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Author Topic:   broken cranks - balancer?
ss99
Member
posted June 06, 2004 08:50 PM
We've broken two cranks recently. Both were used GM cranks - the first steel and the second cast. I'm using a Pioneer 6.25" balancer that is SFI rated and lightweight (5.5 pounds). I'm wondering if I should blame the balancer or write it off to using old used GM cranks. Any thoughts?

Thanks

waltonjr1
Member
posted June 06, 2004 08:54 PM
What size motor?



Raz_900
Member
posted June 06, 2004 08:55 PM
Belts too tight or balance issue. Oh.. or over revving (past 7000)


ss99
Member
posted June 07, 2004 06:30 AM
quote:
Originally posted by waltonjr1:
What size motor?


355 with 2bbl carb. Max RPM is usually 7100

NJantz
Member
posted June 07, 2004 06:54 AM
Unless you have LW rods and pistons, 7100 is REALLY pushing it on used stock cranks.


waltonjr1
Member
posted June 07, 2004 11:55 AM
Raz is right i would try to keep it right under 7,000 and next time use a standard balancer, no need for light weight unless the rest of your rotating assembly is lightweight.



ss99
Member
posted June 07, 2004 12:45 PM
Thanks guys. I should have been more specific - the first motor had SRP flyweight pistons and light rods. The second motor had Keith Black lightweight hypers (Model 231 piston) and lightweight SCAT rods, so I'd say our rotating stuff was pretty light overall. I think from now on, I'll be using nothing but new cranks. I was just wondering if I should s**** this balancer. It's really a nice balancer, so I'd like to keep it if I can justify it. Competition Products sells them for about $135.00.

ss99

NJantz
Member
posted June 07, 2004 01:13 PM
One other thought - both of these cranks were turned 30/30 at the recommendation of our machinist. Now I'm wondering if that wasn't such a good idea. I've heard that crank grinders can cause this to happen if they don't cut the radius correctly.


Okie11m
Member
posted June 07, 2004 07:18 PM
I wouldn't run a 30/30 in my grandma's grocery getter!!!!! If your machinist recomended turning 7000 with that crank, I'd get the yellow pages out & find anouther one!!!


Raz_900
Member
posted June 07, 2004 08:36 PM
I didn't think they made performance bearings in 30? They didn't even make them in 20 until a couple years ago. Did it perhaps overlap a bearing then snap?

Ya... I'd go with an Eagle 4340 or Callies Dragonslayer if you're planning on over 7000 on a regular basis.

BrianW
Member
posted June 08, 2004 09:49 AM
First - use a standard/standard good core that only needs polishing...

Second - if you use the lightweight components but don't calculate the new bobweight and spin balance the crank the thing will vibrate itself to death and end up destroying cranks. I'd say you should be able to make a good steel stock crank core live at a max of 7100 with lightweight components *IF* it's balanced correctly - also - give the shop the balancer, flywheel and clutch (if manual) to completely balance the assembly as a whole.

ss99
Member
posted June 08, 2004 12:06 PM
Thanks for all the thoughts guys. The entire assembly was balanced together in both cases, crank and all. I figured the machinist would take a beating about the 30/30 cutting, and maybe it's deserved. I didn't even know he was doing that until it was too late. We'll stick with new cranks and max. 10/10 from now on.

ss99

Kromulous
Member
posted June 08, 2004 12:49 PM
Could someone explain bob wieghts and whats its purpose in race engines?



autoshop
Member
posted June 08, 2004 01:05 PM
I've turned 7800 on stock cranks just fine.

------------------
http://www.freewebs.com/redneckracingteam00/index.htm


Raz_900
Member
posted June 10, 2004 03:10 PM
quote:
Originally posted by slater00:
I've turned 7800 on stock cranks just fine.



:P

Murphy's Law, you're now destined to break one this week.

Just kidding!!


UNVRNO
Member
posted June 10, 2004 08:51 PM
keep the balancer, its a sweet piece, there arent any that are better.


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