To balance....or not to balance

[03GC]

I am starting to get my plan together and I was curious what what you guys think about spin balancing the rotating assembly. I have built a few engines in the past with and without balancing(SBC) and never really noticed a difference in smoothness. I believe weight matching is important but not entirely sure about spin balance.

Is it more critical on a stroker inline 6 with non stock parts (like 258 rods w/ 4.0 pistons)? Of coarse pro engine builders say its vital but I'm talking about a 5200 rpm Jeep, not a 8000 rpm circle track motor . Any opinoins would be greatly appreciated.

Paul

[Muad'Dib]

Id also like to know if this is something that is worthwhile. I have read that it really isn't needed, but my thinking process tells me that its possible that it will add to the reliability and longevity of the engine. If it does, it may be worth it depending on the cost.

[03GC]

Another problem is finding someone you trust to do the work. It is easy to measure a main bearing OD to see if it is the right size or measure deck height. It's another thing to just BELIEVE someone balanced something . I am almost ready to just buy a whole rotating assy from Speedomotive or 505 Performance. I know that is a somewhat hi dollar approach, but I have been screwed over by local machine shops before. Not that I am bitter about it . Seems like my parts are the ones they train the hi school co-op on.

[gonridnu]

Balancing of the rotating assembly on a straight six does not utilize bob weights as does a V8 so you are essentially talking about spin balancing the dampner, crank, and flywheel/flexplate and weight matching pistons and rod ends. Since these were not part of a cingular assembly coming with at least the factory balancing my recomendation would be to balance or face a potential vibration problem...

[Mgardiner1]

Just my .02..... Did you ever see the equation that showed how much a few gram's weigh in terms of newtons law, at the rated rod/piston speed at 5000 RPM? A few grams difference at 5000 RPM, and the rod is traveling a certian amount of feet per second, grams turn in to almost hundreds of pounds of force (or inertia).....

In terms of reliability of a machine shop... well... that is going to depend more on local favortism. I asked to have my lower assembly spin balanced. I could definitly see the marks where they ground the rod assembly's on the balance pads, but i did not find any marks on the crank to lighten it in any area's. Something in my gut tells me that the crank was not perfect from the factory balance.......

Regardless, if the machine shop is honest and truly does a SPIN balance, then i think the benefit greatly outweighs the cost.

[Muad'Dib]

So if you want an engine balanced, does the machine shop or whoever you take it to do the assembly? Or do you take the completed project to them? How does it work i guess is what im asking.

[gonridnu]

To spin the crank it will need to be ground to size and you will need to have the harmonic balancer flexplate/flywheel clutch assembly you are going to use. To balance the rods end for end they will need to be sized and ready to go and the pistons should also be in their final configuration. Because there are no bob weights you do not need to take them bearing and rings. The rods and pistons are in their final configuration at that point so I would go ahead and have the machine shop hang the pistons.

[Mgardiner1]

Maud'Dib, from my understanding, its a lot like balancing a car tire, you do it off the vehicle. So the crank will not be rotated in the engine block with the pistons and rods assembled into it. Most of the balance procedure occurs like Mr gonridnu has said. The BE of my rods were ground down on the balance pads to match their weights, and the pistons had the same. The use like a 3M scotch brite disk to take some material off the bottom of the piston underneath the wrist pin. I was unable to deliver my balancer and flywheel to the machine shop, and he just recommended having them balanced seperatly before installing. I was unable to do so because i was doing a weekend swap.

[03GC]

Looks like Speedomotive is out. Quite a few horror stories out there.... everything from 383 chevys,408 fords to 4.6 Jeeps. Customer service sucks from the sound of it. I am going to check around a few shops locally and see if I can find a 60 year old mean bastard who has been grinding and balancing cranks for 40 years. I'll put up with attitude if I feel the guy knows what he is doing.

gonridnu- why did you get out of the machine shop business? Sounds like you know whats going on.

Paul

[SilverXJ]

To spin the crank it will need to be ground to size and you will need to have the harmonic balancer flexplate/flywheel clutch assembly you are going to use.

Its better to have the harmonic balancer and flex plate balanced separately in case you have the need to replace one down the line.

[Cheromaniac]

I am starting to get my plan together and I was curious what what you guys think about spin balancing the rotating assembly. I have built a few engines in the past with and without balancing(SBC) and never really noticed a difference in smoothness. I believe weight matching is important but not entirely sure about spin balance.

Is it more critical on a stroker inline 6 with non stock parts (like 258 rods w/ 4.0 pistons)? Of coarse pro engine builders say its vital but I'm talking about a 5200 rpm Jeep, not a 8000 rpm circle track motor . Any opinoins would be greatly appreciated.

Paul

Spin balancing the rotating assembly on a stroker I6 would be desirable for the best smoothness bit it is NOT essential. Weight-matching of the rods and pistons will suffice. That's all I did on mine and it's virtually vibration-free at 5000rpm.

[IH 392]

If you can fit into the budget DO IT!
On a straight six the machinist SHOULD!? , (after the machining is done) balance the crank, then install the flywheel and/or dampener and balance them accordingly, then the pressure plate, the remaining components will be weight matched, then in the future "IF" there are any issues? replacement components can be matched to the "specs" given to you from the balancer.
Ask around for a local shop, go to the local track and ask those guys who they use!

[gonridnu]

Its better to have the harmonic balancer and flex plate balanced separately in case you have the need to replace one down the line.

That is correct but you still need to spin the assembly as it affects the crankshaft balance.

I am going to check around a few shops locally and see if I can find a 60 year old mean bastard who has been grinding and balancing cranks for 40 years. I'll put up with attitude if I feel the guy knows what he is doing.

Personally i would look for a shop that has the newest equipment...the new balancers are more accurate and any competant machinist can drill a hole if the machine tells him precisely where to do it.

gonridnu- why did you get out of the machine shop business? Sounds like you know whats going on.

I had small children at home and was there 10 hours a day six days a week. Was offered a good job with a NASCAR supplier and used it as an opportunity to make a life change and spend more time with the family.

[gonridnu]

I would also like to add that this is not an expensive proposition. We're talking like $150 which is not a lot when you take into account the increased longevity of an engine that has been properly balanced vs the potential for problems. As I mentioned earlier It is especially critical when using parts that did not come as an assembly from the factory but it is still part of any good engine build.

[Alex22]

If you can fit into the budget DO IT!
On a straight six the machinist SHOULD!? , (after the machining is done) balance the crank, then install the flywheel and/or dampener and balance them accordingly, then the pressure plate, the remaining components will be weight matched, then in the future "IF" there are any issues? replacement components can be matched to the "specs" given to you from the balancer.
Ask around for a local shop, go to the local track and ask those guys who they use!

I can't find my 4.0 info sheets at the moment, are the 4.0 and 4.2 engines internally or external balanced? I have a 12 counterweight crank around that I can throw on the balancer to see how it is, but I don't have a flywheel or flexplate.
As for balance spec, >.50 inch ounces is good enough for street use and >.25 inch ounces is the spec for race engines.

As the others mentioned, just weight match the rods, pistons, bearings and ring packs.