Last winter I sold a car with a well running 100,000 mile carbed 305 to some guy. Not two weeks later he brought it back with the motor making a horrible racket. We investigated a little, and found a screwdriver propping the choke open, and the oil was full of gas. So, we figured, he flooded it, propped the choke open, got it started and reved it up to high and spun a rod bearing. Well, we finally got around to taking the motor apart and this is what we found. The first three MAIN bearings were spun, and the crank was snapped in two How could flooding the engine lead to a broken crank? Could he have flooded it enough to hydrolock it? If that's the case, wouldn't it have just trashed that rod?
If your 305 has a mechanical fuel pump I would check the pump diaphram. Look in the block where the fuel is mounted for signs of fuel washing. Sometimes the diaphram will get a small hole in it and flood raw gas into the pan, you then loose lubrication because the oil in diluted and then spin bearings and who knows what stresses are put on the crank when that happens. That is my best guess. Good Luck dave
Its called "hydraulic-ing" it, Bugman......if -as the screwdriver indicated,the carb was severely flooding[BTW his fault] -then it is possible that it could have partially filled a cylinder with raw fuel...then next time he cranked it WHAM! broken crank [and bent rod possible] too.....if he then got it started and it ran [as it sounds like he did]....it would ruin the bearings on that throw for sure.....
Yeah, he broke it, then DROVE it 20 miles to our house. So, how many times do you need to pump a quadrajet to get enough gas into a cylinder to hydrolic lock it? Even 305 heads have pretty big chambers.
I've seen probably 20-25 hydraulically-locked engines, and never seen one break a crank, but most of my experience is with steel cranks, not cast iron, which I ***ume that 305 is (and we all know what happens when you ***ume...) . Usually, hydraulically locking a cylinder just bends or snaps the rod. If the rods are straight, but the crank is broke w/ spun bearings, I'd lean towards fuel dilution, which lead to spinning some bearings. The driving probably did the breaking.
I broke my crank by allowing the dampener to liberate itself. I didn't even have to stick anything in the carb. Maybe that has something to do with this particular case?
Damper is fine. That was one of the first things I looked at after we took the pan off and discovered the crank. How's this for a theory...because the oil was so diluted, it caused the first two mains to spin. the continued running welded them to the crank and block causing the front half of the crank to momentarily lock up, snapping the crank. The third main followed bacause of the resulting pounding. It was also very(very)cold, right around 0, which could have made the metal a little more brittle than it normally would have been. Sound plausable?
If you're really curious about how it broke, and the surface where it broke hasn't rubbed itself smooth, take a good close-up picture of it and post it here. If it failed in fatigue, you'll see lines (called beach marks) radiating out from the failure point. If it snapped quickly, the surface will be whiter, and have less (or more widely spaced) marks. Depending on how good your camera is, it might be hard to get a close-enough shot to see this kind of detail.
Gosh no. I did feel bad for the guy, cause we told him it was a good car(it was), so I bought it back for half what he paid. I still came out ahead, just not by as much. here's the best pic I have of the break. It does have about 20 miles post break on it, you can see the ends smooshed over from the 2 halves grinding against each other. If it's not good enough, I can try to get a better one with less reflection this weekend.
Hey everyone. I'm NO EXPERT in metal failure, but I did take some cl***es on it way back when. Judging by the pics, I have to agree with Hot Rod. The 45 deg angle on that break indicates a brittle failure in torsion. I think you can be pretty certain that somthing jammed up hard and snapped the crank. (Interesting FYI, if you twist a peice of chalk until it breaks, it shows the same kind of angled break) Also, the fact that it was a brittle failure would seem to point to a cast crank. I'm not sure if any one is really interested, but that is my 2 cents. rlb
From what I see, it's probably low-cycle fatigue. Crack origination from the oil hole, which is a higher stress area. I'd bet money it happened after the bearings started to go. Once those bearings started locking up, all sorts of unusual loads were put on that point. Even if you see some lines radiating through the failure surface, if they're widely spaced (1/4" is wide), it just means that after the crack was started, it opened up that amount every time one of the two rods attached to that journal saw a plug fire (***uming it was still firing at that point!). Often, when something fails in high-cycle fatigue, you can only see the little tiny beach marks near the origination point with a microscope. The most common area for cranks to fail in high-cycle fatigue is the small football-shaped area where the first rod and main bearing journals overlap- the other footballs/overlaps are the same size, but the input loads are a little higher at the front of the crank. The last football (between the 7/8 rod journal and the last main) is probably the next most common area, particularly on manual trans cars, since loads can be higher back there on shifts. Yours didn't fail in either place, which was the first clue, even before I saw the pic, that it wasn't high-cycle fatigue. There are exceptions to this rule of thumbs, of course- some cranks have odd natural frequencies that can make them bend and fail in other areas if you happen to run the engine at the speeds that match up with those frequencies. I'm not a metallurgist, so if there is one on here, he can probably say some of these things better than I have. It's been 10 years since I took a metallurgy cl***, but I've seen a bunch of failed cranks since then.
Ouch that dont look good. I'm guessing turning the key on had something to do with it. If it was still shut off i bet it would be in one peice.
Thanks for the info gang. I'm going to go with what you guys said and concider the mystery of the broken crankshaft solved
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