If it were mine, I would replace the defective piston, reuse the rings , and make a small notch to prevent it from happening again. I would also make a similar notch on every piston skirt to help preserve the balance. I personally would not step up to forged because of this problem. Any piston will break when something hits it.
My solution also requires pulling and completely disassembling the motor, but that would allow for thorough cleaning of debris and closer inspection.
That would be my solution, others may disagree.

 

I don't know jack, but it seems to me anything less than what Pete said, and then driving it until Sept. Oct, could result in really bad results. Or not. But that would be sheer luck.

 

My vote is no. It can move back and forth along the crankshaft axis,
but not 0.040"+.

Why? Because the piston is fitted to the cyl at 0.002" and it is
supported by the wrist pin, rod and crank journal. While operating,
the piston clearance will close up somewhat due to thermal expansion.
While there may be more than 0.002 movement possible down at the
bottom of the skirt (in the direction of the crank axis) once everything
is summed up, I still say this will be well under 0.040".

But.

Remember that there is also crankshaft end play. I haven't looked up
what is recommended for a SBC. However, for our purposes here, I
feel safe in guessing that it is somewhere in the range of
0.002" - 0.008" (please check the actual specs, yourself).

Thirdly.

Pistons are designed to rock around the wrist pin. In this direction
(at 90º to the crank axis), the rocking action means the piston skirt
is free to travel over a wider range of movement than it is side-to-side.
How much movement? Don't know, never remember needing to check it.

The reason I mention the rocking action is I wonder whether this
motion, along with crank end play and piston side movement, could
have led the skirt to contact the crank counter weight?

If so, and if this hadn't been taken into consideration then the
clearances might have looked tight but acceptable during assembly
only to lead to contact during operation. It wouldn't take much
contact, I believe.

Understand that I have no experience with strokers and I don't have
an SBC short block to look at to see whether what I am suggesting is
possible.

.

 

Thanks,
What is make me nervous (forever...) is that I will never know the cause of this failure.
Probably a defective piston,
probably a clearance problem,
probably both.
I will never know
-Beppe-

 

Pistons are almost never defective. Don't be afraid of the kb piston. They have a proven track record.

 

I doubt its either, KB can handle a thou and a half to 3 thou clearance, they are a better piece than people give them credit for.

 

Beppe, I apologize to you and all for my earlier, wet-eared comments. The guys that are helping you here really know their stuff. I know just barely enough to be awfully dangerous, and should keep my mouth shut.

But I do feel rather strongly about this......what Pete K is post #41 saying about disassembly, a thorough check, it seems to me would be advice I would follow. I would not drive it without full disassembly. I know its hard, after all the wait, and here there's more downtime. Here comes summer too. I, we, all feel your pain there is no question about it. However now that you know there's a problem, the risk is not worth the reward of continued use by just throwing another #8 piston in.

 

When a piston is defective, I don't believe it shows this by shedding
the tip of the skirt on the side closest to the counter weights. In
other words, my vote is that this occured because of a clearance
issue.

On the subject of what to do now. Consider the words of our friend,
Detective Harry Callahan:
If you can afford to lose the majority of the engine, then try driving the
car as it is for the rest of the season. I wouldn't go on any
cross-continental tours or on laps of the Ring but I think that there
is a possiblity that the break at the bottom of the skirt is a clean
one and that the piston isn't cracked. If the thrust side is still good
then I don't think the cyl walls will be hurt any worse than what you
can see now.

The engine could live happily under mild conditions for some time this
way. It might give advance warning that it is becoming unhappy by
gradually developing more piston slap noise.

Or not.

And if the piston failed catastrophically, it would likely take out the
rod and then the wall and so on and so on ...

Some people can afford to replace an engine and would take the
odds in order to get out and have some fun. Other's can't or won't
do this. You are the best judge of your circumstances.

Harry was out of rounds, IIRC.

.

 

The theory is; torquing the head in place distorts the cylinder bores.
By using the torque plate, the cylinder bores are distorted during machining and are then machined true.
This means that the bare block now has distorted, machined bores in its free state, BUT when the head is torqued in place the bores will be pulled back into perfect roundness.

That's the theory, anyway. As for myself, I doubt that the distortion in the block is significant enough to worry about...maybe a few ten-thousandths. Maybe. I believe distortion from full throttle power probably is more than any from head bolt torque (where the stresses are aligned axially with the cylinder bores.)
One thing is unarguable: using a torque plate sure won't hurt anything!

Larry
code5coupe

 

I think the arrival of the Sunnen Dial Bore indicator led to increased
awareness about cylinder shape.

While it is probably not common shop procedure for your block or mine,
some people go to the trouble of plumbing the blocks for hot water
while these are being machined.

When rules or other constraints limit what can be done to gain an
advantage over the competition, one uses whatever means is at one's
disposal to get that last bit of performance/durability/fuel economy.
If my machinist is better than yours, I win.

.

 

silly question... if one piston broke, why not the other 7?

I mean, aren't most crank throws about the same size? Or does all get into the esoteric science of engine timing, and I am just confusing myself....

 

Not a silly question.

This is a Crower unit but it still serves to illustrate the general
idea



You can see that the width of the weights varies and also the 'clocking'
of their leading/trailing edges on a given throw (not to mention across
different throws.)

.

 

I only had one on the last stroker I built. I notched them all the same though.

 

I can't afford a new engine now. for sure. the problem are $$$!
BUT what I hate is to spend 1200-1500 only to remove the engine and to reinstall ONLY to change one single piston..without upgrading it.
I will end spending about $2000 only to have the same engine as now and be always afraid to have a failure as now...because I can't be 100% sure about the clearance issue.
I really prefer spending 3000 (that I havent... ) knowing to have a new UPGRADED engine.
So, HOPING to have the engine working up to september or october in order to HOPE (again) to have the right amount of $$ to perform an upgrade.
And now...which upgrade?
I will NOT have problem to sell my actual rotating assembly to someone else here in Italy after the replacement of the cracked piston and supplying new rings....recovering some cash to use in the new rebuild.
A new balanced rotating assembly with forged parts?
A new bore-stroke combination using the same block? which possibilities on this??
Could I go up to .040 .050 .060 overbore with the stock block?
I need a new combo wher I have NOT to put in the trash all the parts i'm using now.
It seems I'm in search of the ideas to improve the output of my engine....
-Beppe-
P.S. It's really absolutely impossible to put a new piston from UNDER the car???
Im speaking of removing the rod cap then the rod with attached wristpin and piston...
I read somwhere doing a complete REring job without removing the engine...

 

I replaced 2 pistons with the engine in the corvette. I removed the heads and the oilpan.