Piston broken in half(top-bottom) 5

[dirk489]

I reconditioned a friend's engine which broke later on during running in.The piston broke just on the pin line,conrod bolts snapped clean off, smashed the whole block.
My question is: is this always due to overreving or is there another possibility?
Dirk/Antwerp

 

[Panther140]

What is your theory for those asymmetric markings on the wrist pin bores?

"Formal education is a weapon, whose effect depends on who holds it in his hands and at whom it is aimed." ~ Joseph Stalin

 

[Panther140]

If the writs pin slid over, the tension on the piston would stay the same, but be much more focused on the aluminum surrounding the inside half of the bore. So each side of the piston would still get 50% of the tension, but the right side would only be using 25% of the strength it could be using. Then that side breaks and gives the other side 100% of the tension which would make it fracture more dramatically



"Formal education is a weapon, whose effect depends on who holds it in his hands and at whom it is aimed." ~ Joseph Stalin

 

[LionelHutz]

With the claim the retaining clips were in place and with the wrist pin not contacting the bore, it makes one wonder if the wrist pins were simply too short.

 

[dirk489]

I went to my garage for another check on the block:there are no scratches or marks at all on the cyl. wall,exept honing marks.
As for the asymmetrical marks:the owner tried to restart the engine to see what was wrong;there was a lot of debris that could have been touching.Most of the bottom half of the piston was thrown out trough the big hole...Piston pin was OEM new,looks right length to me.
Dirk

 

[Panther140]

have you contacted the piston manufacturer? Do any of the other pistons have wear marks in the writs pin bores even between the clips?

"Formal education is a weapon, whose effect depends on who holds it in his hands and at whom it is aimed." ~ Joseph Stalin

 

[dirk489]

Piston is Hepolite,made in 1958 and NOS. Difficult to make a claim...
No marks on the other pin bores,I measured the bore with a tripod mike, no deviations,wrist pins are quite heavy,very little movement axially possible even without circlips.

 

[Tmoose]

Are the pistons marked "Heplex" on the inside?

 

[dirk489]

Hi Tmoose,
Pistons are marked in the four corners:
Hepolite
AHH402
641
AEHF59

 

[Panther140]

have any of the other pistons been sent out to be analyzed? A brand new piston should not abruptly fail under any RPM that the engine can rev to on its own

"Formal education is a weapon, whose effect depends on who holds it in his hands and at whom it is aimed." ~ Joseph Stalin

 

[tbuelna]

Based on the amount of damage shown in the photos to the block/conrods/pistons/etc, I can't imagine the engine is worth salvaging, unless it is an extremely rare model. Fixing the massive hole in the side of the block would require extensive weld repair and re-machining of all close tolerance features. The picture of the block appears to show the cylinder bores have already been over-bored and sleeved. And given how badly the conrod was deformed, it seems likely the crankshaft was also bent out of shape.

While this is an interesting topic of discussion, I think the only course of action is to find serviceable replacement parts.

 

[Panther140]

tbuelna we are focused on finding a root cause for this failure! We need resolution of some sort. Mail me the parts please

"Formal education is a weapon, whose effect depends on who holds it in his hands and at whom it is aimed." ~ Joseph Stalin

 

[dirk489]

Just wondering,as the rod is angled,and the piston broke first,whether there could not be shear implicated.
When the rod is blocked there would be side pressure on the cap lips as well.They are not designed to take those high loads and could move a bit sideways to create shear on the rod bolts.I will examine the cap's lips again.
I talked to some other guys and the piston breakage on those older engines is not uncomman when overreving.

Dirk

 

[chief]

i have experienced this problem. Root cause was casting sand which had congealed and remained in the piston internally. Worked loose and cause grinding effect on liner , eventually seizing piston on TDC and separating at gudgeon pin area.


Offshore Engineering&Design

 

[dicer]

What motor oil was used?
And how about the wrist pin? Any pics of it?
If it was going to be hot rodded should have used some good forged pistons. Is that an inclusion right hand upper
wrist pin hole? I think it was a bad material-casting problem.

 

[dirk489]

Hi all,

Dicer: 20/50 oil,no trace of the wrist pin. Might have thrown out with the debris of the underside of the piston.

I have some pictures of a similar engine desaster,also on cyL nr.2.
Rods are Phoenix,bolts ARP 2000 at 45 lbs.ft. Pistons were a lot lighter(Triumph Bonneville Powermax) and also broke top/bottom in two.
Rod being 1/2" longer went in the waterways.Happened at about 9000 revs.

 

[dicer]

In both cases were the pistons cast? What kind of load were these engines under when this happened? 9K free rev?

 

[dirk489]

Both pistons cast.The 9000 one I did myself.... Thought I was in 4th gear but was in 3rd.With the longer and lighter rods the engine is much smoother.I noticed something was wrong when I saw the rev needle pass the ignition light......After that it was silence.

The first engine was in 1st gear as he was driving away from the lights and trying to pass left before another arriving car.
The MGA Twin cam was known for easy overrevving .
Dirk

 

[FredRosse]

"The 55 lb/ft installation torque you note seems high for a 3/8-24 UNF bolt having 110 ksi UTS, especially if the threads were lubed at assy. You might want to do a quick calculation to check if the combined tension and torsion forces at installation would be enough to overstress these bolts. "

I put this torque (55 LBf-ft onto 3/8-24 stud/nut) in my thread stress software, inputting nut to base friction coef = .18, nut to stud threads coef = 0.18, (lubricated threads and nut seat)comes up with stud princ. stress = 136ksi! Not lubed, friction coef = 0.3, gives less than 100ksi stress.

 

[dirk489]

To FredRosse:I mentioned 45 ,lubed as given by manufacturer of rods,and I think ARP as well.I have a racer with Cunningham rods,same spec.
Dirk

 

[jimwolf]

Your close up of the failed bolts clearly shows that one of them has the tell tale "beach marks" indicating that it failed over more than one cycle and not catastrophic like the other. That bolt fracture went catastrophic after about 3 cycles based on the beach marks and the other bolt is a pure catastrophic tension break, likely from the rod cap leveraging it in tension after the first bolt broke. The first bolt that broke also has a pulled thread and a slight lean opposite the failure origin (center OD of 1st beach mark) indicating that the cap was bending about the remaining bolt, causing the extreme tension that failed the second bolt catastrophically. BUT, pictures alone can be a bit deceptive.