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Internal balance VS. External balance

2140 Views 4 Replies 3 Participants Last post by  camachinist
I have a 289 for my 67 Fastback that has been bored and balanced externally. I am going convert it from manual to AOD. So it has been balanced with the flex plate. I would like to have the option of going back to manual if I decide that I don't like the AOD. What are the opinions out there on internally balancing this motor, so I can just zero balance the flywheel or flexplate/ harmonic balancer combination and not have to re-balance the engine to go back to manual. From talking to a machinist his opinion is that externally balancing an engine is a cost saving procedure for a manufacturer and not necessarily the best way to balance an engine. It seems to me it would give me more flexibility to internally balance.

Jon Sherar/ (67 Fastback)
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Your machinist is 100% correct. Ballancing each counterweight with Mollory metal against the weight of the appropiate piston/rod assmbly is the best way to ballance an engine, (internal ballanced). When a crank is ballanced externaly, each counterweight is still partialy unballanced and the difference is taken up at the flywheel/front ballancer. This puts the "assembly" into ballance, but the unballanced loads between the crank pins and the ballance weights have to travel through the crankshaft to get there. This puts more load, (and harmonic stress as the shock loads travel back and forth down the length of the shaft), on the crank then just having to transmit the torque from the crank pins to the flywheel flange.

That being said, automotive crankshafts are designed to carry this under normal circumstances. Unless you are building more HP into the engine then the crank was designed to handle, external ballancing is fine and acceptable under most circumstances.

0 ballance is just that, stock ballance is OFF ballanced 20oz on your 289. You can buy 0 ballanced aftermarket flywheels for use on internaly ballanced engines, but if what you are looking for is flexibility, stay with the 20oz flywheels. That way you can use anything that is designed for a SB ford and just bolt it on.

If you are building a very high RPM race engine, go with the 0 ballance.

Love hard, drive fast, wear your seat belt.

PS, that's my 'bird...... My Mustang is too ugly to take pictures of yet........*G*.
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When I built a 428 to put in my Shelby for driving, I went the 0 balance route for just that reason. Car is currently a toploader, but when I pull that engine and put the original back in, I wanted the best possible market to sell (the 428) it in. With 0 balance, you can change to an automatic for the cost of 0 balancing a flex plate. On the down side, it is more expensive to 0 balance than external balance.

68 GT500
68 1/2 CJ Coupe

MCA# 18519
I've had this car since 1976, it's been out of commission since 1985(I hate to admit that!). Anyway I had to rebuild it right after I bought it. Being in the service at the time and not a having much money to fix it, I did a low buck overhaul. When it came to the crank, the machine shop had one already turned and ready to go. Being a young whippersnapper at the time, having no idea about it needing to be balanced, put it together and the engine wobbled not enough to hurt the engine but the irritation factor was pretty high. One thing for sure, I don't want to have an out of balance motor again!

Thanks for replies, I think that I will internally balance it this time.

Jon Sherar/ (67 Fastback)
Short answer...agree with Hal...

FWIW, I've never built an internally balanced 289/302 or W and have routinely spun the little engines over 7K in the lights and the W to nearly 7K....

Smooth as a baby's bottom...I'll give credit to the balance shop for that one...

If I was contemplating a multi-purpose combo and was switching back and forth on trannies...I'd just have a flexplate and flywheel matched to the engine when doing the balance work...

A balance shop can also match the current unbalance to a new item....they'll need the flexplate/flywheel off the engine to set up the machine...

Personally, I'd leave the internal balancing of external balance engines to the hard core racers.....maybe getting a quote on such a job might be telling...

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