Piston Break

[PeteA216]

Hey guys, My buddy's got an '79 caprice w/ a 305 he just bought with 79,000 original miles. It had no power, but ran GREAT. Turned out the cam was worn. He pulled the motor to put a new cam in, and just for sh*ts and grins he dropped the pan to check things out. The thing's CLEAN, and I mean really clean. Almost no sludge at all on the bottom. Not metallic particlals. But, there was one thing. A chunck of metal about the size of a dime. Well, it turns out that a piece of one of the walls on a piston broke off. The part of the piston thats lowest, kinda protrudes out... in the # 5 cylinder. The cylinder wall looks and feels flawless, the motor ran fine. How bad would it be to just leave it alone and run it like it is, this dude isn't exactly made of money right now. I'm also kind of curious as to what would cause that kind of thing. I was almost thinking severe detonation, but when it ran, it seemed fine, and I didn't hear anything even remotely close to a ping. What do yah guys think?

[silicon212]

Well, it can PROBABLY live if he doesn't push it. I ran a motor that way once (incidentally, a 305 btw) for about 30k miles - no harm, no foul. However, that part of the piston skirt is there to keep the piston square within the bore, so it's not a great idea to keep it running that way.

[PeteA216]

Ha, he's on the phone with me now asking me questions left and right. "Can I try to weld the piece back on?" "How did it happen?" "If I get on it what could happen?" "Can I replace just one piston?... probably not without major work."

My guess is, welding it is out of the question. I told him that I kinda figured some mazzive piston slap caused the break, but then again I've never seen this before. I don't see why he couldn't replace just one pistion, but if he's gonna take everything apart, he might as well go through the whole thing.

[silicon212]

It was probably caused by detonation.

It cannot be welded. A new piston would be the best course of action, but will require removing the pan and the affected bank's cylinder head. It's some serious work to do while the engine's in the car, but it can be done. New piston and new rings - but ONLY do this if there is no top cylinder ridge (indicating cylinder wear - taper). If there is a ridge, you will have to remove it and reuse the original rings provided they're not broken.

The broken piece, depending on how it broke, could cause a stress point on the piston which is where a crack will develop. This will cause an eventual grenade effect, resulting in a dead engine.

If he already has the pan off, it can't hurt to pull the head off. You likely already have that exhaust manifold off as it is.

[PeteA216]

Grenade effect? Like the engine blows up... literally? If a small file was used to clean up the break could it remove that stress point for a potential fracture? I know my dad used to de-bur the holes in flywheels/flex plates to remove any weak points to help avoid any cracking. I tried calling this guy back, his phone's off though.

[Blue Bowtie]

Actually, it can be welded. The piston would be removed, jigged in a sand bath, the piece fit up, then TIG welded. Then the whole thing would be machined smooth and balanced back close to original mass.

In short, it will be cheaper, easier, and faster simply to replace the piston. Since the pan has evidently already been removed, the head on that side would also have to be removed. Remove the rod cap nuts, then push the piston up ad out of the bore (getting past the ridge could be fun, but you don't care about breaking rings on that piston). Get a new standard piston, new set of rings, and new bearing shells for that rod. Heat the rod small end and press the pin from the piston, then assemble it to the new piston. Install the rings, oil it up, compress the rings and install the piston/rod assembly.

Of course, and cylinder damage discovered in the process would indicate further work, as well as any journal damage on the crank.

[PeteA216]

Yeah, I'm gonna try to convince Dan to replace the piston. He's kind of set on the idea of simply cleaning up the break. He's telling me to ask what the worst and best case scenario is if he does this. I think his biggest fear is opening up pendora's box by changing the piston. Personally if it were me, I'd use this as an excuse to get a 350.

[silicon212]

Originally Posted by Blue Bowtie

Actually, it can be welded. The piston would be removed, jigged in a sand bath, the piece fit up, then TIG welded. Then the whole thing would be machined smooth and balanced back close to original mass.

In short, it will be cheaper, easier, and faster simply to replace the piston. Since the pan has evidently already been removed, the head on that side would also have to be removed. Remove the rod cap nuts, then push the piston up ad out of the bore (getting past the ridge could be fun, but you don't care about breaking rings on that piston). Get a new standard piston, new set of rings, and new bearing shells for that rod. Heat the rod small end and press the pin from the piston, then assemble it to the new piston. Install the rings, oil it up, compress the rings and install the piston/rod assembly.

Of course, and cylinder damage discovered in the process would indicate further work, as well as any journal damage on the crank.

And don't forget the pieces of hose over the rod bolts to prevent them from nicking the crank on the way out or in!

[PeteA216]

Well, He's convinced. We pulled the head yesterday and today after work the piston and rod will come out.

[bobss396]

A new piston is the way to go, I'm just not sure if he can buy a ring set for just one piston. It may pay to re-ring the whole thing while he has it apart. But read up on fitting rings to the cylinder, the ends have to be filed so they won't butt together after it warms up. Look at the bearings closely too, if the faces are not scored or marked up, leave them in the rods and oil them up for reassembly. The best thing to clean off a crankshaft is a woman's sanitary pad and some solvent, not a joke, they work great.

Bob