O/T Balance Question


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By Butch Lawson - 16 Years Ago
This is not a y related question, but this is the  most knowledgeable and friendly place I know to ask.  I have been having a discussion with a fellow Ford "nut" about crank balancing.  My question is .... we have a stroker crank for a 351w engine that has been balanced (3.85 stroke).  The rods and pistons are long gone.  Could a set of rods and pistons that have been weight matched be installed on the crank for street use without re-balancing, or will the weight difference between the old balance and the new assembly require the crank to be balanced to the new rod/piston weights?  Inquiring minds want to know.

Thanks to all of you for sharing your knowledge so freely and here's hoping you all have a safe and happy Thanksgiving!!

By Roy - 16 Years Ago
If the new parts are anywhere close to the weights of the old parts, you should be fine, but I'm no expert. Balancing will certainly enhance your set up. Typically you'll get more, and faster, rpm with a good balance. Also your engine should last longer. The balancing I've had done, the shop wanted everything including the clutch. All the pistons, rings and pins were equalized in weight, then that weight was added to the crank for the balancing. I don't believe manufacturers balance engines, but in your case where you've got a stroked crank, it may not be balanced for the new pistons. Hope that makes sense.
By mctim64 - 16 Years Ago
Get it re-done!  If you don't know what it had before you will not be able to get it close.  We charge $150 for a balance job, I think that is cheep insurance that your engine will run smoothly.  You don't want to get it together just to find out you should have done it. w00t
By pegleg - 16 Years Ago
Absolutely get it done. When Ted (Eaton Balancing) see's this question you're going to get an explanation from him too. Resons why.........Longer life, smoother operation, and more power because less is wasted moving the engine up and down and/ or side to side. Ted or Tim or Mummert can also change the balance to reflect the RPM range you'll be using, along with other tricks.
By Butch Lawson - 16 Years Ago
Thanks to all of you for confirming this!!!  It means more coming from you guys who do this all the time than from someone like me who has only built less than a dozen engines in my life, but I still win the steak dinner.  Thanks.
By Ted - 16 Years Ago

To add to what Frank and Tim have said…..

Your crankshaft was originally balanced to a specific bobweight value which means if you’re trying to maintain the same state of balance on the crankshaft, the replacement connecting rods, rings, bearings, and pistons need to add up to a comparable bobweight value.  As a general rule, the bobweight value for the new combination of parts can not be any heavier than was originally in the engine but can sometimes be as much as 30 grams lighter depending upon your combination of parts.  Talking overbalance here.  If you don’t know what the original bobweight value was, then rebalancing the whole assembly is going to be necessary.  Similarly, if the engine was externally balanced and the original flywheel and/or damper is missing or have been changed out, then rebalancing is again recommended.

 

Here’s a link to understanding what makes up the bobweight value.

http://www.eatonbalancing.com/blog/2007/11/17/engine-balancing-part-5/

 

The oem’s do balance their rotating assemblies but they are typically not balanced as closely as you’ll find being done in an independent shop.  This is part of the reason where two identical factory cars do not perform the same and likewise, do not get the same fuel mileage.

By Butch Lawson - 16 Years Ago
Thanks Ted.  I went to your website and read several of your articles.  Very informative and interesting.  And better yet, they are written where even I can understand them.  Thanks for sharing your knowledge with those of us who are still learning.  Thanks again!!