need opinions - engine swallowed a nut

The first picture shows the ding in the piston. All the pistons look hammered to me, looks like something has been in every cylinder which makes sense since all three of the spark plugs were bent. That accounts for the engine running poorly.

Now all I need to find out is what the hell happened...lol

 

If you've had issues with the timing where it was too far advanced it would be pinging, that can beat up the pistons.

 

What probably happend is that the nut boinced back & forth through the intake valves on that side. IIRC the intake manifold has basicly 2 levels one to each bank of cylinders. While running the nut was light enough to be pushed out of one intake valve into the manifold and then into the next cylinder via its intake valve. This is possible because there is a small amount of valve overlap between the exhaust and intake strokes. Thus a small amount of the exhaust gasses push back into the intake manifold until the pressure equalizes due to piston travel (high speeds) or due to enough time passing (at idle). The nut was probably pushed around by these gasses.
I have seen this on other engines. I had a SBC do this because the jam nut holding the air cleaner stud tight lossened up allowing the stud to back out and then the jam nut went trough the engine. We only found this because the engine started to run really rough. When we pulled the spark plugs to do a compression test, all four of the on the left bank were peened closed. Then we noticed the loose air cleaner and missing jam nut. The compression was good, but we pulled the heads to check for damage and found exactly what your pictures show! We had no other piston damage, so we put it back together with new gaskets & sparklers. It ran fine for another six years at least.

 

That may have happened but it seems so unbelievable to happen. I can't think of another explanation and there is only one nut missing. The other side looks fine, no damage at all.

 

Howdy, you should also pull the oil pan and check the rod bearing clearance on that cylinder. Had a 440 swallow a carb stud years back. Chewed it up and spit it out, no fuss, no muss, no knock, no noise of any kind. Some time later it spun the rod bearing on that hole. Am convinced that the shock of hitting a solid object killed the bearing and a nut is harder than a piece of 1/4-20 All Thread. Know it is not what you want to hear, but park it, pull the engine and check it out.

 

I would drop the oil pan and pull that piston with the big chip on the edge just to make sure the top ring groove hasn't been deformed down against the piston ring. A quick check for bent valves on the head is to turn it upside down and fill the combustion chambers with gasoline. If the valves are bent the gas will run right out.

 

That is some good advice, thanks.

 

I can not swear for absolute certain that FOD was what caused my rod journal bearing to fail. It was however the only cylinder where the piston head showed evidence of hitting the carb stud. Am convinced that during engine assembly, the bearing spread should not be removed with a hammer, which is basically what is happening in the running engine.
While checking for bent items, roll your pushrods across a piece of plate glass. Sometimes they are weaker than the valve stems.

 

just my $.02 pull these pistons and check where the wrist pin goes thru the piston for cracks, a sheet metal screw was enough to put the rod thru the cylinder on a 302 ford, came loose at the wrist pin, motor was junk after that. the gm engines seem to have a little more tollerance, heck the ford grenaded @ idle, you drove yours almost home, i am really supprised it didnt grenade

 

I did what you suggested, kaiser_willys and pulled all three pistons on the passenger side. My first sign something was bad was the bits of metal I found in the oil pan . I pulled out the number 2 piston (the one with the visible crunch) and found what you see, a completely destroyed piston.

There was a good chance I was just going to put it back together without checking the pistons and if I had done that I would be looking at a whole new engine. So, I owe you a big thanks!

My next question is; how do I know what piston to buy? Oh, I pulled the other two as well and they are marginal so I should probably do those too. I see no reason to pull the other bank and replace those. Am I right about that?

 

Are there any numbers or identifying marks on the piston?

RSN

 

At this point I think I'd rebuild the engine. That way all pistons will have a closer compression ratio.

...and you've already got it half way apart.

 

have to say i would just buy a set of pistons, you can buy them one at a time but the weight of the pistons and compression ratio may be different, thus resulting in a strange running engine that would be out of ballance both weight wise and compression wise. this is one expensive nut as mentioned earlier. also check the bore on the cylinder with the busted piston for damage. i hate this for you. but have been there twice in my early 20's and the worst part is it was on a customers vehicle. both came out of my pocket, it was my fault and that was the right thing to do

 

Decision time- is this Jeep something that you will keep the rest of your life and thus cost is irrelevant, or is this just another project? These motors are still widely available- I picked up two in the last month carb to pan for $50 each.

 

I may put a new set of pistons in but I am not rebuilding it any further than that.

I understand how expensive that nut was, got it.