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Which broke first, piston or conrod? [B230F+T]

D

devolutio

New member
Joined
Apr 19, 2015
Hi All

Broke a skinny rod B230F on the weekend. I have a later engine ready to go in as this was expected.

Engine spat out the bit of conrod in 3rd gear at pretty low speed, so assuming right around the peak torque rev range. I've been giving this engine hell for a quite a while, but I'd just finished T5 trasmission conversion. Before that the M47 clutch would slip in 3rd.

Specs:
1987ish B230F +T
B230FT head (for cam and valves)
E85 fuel, MS3, 1000cc injectors, Wideband O2 etc (fueling shouldn't have been an issue).
Wasn't logging, but estimate around 15psi from a stock T3 turbo.

Any thoughts on what broke first?

8AD9YSG.jpg


cYVMKVy.jpg
 
What does the wrist pin look like?

Is the big end of the rod blue?

All the piston breakage seems to be fresh (no fretting in the cracks),
so I would guess that the rod beat the piston up.

However the pin bore of the piston looks like the pin was temporarily only connected to one side.
The edges of the piston pin bore on one side looks like the pin was wobbling?
Maybe the pin broke or slid to that one side.
 
Wow....

Having no first hand experience here (fortunately), I'd guess the rod twisted and snapped off the piston at the wrist pin. I'm surprised how little abuse there is from broken metal flying around and getting pounded -- was the cracked piston wedged at the top of the cylinder when you disassembled it?

The only thing close in this Mahle guide is pg. 11 Hydraulic Lock:
http://epaper.mahle.com/mc/onlineCatalogue/2062863636375&lang=en
 
I dont think the rod would be that bent if it just snapped.

Also the bent is not the plane of the rods movement (so the bent has to have happend while not "connected" to the piston).

Dirty Rick said:
However the pin bore of the piston looks like the pin was temporarily only connected to one side.
The edges of the piston pin bore on one side looks like the pin was wobbling?
Maybe the pin broke or slid to that one side.
Click to expand...

I think he could be quite close here. :nod:
 
the rods bend so easy anyway, I'm not sure you could draw much of a conclusion based on how bent or not the rod is at this point. I would guess that the rod has been bent for a while, side loading some things that weren't meant to be side loaded.


engine have a lot of piston slap?
 
Mine was surprisingly quiet with a broken wristpin and a one-legged piston.

I even engaged in some high revs/high boost shenanigans on the way home *after* I first heard the slight tapping noises.
 
linuxman51 said:
I'm not sure you could draw much of a conclusion based on how bent or not the rod is at this point.
Click to expand...

Well, tell me how the rod can bent that much AND snap without the piston failing before that. :D

linuxman51 said:
I would guess that the rod has been bent for a while, side loading some things that weren't meant to be side loaded.
Click to expand...

That would explain why the bore looks different on one side.
 
I def. broke a rod in my 13mm rod motor, since the piston was still (mostly) intact.

IMG_0986.JPG


Here's another one of the rods from that motor - bent but hadn't broken yet:
IMG_0991.JPG
 
JohnMc said:
The big end of the rod could have smacked the crap out of the short end after it broke.
Click to expand...

punching a hole in the block is also said to not be healthy for the rod(s)

I'd be curious if the piston from that busted rod you posted had damage on the bottom of the skirt (for instance, bent bad, bottomed the piston out against the block or the crank, and then broke the rod)

as to how the rod can bend before the piston fails.. the rods almost always bend before the pistons fail. it's a pretty rare occasion that someone with a stock bottom end says "I busted a couple pistons but the rods are mint" in the b230 world. loading the motor up at low rpms with a little turbo further exacerbates things due to the relative speed of mechanical things vs the speed of combustion.
 
As there isn't a hole burned in the top of the piston, it looks like the rod broke and then took the piston out from below.
 
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