piston chips?

[jwspin]

the friend that i posted about the other day who had a runaway motor due to the turbo seal has some other probs. i went down to the shop today and checked his motor out. he has cracks between the valves on each cylinder, which seems kindof typical around here. he also has carcks on both sides of the port which leads from the prechamber to the cylinder, and his number 2 and 3 pistons have lost some metal at the front edge where the gasses first hit the pistons. does anyone have any opinions on diagnosis other than running the motor way too hot. it looks like the pistons have been worn for a while it doesnt seem like it was just from his last incident.
i also had another question. has anyone ground off the sharp ridges on their pistons to eliminate the hot spots? i was taught buy an old hot rodder to take away all of the sharp ridges in my gasser heads when porting to eliminate hot spots and decrease the potential for detonation. could this apply to diesels?
my friend figures he is just going to leave the bottom end as is and run it untill it dies. i figure if we can "fix" his pistons up a little bit then maybe he can get it to last a little longer???

-jared

[Master ACiD]

are the piston tops actually chipped or are they melted?

[jwspin]

melted. i guess i meant a significant chunk is missing by "chip" but it is very local on the piston and its the same for each one.

-jared

[wyldman]

Once the piston starts to burn,it will continue.

While removing sharp spots on most things will help,it's not recommended on diesel pistons.You want to keep the flame front up top,on the piston,If it starts to get around the outside,the piston will burn quickly,and it's real hard on the rings.

If he's just going to run it into the ground,then it may last for a while if you keep the fuel down,and don't drive it hard.

[Master ACiD]

i would guess that it was from excessive temps caused by the oil in the intake acting as a fuel. diesels cant have fueling before the top of the stroke because of detonation.  i wouldnt run a block with damaged pistons like that if there was any way to avoid it.  if he has to do it because he cant afford to fix it or has absolutely no time then hey i understand that you gotta do what you gotta do to get by. sence they got to hot as to actually melt out a section of the front piston crown then there is most likely ring damage as well.  having a cracked or scored or partly melted ring would do alot of damage to a cylinder. even if the rings are ok, losing piston crown means the rings in that area will be subject to excessive combustion tempatures and carbon/soot and will shorten their life in a hurry. i doubt the engine would last very long in this condition.

i would suggest at this point to atleast tear it down and install new pistons. if he is very cheap (which i believe him to be sence he was considering running it like it is). then at the very least lightly hone the cylinders and install some cheap normally aspirated pistons and rings. you can trim the squirter slots with a dremel.  then turn down the boost to a couple psi and he should be able to get by OK for a good ammount of time while also saving his cylinders from certain doom caused by cracked rings etching out their names into the cylinder walls. a partial rebuild can usually be done with the engine in the car and shouldnt cost more than 200 or 300 american dollars. i have done block_in_the_car_ rebuilds in a1 vw's before.

i had a dirtbike one time that has this problem, albeit caused by something different but none the less the piston crown by the inlet melted partly exposing the upper ring. the engine had good compression even after the melt down but a days later it seized from a cracked ring and scored the cyliner so bad it was beyond servicable.

[bhtooefr]

Tell him to get a $300 Geo Metro and run THAT into the ground.

[fspGTD]

I have seen localized heat damage like this before on really old and abused VW diesels.  IE: cracked prechambers, or cracked head around the prechambers, pistons starting to melt, etc.  I would add poorly maintained fuel injectors to the list of possible causes.

One or more eroded glow plug tips would be a dead giveaway that the fuel injectors are either dribbling, or are pissing a stream of fuel, rather than a finely atomized mist.

Injectors should be checked if not rebuilt routinely using a pop tester to verify the spray pattern is good, the tips don't leak, and the breaking pressures are within specs.  Hope this helps!

[jwspin]

good comments, just to clear things up, he isnt cheap. it is just that his jetta is like a geo metro to him. nothin special just an economical ride to work. he has a whole garage full of vintage muscle cars, corvettes, rare 4x4s, but he does like his shi*box jetta. i think ill convince him to put some pistons in or sell me the block and ill do it. his head is pretty bad. he sent it off to his machinist to see what his opinion was. maybe he could weld the cracks up a little... who knows.
another question? i believe the prechambers are replacable right? how do you pull em out.

-jared

ps he is def doing some nice new injectors too....