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One of the critical differences between the correct Mitsu 6bolt rod and the 7 bolt version is the width of the big end. Eagle/Mitsu 6bolt rod is 28.3mm wide. The incorrect 7bolt rod is too narrow.
The big end width I have are >1.117 by according to my calipers. 1.117-1.121 assuming inaccuracy in me and the calipers.
These are the same rods and pistons for the original 2.85 liter build. The rods are a hybrid design of sorts using a 6 bolt big end and a 7 bolt pin end with a 22mm pin hole. I had mine bored and bushed for the standard Chevrolet pin size but the bushing is not completely necessary as Shawn will attest.
I would be curious to see the condition of the crank thrust bearing surface upon inspection as well. I had a customer Audi 5 cylinder with a similar concern about clutch issues that turned out to be a worn thrust surface on the crank.
I was pretty positive all was good. Originally the wrong rods were shipped. These were the replacements. Good for others for future reference if a 4G63 rod vendor sends the wrong rods.
So thinking about how much a crankshaft twists under hard acceleration do you think it may have to do with timing at some
point? If your crank sensor is in the rear and that's all then it can't compensate timing for crankshaft twist. Just because they are forged doesn't mean they don't twist just a little. And a little over a long time is wear. And throws the spark timing off. Maybe you want to think about running a crank sensor front and rear to calculate the crankshaft twisting for better timing. You did take material off the crankshafts rod bearings which help it from twisting, lets not forget that. Oh and the fact that now it's a extremely huge 3.2L four cylinder spinning at high RPM's a lot. I'm kinda suprized it lasted as long as it did. If it was a 6.4 Flat 8 you wouldn't see that kinda crankshaft harmonics mainly because the firing order power band overlap each other. If the power band of the piston is 120 degrees from firing the 8 cylinder crankshaft never slows down due to a 90 degree firing order. But on a four cylinder it fires every 180 degrees allowing the crankshaft to slow down just a little mainly seen at idle, above idle there is a point where inertia takes over. And we are talking about the crankshaft on a four cylinder slowing down maybe 50 RPM at idle between piston firings. So that's why I asked if your using a harmonic balancer.
I do not believe this is a torsional failure. Looks like a load based fracture that then propagated until final failure. Initial fractures looked to have started in the journal radius. Could have been the press but I am betting not at this point. The oil hole on the 94.7mm stroke looks concerning to me. Done a lot of reading. I definitely should have done nitride treatment as it may have helped prevent the initial fractures. The radius needs to be a big as possible and if I had this to do again I may not go 95 mm but probably something like 92 mm. Here is some interesting info:
Pretty sobering photo! I am absolutely not an expert, but all the hardcore racer/machinist types I've spoken to preach the need for nitriding after machining these cranks. Michael Stimson probably went on for 30-45 minutes about it one day, way over my head, when I called about drilling a crank or something... Your story certainly does nothing to prove them wrong, but on the other hand, you got plenty of power and miles out of the motor and did it with one of the higher HP to $$ ratios out there, so I'd chalk it up to the pioneering spirit and make it bigger, stronger and faster next time around!
Pretty sobering photo! I am absolutely not an expert, but all the hardcore racer/machinist types I've spoken to preach the need for nitriding after machining these cranks. Michael Stimson probably went on for 30-45 minutes about it one day, way over my head, when I called about drilling a crank or something... Your story certainly does nothing to prove them wrong, but on the other hand, you got plenty of power and miles out of the motor and did it with one of the higher HP to $$ ratios out there, so I'd chalk it up to the pioneering spirit and make it bigger, stronger and faster next time around!
Thanks Tom. Yep, no regrets My over confidence in the crank is a lesson learned. Rebuilt motor will be 92 mm stroke with nitride treatment and largest fillet radius possible. It will be a 3.0L vs 3.1L but should be fast and a hopefully last even longer. Hoping to have it back together by early June.
Thanks Tom. Yep, no regrets My over confidence in the crank is a lesson learned. Rebuilt motor will be 92 mm stroke with nitride treatment and largest fillet radius possible. It will be a 3.0L vs 3.1L but should be fast and a hopefully last even longer. Hoping to have it back together by early June.
If you get a new motor in by June, after I've been sitting on my next-gen motor parts for way too long, I might actually be shamed into action...
Strange failure, even with stroke mods.
Could be torsional but I doubt as someone commented above that opposing end mounted sensors would have resolution to document it.
Out of curiosity, do you run a non spring damped clutch disc..?
And solid engine mounts..?
On track, short of natural backlash in CVs and transmission gearing, only the length of the long input between the clutch spline and the transmission input shaft coupler absorb torsional forces through the drivetrain when you remove the spring center from the disc and the isolator (rubber mounts) that both absorb torsional forces when you hop the rear tires on an errant or missed rev matched downshift with high grip tires.
BTW - re your video, it's hard to do with traffic, but in a car with your power, after out lap, try making some laps in 5th gear only...., never downshift.
This will teach you momentum faster than anything else.
looking at the metal grain patterns in that photo that looks like a crack->snap caused by crank whip rather than torsional stress.
probably could've used a revised (heavier?) harmonic damper along with the larger radiuses as mentioned.
if it were torsional failure...easiest to picture a splined driveshaft twisted from mega torque input, the metal twists on itself as if it were a a towel being wringed out, if smaller scale.
04-01-2018, 10:35 PM
My over confidence in the crank is a lesson learned. Rebuilt motor will be 92 mm stroke with nitride treatment and largest fillet radius possible. It will be a 3.0L vs 3.1L but should be fast and a hopefully last even longer. Hoping to have it back together by early June. 
