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Old 03-17-2008, 11:31 AM
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305 crank in a 350 motor

alrite, i recently built a 350 new pistons rods etc the only thing used is the crank wich i pulled out of a 305 that was built less than 1000 miles before that. after i had put the motor in and am about to start it the friend i bought the crank from was telling me he read that the 305 crank will ruin everything and wont work good in a 350 because of the balance is that true? if so whats it gonna do

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Old 03-17-2008, 11:47 AM
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Yes that true... It will have some vibration. The 305 piston are lot smaller and lighter than the 350.
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Old 03-17-2008, 11:57 AM
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so it will ruin tha motor or will just have vibrations
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Old 03-17-2008, 12:00 PM
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It will be hard on the main bearings and transmission.
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Old 03-17-2008, 12:01 PM
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That why is a good idea to balance a ratator if your mixing parts as we have had calls about an engines vibrating because they have used a 305 crank in a 350 or vise versa.

And you have to watch out for remaned cranks as well>
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Old 03-17-2008, 12:06 PM
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the motor is gonna be mostly for the weekends and sometimes at the drag strip. is it gonna hurt performance by much?
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Old 03-17-2008, 01:27 PM
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yes it will hurt your performance because after a few runs to 6000 rpm it will blow up and then you will have 0 horsepower.
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Old 03-17-2008, 02:09 PM
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That 'vibration' will destroy your motor. OK maybe it'll live, might live a long time, might wipe out the mains in a few days...and then like said above. There is more reason to balance your assembly than $$$. It's called doing it right.
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Old 03-17-2008, 02:17 PM
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well the crank was never balanced for the 305 it was bought brand new and was only in a 305 for about 1000 miles does that matter? is it cuz its from a 305 or wld it be the same even if it came frm anther 350

Last edited by Casperdude898; 03-17-2008 at 03:12 PM.
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Old 03-17-2008, 02:44 PM
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Like mentioned, the 305 pistons are likely lighter. They may have been the lightweight rods too. Whatever it was, it isn't any more. To balance a crank, the parts that attach to it are weighed. From these weights, combined, added, divided, and figured for a balance factor etc, the crank is then balanced to a determined 'bobweight'. This ensures smooth operation and eliminates any possibility of an out of balance condition - if done properly. It is possible you are close, but I doubt it. It's your call as it is your motor. The only way to know is to have it balanced. the amount of out of balance would have some effect on the longevity of your motor, as would it's application. You can do a search on bob weight, balance factor, crankshaft, rotating, reciprocating and I am sure you'll find plenty on this subject. Really, for you, it's not an option, you need to have the assy balanced.
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Old 03-17-2008, 03:28 PM
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would it change anything if i said my rods and pistons are balanced
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Old 03-17-2008, 03:41 PM
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ive already built it and its already in the truck and all i have to do is put the starter on and start it up. should i keep goin and see how it runs or would it be worth pullin it back out and takin it apart and goin and gettin it balanced?
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Old 03-17-2008, 05:01 PM
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It needs balanced or otherwise you're going to end up with a pile of junk in short order. The last engine I built for myself used lightweight rods and pistons. This allowed my machinist to remove 3 lbs. from the forged crank when balancing the rotating assembly. While this is kind of the reverse of your situation, it illustrates how much parts swapping can change things.
I can't even imagine doing a performance build and not having it balanced. I always recommend it even when doing a stock rebuild for someone. The factory's balancing job nearly always leaves room for improvement. And for those buying stroker kits or other aftermarket rotating assemblies, don't believe or pay extra for the "pre-balanced" kits. Have your own machinist, who's hopefully one who specializes in high performance work, do it. Mine is a Godsend, he always takes time to explain in great detail and why, anything he does for me. And his results are always flawless with every spec dead on.
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Old 03-17-2008, 06:18 PM
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Quote:
Originally Posted by Casperdude898
ive already built it and its already in the truck and all i have to do is put the starter on and start it up. should i keep goin and see how it runs or would it be worth pullin it back out and takin it apart and goin and gettin it balanced?
If you are that far you might as well start it and see if it has any bad vibrations. If you used lighter pistons than the original the imbalance may not be that bad.

What kind of pistons and rods did you use?

tom
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Old 03-17-2008, 06:19 PM
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Originally Posted by Double_v23
yes it will hurt your performance because after a few runs to 6000 rpm it will blow up and then you will have 0 horsepower.
Nah, if it's that far out the rear view mirror will fall off first.

tom
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