I'm doing a re-ring job on a 292. I was trying to get this engine going on a tight budget (thanks to Tim McMaster for his help in that regard). I read Ted Eaton's articles on balancing engines and wondered if I could get away with just doing a static balance and skip the dynamic balance. The idea is that as long as the reciprocating assembly is balanced, the rotating assembly will be ok.



I have a 1Kg scale coming that's accurate to .1 gram and I'll follow the instructions about matching pistons, rod

ends, etc.



Thoughts?

Best regards,



Paul Menten

Meridian, Idaho

Paul,

Most production rebuild shops don't go to the trouble of balancing every engine they do, as long as you are using the stock components and the pistons are within 3-4g I don't think you will have a problem. Dynamic balance is a nice way to go, I do all My engines but I have a very nice machine at my disposal , it's not always necessary though, if you can make all the pistons weigh the same and get the big ends (rotating) and then the small ends (reciprocating) to be the same you should have a pretty good running engine.

   God Bless.   Tim                           http://yblockguy.com/

350ci Y-Block FED "Elwood", 301ci Y-Block Unibody LSR "Jake", 312ci Y-Block '58 F-100, 338ci Y-Block powered Model A Tudor

tim@yblockguy.com  Visalia, California    Just west of the Sequoias

On a V8 engine, the factory balances the crankshafts so as to match the weights of the rest of the rotating assembly.  If the weights are changed on either the rods, pistons, or peripherals, then the crankshaft itself becomes out of balance by lieu of a different amount of mass that is being spun around and the crankshaft counterweights are no longer being correct in which to compensate.  The degree of imbalance simply varies by the amounts any of these components are changed in weight.  As Tim mentions, 3-4 grams is not a significant difference but it’s not unusual to find 30-40 gram differences in replacement components which is definitely significant.  But stockers have been and are still being rebuilt with absolutely no regard towards the balance.  It’s only in those instances where there is a noticable vibration that it’s realized that the weights of the parts being used have deviated too far from what the crankshaft was originally balanced to be compatible with.  But it’s those imbalances that are present but not being felt that are destructive to internal components over the long haul as well as taking away power that would normally be directed towards the flywheel.

 

But simply match weighing the rods end weights and piston weights will be a step in the right direction but the job is not complete until the remainder of the  parts have been weighed and the crankshaft is dynamically balanced to match the bobweight calculation for those components.  The exception to this is on a stocker where the new pistons are match weighed so that they match the weight of the originals.  In this instance, the rotating assembly is in roughly the same state of balance as delivered from the factory.  I say roughly because even the factory balance jobs varied with some being better than others and with only a very few being ‘on the money’.  Blame it on the day of the week, breaks, the particular guy doing the balancing, or whatever.  For this reason, even rebalancing stockers with all their original components to a much closer degree than was being performed at the factory is of a proven benefit.

Lorena, Texas (South of Waco)

Ted,

    I hope you don't think that I am trying to imply that there is no "benefit" to balancing, there is, all I was telling Paul is that if he is replacing one piston with a used one to try and save a little money and He gets the weight of the used one to match the rest in his engine He will be fine. In the case of a complete fresh, all new parts, rebuild a $150 balance job is money well spent.

   God Bless.   Tim                           http://yblockguy.com/

350ci Y-Block FED "Elwood", 301ci Y-Block Unibody LSR "Jake", 312ci Y-Block '58 F-100, 338ci Y-Block powered Model A Tudor

tim@yblockguy.com  Visalia, California    Just west of the Sequoias

Ted, Tim:

I have heard somewhere that factories back in the '40s, '50s and 60's would weigh "representative" parts, balance a crank to those parts, then drill the next batch of cranks exactly the same as the sample.  That means factory balance is iffy from the start.  They also had the luxury of 90 day warranties and most customers only keeping the car a couple of years.  I doubt if the factories individually balanced each engine they manufactured.  Too labor intensive.  Luxury cars of the period may have received better balance jobs.

John in Selma, IN

John - "The Hoosier Hurricane"

I was guessing what John was suggesting; the factory sampled the weights of the parts and balanced to that sample weight. That might be part of the reason that piston weights are published; to ensure that the factory balance is not thrown too far off when buying replacement pistons.



Thanks to all for their wisdom. I'm going to cut this corner this time because I need to get my F100 back on the road as cheaply as possible. I'm going to static balance the parts to each other and call that good. (One of the rods looks a little different than the others - the balance pad on the big end was not milled down at all. I'm guessing that the previous rebuilder grabbed a replacement rod from inventory and didn't worry about balance. I suspect that rod will be a tic heavy on the big end.)

Best regards,



Paul Menten

Meridian, Idaho

It could have been the lightest rod.

You'll need to devise a fixture for weighing the rods. They don't simply get matched for weight. You match the big ends then do the small ends (or vice versa - it doesn't matter which end is balanced first). Find the rod with the lightest big end, match the others to it. Find the rod with the lightest small end, match the others to it. When done correctly, all rods will weigh the same.


Lawrenceville, GA

That's what the "extra" material is on each end… for you to subtract from.

       A  L  A  N   F  R  A  K  E  S   ~  Tulsa, OK    

Here is how it's done.

   God Bless.   Tim                           http://yblockguy.com/

350ci Y-Block FED "Elwood", 301ci Y-Block Unibody LSR "Jake", 312ci Y-Block '58 F-100, 338ci Y-Block powered Model A Tudor

tim@yblockguy.com  Visalia, California    Just west of the Sequoias

Tim.  Feel assured that I didn’t take it to mean that you are not in favor of balancing.  I probably came across sounding like it has to be balanced or it will fall apart right away.  In Paul’s case, he is to be commended for at least trying to minimize weight variances in the rods and pistons himself while knowing that he’s not in a position to rebalance the crankshaft at this time.  This is much more than what happens on a majority of rebuilds and at least match weighing what he can will still help in the long run.

Paul.  Don’t be overly concerned about the differences in balance pads as a variance in balance pad heights within a given set is common.  And it’s not unusual for the rod with the largest pad to actually be lighter than the heaviest in a set.  This has to do with how the rod was initially laid out when the machine work was started and the distribution of the weight on the rest of the rod as a result.  Let us know what you actually find on the rod weights in regards to pad height versus the weight and how much variance in grams you actually find from the lightest to the heaviest.

John.  I’ve been told the same scenario on the factory balancing of crankshafts in regards to hole drilling without actually balancing.  Essentially, a rack of cranks would all be drilled the same based on a sample crankshaft that was actually balanced.  I tend to see this on the 350 scrub engines as a majority of factory balance holes appear to be identical both in depth and location.  In all the years I’ve been balancing stock engines, there’s only been a few (three come to mind) that actually would have been okay as delivered from the factory.

Maybe Dennis K. can chime in and give the real story on what at least happened at Ford in the balancing department during engine assembly.

Lorena, Texas (South of Waco)