cut 6 bolt crank

[bmil13]

Yes I know there are hundreds of threads of people arguing about cut cranks. Thats why im not asking whether or not I should or how much power people have made on cut cranks. But does anyone know of anybody actually breaking a cut crank?

 

[stars]

Only reason you want to cut the crank is to fix the bearing where it had spun. Now the bearing has spun you have to get an over sized bearing and cut the crank to match like the rest. I don't know why you want to remove the metal from a 6 bolt? If you want a lighter crank then get a 7 bolt engine it has a lighter crank and rods. If you want to waste your money go ahead with your plans. I don't know what you want to accomplish but have at it, and post your results.

Crankshaft break due to imperfections within the metal alloy itself. That has happened at times but it does that to every vehicle on the road at one point or the other. Stuff breaks and wears down over time with extensive road use with a daily driver. Normal oil changes on a regular basis will help much more than cutting your crankshaft. After all the bearing had probably spun because it was starved of oil. That's why you do oil changes.

 

[bmil13]

So all of a sudden you know all my plans. I never said I want to lighten the crank. all I wanna know is if anyone has broken a crank that was cut .010

 

[WES_393]

All I wanna know is if anyone has broken a crank that was cut .010

You probably wont find anybody who has legit evidence of a cut crank breaking or being the main cause of failure. Most likely because there is nothing wrong with cutting a crank unlike popular theory.

 

[boostingsx98]

Only reason you want to cut the crank is to fix the bearing where it had spun. Now the bearing has spun you have to get an over sized bearing and cut the crank to match like the rest. I don't know why you want to remove the metal from a 6 bolt? If you want a lighter crank then get a 7 bolt engine it has a lighter crank and rods. If you want to waste your money go ahead with your plans. I don't know what you want to accomplish but have at it, and post your results.

Crankshaft break due to imperfections within the metal alloy itself. That has happened at times but it does that to every vehicle on the road at one point or the other. Stuff breaks and wears down over time with extensive road use with a daily driver. Normal oil changes on a regular basis will help much more than cutting your crankshaft. After all the bearing had probably spun because it was starved of oil. That's why you do oil changes.

WTF is this guy talking about^^^^

I have ran a cut crank on one of my old 1g's pushing 25psi on a daily basis with no problems whatsoever. I have also ran multiple cut 7 bolt cranks with no problems thus far. You should be fine, if you do a little research on the forum there are multiple guys on here running cut cranks and achieving bigboy HP levels. Also the amount cut off a crank is very minimal and alot depends on properly machining a crank.

Good luck

 

[midoko]

Just broke this one. Couple machinists said the notation on the counterweight indicates rods & mains had been cut .50mm. Just rough measured one of the rod journals and it's 44.62mm. The manual says stock is 45mm, and service size is .75mm undersize...so it was within acceptable service range. Told factory nitriding was shaved off, resulting in weakening. Checked around and typical nitriding is .10 mm to .20 mm thickness. Was casually coasting into a parking lot when it started knocking. Wasn't oil starved, wasn't racing. Engine probably makes less than 400hp (guess). Don't know for sure why it broke, just know it did.

 

[stars]

Told factory nitriding was shaved off, resulting in weakening.

Metal alloy imperfections, example of

WTF is this guy talking about^^^^

boostingsx98 Do you know what Magnaflux is without using the Internet to do a search?

I'm not against machining if it needs to be "trued". If you’re worried after you do the math, whether it's within specs, then buy a "new" one.

 

[viperlp01]

I run a crank cut .50 on mains and rods. Mid 10 sec power. No issues.

 

[bmil13]

Yup I've read threads where people make a good bit of power on cut cranks without problems. I just wanted to see how many people have actually broken one

 

[WES_393]

How can taking a coating off of a metal surface weaken the inside? It doesn't add up. That's like saying stripping the paint off your rims will cause them to crack.

And .10 mm is 1/10 the thickness of a credit card. Were not talking huge differences in size.

 

[stars]

Let's go back to Metals Class back in High School. Iron ore melting down with other ingredients into a liquid form and transferred into a liquid molding of sand. The mixture is poured in and allow it to cool. Workers then come in and remove the mold and now it is considered a "casting" Sometimes in that process air bubbles...little tiny ones you need a microscope to detect can form. Any impurity can remain hidden just underneath the surface. As the mold cools it hardens. They then machine that hunk of "casting" into what is known as a "crankshaft". All it takes is a slight drop anywhere along the assembly line for a "chip" to happen. Don't fool yourself, even case hardened material has it's limits. That's why you use bolt cutters to cut off a Master Lock, try sawing thru it and see what happens, Einstein.

When I was in aviation maint, back in the day, we had to use a Magnaflux machine. It uses Eddy Current and specialized "metallic spray powder" to help identify where a potential flaw is located. The crankshaft is energized and the metal filings will align where the flaw is at. You then have to have it sent off and have it X-Ray'd to inspect the flaw further. There are various other methods including spray on UV dyes that can locate "superficial" cracks. Those type are hard to see with the naked eye.

I disagree that .10mm does make a difference depending where it is located. Yes, you can't see it but it is there, just look at that pic of broken crank and tell me how it got that way? That should be proof enough for some people. Look up the term "nitriding" if you still can't figure it out.

 

[WES_393]

The picture of that crankshaft is not emperical proof of anything other than the fact that the OP's camera works. If I show you a picture of Scarlett Johansen, will you believe that I'm dating her? No offense to midoko, but without a legit test showing that crankshafts are structuraly weakend by machining, it's an assumption at best.

Very many people here are running cut cranks, including myself, with zero issues, at high horsepower levels, for many miles, and for countless races. That is empirical evidence that cut cranks work.

/thread

 

[midoko]

So 12 years later I come back and find this post again. With only seven posts total, you can see I didn't get on here much. Was on the GVR4 forum more since that was my focus.

Anyway, I read the few replies and realized I hadn't given some information that may or may not shed light on why it broke. That crank was from Jack Culotta's Eclipse he drove for Extreme. Based on information I got from Sean Glazar, it is the same motor and driveline. Obviously they were abusing their cars, which seems to me, explains why the crank was turned in the first place, and increasing the likelihood of over stressing engine components. I put 5K+ on it daily driving before it happened. I did occasional pulls but never took it to the track. It never had any knock until then. And oddly, to me, it didn't come apart, but just started a bad knock after a pull. IIRC rev limiter was at 6500. Didn't realize it was two pieces until it was apart.

So I'd like to hear if anyone has any further ideas why it broke. Is it really possible a micro-hairline crack held up under Extreme's last races, through the two owners after that, my 1000s of miles daily driving and pulls, and then finally gave out? I mean, I didn't drive it at the level Jack did. It had a smaller turbo and lower boost up to that point.