R0tB0x70

Hey All,

I just purchased a crank for my lq4 that's been cut .030 under on the mains. I'm being told by several sources that .030 is too big of a cut to run in this engine. I was going to run a 50 shot on the motor. Would you feel confident running this crank or should I be looking for a new one? Thanks!

99 Black Bird T/A

iTrader: (4)

R0TBox70,

Please read this link it directly addresses most of your questions.

Crank Grinding Many Service Technicians Still Cling to Myth

I suspect .030 under concern is based on obsolete information from the 1940's & 1950's when more Babbitt material was required to make a bigger & weaker bearing for an under crank. Today's bearing are made differently.

Back in,the 1960's & 1970's offset grinding as much as 1/4 inch, that's .250 was accepted for many cranks to build a stroker motor. Cutting .030 typically reduces strength by about 4 percent. In an extreme applications that might be a concern like a 1,500 whp twin turbo build with a stock LS Crank. Probably not much of a concern for the typical NA engine build with a small 50 shot of spray.

With that said the undercut and polish work needs to be sport on otherwise a stress riser can be introduced. I've seen stock LS cranks for $125 in the last month so it might be just as cost-effective to find another crank. I've also seen them for three times that.

Most importantly make sure your dealing with a qualified machinist and engine builder.

Best regards

RB04Av

Strength reduction, or bearing thickness, is a minimal concern.

What I'd worry about more is, the hardening is only skin-deep. Only the surface gets hardened completely. Can't tell you haw deep it goes, but it's not real far. Taking off too much can get down to the softer metal underneath, in which case, it will wear a good bit faster.

Personally I'd recommend getting another one as well, and chalk up the cost of this one to the dumab$$ tax. Hopefully it wasn't too much of an installment payment...

Enigma540

This. Strength is rarely a concern. Turning a crank down can, oftentimes, make it even stronger........by adding more radius in the filet area. Bearing fitment must be taken into concideration here. My issue with running cut cranks is, as said above, the hardening usually only penetrates to 20-30 thousandths. And, unless it's a high dollar crank, rarely does anyone bother having it nitrided after the machining. A $150 crank just isn't worth that. So accelerated wear can become an issue. Also, I don't trust a lot of people's machining, these days. Too many "high volume shops" pumping parts out the door as quick as possible just to keep revenue coming in. Turning down a crank isn't a simple bore job, and runout can be a problem. If you already have the crank, check the filet radius, mic it everywhere and check the runout and concentricty, and decide if you want to take the gamble of using it. Personally, I'd question the morality and integrity of someone who'd just knock 30 thou off a crank without addressing the radius, and re-nitriding it, then selling it to someone. And I'd throw it right in the trash.