I have a 2001 standard cab automatic with the 2.5 4 cyl. It only had 135,000ish miles. I changed plugs last year because of poor performance and check engine code that was something like error at bank one or something like that. The mechanic said it could be couple different things, plugs or MAF or worse. put medium grade plugs and new wires in around August 2014(which isn't hard except the two on the right towards the cab, hard to get at and the fact that you got 8 plugs and wires equaling around $100) Noticed the two on the right towards the radiator were especially fouled. Truck ran better then started having check engine lights a few hundred miles later again pertaining to same error. I let it go for a little while(mistake most likely but doing road construction just never had time)driving it through its little stalls were I'd have pedal down and no power then let off pedal and then press pedal and it would go. After an oil change(which I always do at 3 k miles) I decided it may be throttle sensor but more likely the MAF sensor and decided to clean it with MAF cleaner before buying a 130 $ dollar sensor. Truck ran better than ever. Then a month and maybe 5,000 miles later while running about 70 mph a loud pop and then very loud popping/cracking occured but I drove it off the interstate a few miles slowly. First thing I check was plugs, sure enough the second to front plug on right pulled right up when I pulled the plug wire. Only the porcelain part(I mean the top of plug pulled off with wire, crazy), the metal thread part of plug remained in engine block. So I went and got new plugs and wires to see if i could get it drivable cause I was 200 miles from home and didn't need a tow bill of that magnitude and changed the dang things again(less than a year since last time)this time in a nasty motel parking lot instead of my dad's comfy accommodated garage using a ratchet set I bought at wal mart and then returned afterwords not needing them at home. Well when I got that other half of the plug out(which threaded right out, I was afraid I'd have to drill it and extract it and re tap engine block threads but no)it was hollow, the inside of the plug went into cylinder and caused major damage(bits of small chunks and shavings present inside that cyl). Changing plugs and wires made it sound better but only drivable enough to load on trailer. Obviously the motor is shot(i.e. Metal shavings and all)or needs rebuilt but what caused that plug to get so hot that it melted and came apart like that. The other plug I mentioned being the front right one was very dark and burnt looking as well just like when I changed them a year ago. Any ideas why this happened? Everyone I talk to says they never seen a plug come apart like that and get in the cylinder.

 

Maybe cleaning the MAF sensor(or more likely not doing it sooner), But it improved performance 10 fold, and I used the correct MAF cleaner and allowed extra time to dry filaments even though the cleaner evaporates instantaneously, and also the way those two plugs looked like they had been cooking for a while like months not just the couple weeks since I cleaned the MAF?? I don't know Ranger gurus give me your input, I know the motor is done but just have to know what happened for future.

 

2001 2.5l should be a Lima engine, last year of that engine, 2.3l Duratec was used in Rangers after that.

2.0l/2.3l and 2.5l Limas were in Rangers from 1983 to 2001 and they were very reliable engines.

I have seen spark plugs break, not common but does happen, and that wouldn't cause metal shavings in the oil.
The bits of spark plug end will bounce around causing dents in piston and cylinder dome but will generally be cleared out the exhaust valve port.

After you get the head off check if valve is bent or chipped, a chipped valve may have cause spark plug end to be hit and broken.
And with head off rotate crank and check each pistons height at TDC, the #2 piston especially, it could have bent Rod from debris in cylinder so won't come up all the way like the other 3 pistons.