollution/energy-production/fuel-regulations/clean-fuel-regulations/about.html

Robert.