What caused this? (Broken Rod)

[Harley PHD]

Hi, I'm new to this forum, and would like to thank you in advance for your help.






The first picture is a hole in the crankcase just in front of where my wife's foot was.  This happened going to work and after 30 miles of 70mph,stoped at trafic light, when pulling away Boom!
The second picture is the broken rod,  looks like fatigue fracture halfway across rod cross section. 
Its from an 1980 V50II with 16,000 miles on it.  We bought it used but have had it for 20 years.
What would cause this, and how common is it?
And what should I do to the donner engine to make it reliable?

[oldbike54]

 Seem to remember you posting this before , apologize if no one even offered up an opinion .

 One of our real experts will come along shortly , but in the meantime . Yes , a stress fracture at the weakest part of the connecting rod , probably just one of those things . It might have been a manufacturing defect , you could have the rod examined at a lab , have them look for a flaw , likely not worth the trouble unless you just have to know .

 What to do to prevent it from happening , well , have the replacement shot peened to remove surface imperfections might help , or if aftermarket rods are available for that engine they might be better .

 Did you notice any unusual vibration before this occurred ? Might have been caused by a wrist pin going bad not allowing the piston to pivot properly .

 Dusty

[Harley PHD]

If its not common I will just leave the rods in the donor engine (1982 V50 Monza) alone.  I was going to grind the beams smooth, but the casting is so rough there wouldn't be anything left.

[oldbike54]

If its not common I will just leave the rods in the donor engine (1982 V50 Monza) alone.  I was going to grind the beams smooth, but the casting is so rough there wouldn't be anything left.

 I really don't know all that much about small blocks , one of our SB experts will weigh in shortly , just working from general knowledge about engines . Shotpeening is a better option than grinding if you are worried .

 Dusty

[mtiberio]

fore aft crankshaft play.

[lucky phil]

Hi, I'm new to this forum, and would like to thank you in advance for your help.






The first picture is a hole in the crankcase just in front of where my wife's foot was.  This happened going to work and after 30 miles of 70mph,stoped at trafic light, when pulling away Boom!
The second picture is the broken rod,  looks like fatigue fracture halfway across rod cross section. 
Its from an 1980 V50II with 16,000 miles on it.  We bought it used but have had it for 20 years.
What would cause this, and how common is it?
And what should I do to the donner engine to make it reliable?

It's hard to be 100% from an image but I suspect the rods been defective. See the Beach marks on the bottom edge of the failure line. If this area has as it looks a smoother texture then thats the classic beach mark or a fatigue failure originating from a surface defect. It generates this effect over a period of time until ultimate failure which generates the other rougher surface from the tension failure.
There is also a section at the top r/h area around where the "I" starts that is a darker semicircular area but its hard to tell if the darkness is due to oil and if the surface is less granular and semi polished.

Ciao   

[John A]

fore aft crankshaft play.

I believe Mike is right,  check your crank end play.

[Murray]

On a bike that old 16 000miles, 160 000miles, 60 000miles in another bike before been plonked into that one or the miryad of mechanical and mainteneace abuses that could of been inflicted before it came into your possenion. The earlier V50's were built to a price back when quality control wasn't its finest. Making it reliable, simply build it to its designed specs the design while it has a few compromises isn't inheritly unreliable, the other solution is look at bolting in a newer version of the small block motor.

[lucky phil]

I believe Mike is right,  check your crank end play.

I dont think so. If the crank floated for and aft a little more than spec it's not going to cause this. Even if it was a lot more than spec.The little end will just slide along the pin as it has a ton of side clearance like all engines that dont use the rod eye end as the rod side clearance control.

Ciao

[Rough Edge racing]

Is there a photo of the top of the piston?

[nbags]

looks like scoring on cylinder wall lack of lubrication ? is the pin loose on piston wrist pin could have seize

[Chuck in Indiana]

What would cause this, and how common is it?
And what should I do to the donner engine to make it reliable?

(1) Dunno. I'm guessing factory defect. Certainly *not* common, though.
(2) Fling it in there and be done with it.

[Harley PHD]

I'm glad its not common.  The exhaust valve on the new motor had high leakdown so I pulled the motor apart.  While waiting on backordered parts from Harpers (piston pin clips) I decided to clean and paint the stained aluminum. 

Old Motor




"New" Motor




Thanks, I will leave the rods alone, I'm glad its not a common problem. 

[acogoff]

    All I can say is, when you put it together, check that the crankshaft endfloat is limited by the main bearing being against the crankshaft itself both front and rear and not the connecting rod small end sliding up against the side of the piston. Similar to what others have said, only stated differently.   

[guzzisteve]

The end float WILL cause the wrist pin to be bound, it tears up the piston shirt but usually not the rod. For this to be what happened you wouldn't have had a usable clutch. I'd say bad rod.  Nice job on motor, V50's are some of the best smallblocks and reliable w/Dyna S ignition.

[Chuck in Indiana]

The only "common" failure on the small blocks is dropping a valve.. not that it is all that common.
On the 4V engines, yes.. that is fairly common.

[huub]

i've only seen one snapped rod on a smalllock , it was a 1978 V50.
the later engines had upgraded rods , so the failure of your conrod is a mistery .
one of those things that happens for no apparent reason ( usually happens to me  :sad)

just ignore it , change the engine , and enjoy the V50....

[malik]

I acquired a carbureted Nevada with similar, if much more drastic crankcase damage. A lot more of the casing around the dipstick had been punched out. When I took it apart, I found the cap on one of the rods split in two, both bolts lying loose in the sump, and half of the cap embedded into the casing, the rod had jumped the crank & then had been jammed into the cylinder by the other rod. The rod was whole, if bent, the piston looking in a similar state to yours, but jammed into the edge of the cylinder. The cylinder looks fine, all but a mangled quarter inch at the bottom on one side. Mangled bearings, more damage inside the crankcase, bits of crankcase & piston all over the place. Nasty. FWIW, there was 67,000km on the bike, and evidence of what appears to be serial abuse. All the valves look good, although my lack of experience & expertise means I often don't know what I'm looking at. To me, it looks like the rod jumping the crank was the proximate cause of the severe damage, but how the rod's cap came off in the first place (if that's what happened first) is baffling.

[Chuck in Indiana]

just ignore it , change the engine , and enjoy the V50....

My sentiments, too.

[Harley PHD]

Here is some more carnage, I think the rod went first, the lower half of the rod slinging around.  I think one of the lifters was still usable.

Here is the top of the piston




The rod, the bearings still look good.




Cylinder, crankcase, and other parts.




It even bent the engine mounting bolt.

[Chuck in Indiana]

I will file this in the "Some things aren't meant to be understood" pile.  Strange.