Piston failure on '94 750 Casting #8AX

[bmxmike90]

Hey everyone, I've got a 94 Vmax 750 casting #8AX. Totally stock, and well maintained (For the 3 years I've had it anyways...)
Had a piston failure, and I'm curious if anybody else has come across this, and has a clue as to what causes this particular failure.

The details:

Cylinder #3, The pin that holds the top ring from rotating actually came loose, and began to wear the bore in which it sits. Eventually it allowed the ring to rotate enough that the edge of the ring got caught on the exhaust port & lead to the catastrophic failure.

I pulled apart the remaining cylinders and found that Cyl #2 was not far from failure as well. Holding the piston in my hand, I was able to drop the pin out into my hand.

Anybody had this before?

What are the likely causes?

Thanks!

P.S - I'll upload some pictures of the damage to help you see and understand what I'm talking about.

 

[toydoc]

Take the ring off piston 2 (good piston), put ring back in cyl 2. Check ring gap at: TDC, center and just under the ports

 

[bmxmike90]

Photos are up!!

Take the ring off piston 2 (good piston), put ring back in cyl 2. Check ring gap at: TDC, center and just under the ports

I compared new rings to old rings (just by eye.. didn't measure them yet). Old rings are about 0.030" gap or more.. big enough to fit the end of a small pocket screw driver. New rings are about 0.010". Not sure what tthe spec. is..

Also, in reference to the last photo, What are the holes in the skirt for, and why is the new one smaller than the old?

Thanks!!

 

[bchita]

Boost port. Your former piston, and I imagine all pistons have this size of hole, which has been modified by someone over the life of this machine. The new one is stock size. The bigger hole is the intention to increase HP, which it will, but will need other things with cylinder to take advantage of this port. Looking at the one cylinder picture, doesn't look like port work was done. My experience anywho.... someone can chime in which much more than I have....

 

[toydoc]

Check it with a gauge. The pin cant come out with the right ring end gap. Your rings (and maybe bore, gauge it) are worn enough to let the ring spin.
Or by the looks of the piston in the back ground of the first pic, it ran lean and stuck a ring, then cought the edge.
What does the exhaust side of the other pistons look like?

 

[toydoc]

Also looks like a crack in the face of the old good piston, from the hole all the way down.
High miles on the motor?

 

[bmxmike90]

Check it with a gauge. The pin cant come out with the right ring end gap. Your rings (and maybe bore, gauge it) are worn enough to let the ring spin.

You are correct, however, 2 cylinders were still intact, and the one(s) with failures had pin bores worn out, which allowed the pin to come out or move enough to let the ring rotate.
Basically, I don't believe it was the result of too much ring end gap.
Ring end gap on old rings is +0.030". I don't have an accurate means of measuring larger than 0.030" unfortunately. New rings are 0.024" +/- 0.001".

As for the exhaust side of the pistons, they have scuff marks where it appears to have had carbon (or some kind of debris) caught between it and the cylinder.

Angine has 11 000 kms

 

[toydoc]

Well a crack is not normal if the piston is intact or not. Debris doesn't fall in front of the exhaust face of all the pistons.

If you want to say it was a pin issue and move on, Im good with it. If you want to find and fix anything wrong / worn your going to dig in and sepc out whats worn.

Good luck with your build

 

[bmxmike90]

The pins are a press fit into their bore. Even with too much end gap, the pins wouldn't come loose. YES, the ring could potentially rotate, but the pins wouldn't (and shouldn't) become loose.

As for the scuffing, It appears the engine was over heated at one point in it's life. This would explain the crack between the boost port and the skirt. It COULD also be the underlying cause of the pins coming loose.

I picked up a "reconditioned" jug. A brand new ring in the new jug measures exactly 0.017" end gap. Old ring in new jug measures exactly 0.023" end gap. As far as I'm concerned, this is not enough to cause an engine failure.

Lastly, I stuck an old piston, into an original non-damaged jug with old rings and tried to spin the piston in the bore (which would indicate that the rings could indeed spin). This allowed me to determine that ring end gap was not the culprit of the failure.

I therefore conclude the pin came loose as a result of overheating (weather it be over expansion, or weakened metalurgy.. Doesn't matter), wore it's bore deeper into the piston and allowed the ring to rotate and get caught on the exhaust port.

Thanks for your input everyone!! (Especially you Toydoc!!)