Technical Engine Balancing question

The machine shop I use doesn't do rotating assembly balancing, they told me "it should be fine"...

Background it I'll be having them change the stock pistons, for a set to gain some CR.

They told me even if they didn't resize the rods, they would match them, as well as verify the pistons matched weight, and the crankshaft would need to be touched.

Got me to thinking-sometimes it's a good thing-how a factory crankshaft is balanced? i/e if the factory is making 1000's of XX type of engine, using the same rods and pistons for each, would the balance change if a lighter piston was used?

I mean if opposing weights on a rod were increased/decrease the same amount, I'd see no reason to balance anything, but a crankshaft isn't a rod.

So after they take care of changing the pistons for me, should I find a shop to take the crank and rod assemblies to and have everything checked? Or if it was fine as a factory engine, should it be fine now?

Thanks.

 

How did it run before taking it apart?

What is the change in individual piston weight ?

 

Rather that answer all your questions let me say this. Factory balance will work in most situations.

When you properly balance an engine first the rods are balanced, then in some cases the pistons weight is matched but if not each pistons rod and ring assembly is weighted and the weight added up then a plumb weight is hung on the rod journal and the crank shaft is spun with weight either added or removed as needed.

Now thiMk and see what you come up with.

 

Then that tells me, if the weight of the piston changes and even if all rod/pistons were matched, it could/would change the balance. based on what the crank was balanced to originally.

 

It ran fine before, I haven't checked deep enough on piston weights yet, I'm running on the assumption aftermarket pistons would be lighter than factory.

 

If you're building a go fast motor and plan on spinning it pretty tight balancing is worth the money if you're just building a drivin around car most of the time you can get away with leaving it alone.

 

Went from forged flat top to a forged 13.4 to 1 dome piston in my big inch small block and did not re balance.
There was some difference in piston weight (lighter) but had good results (spin er up to 6900 between glide shifts)
So far so good

Machine Shops make money every time they touch your stuff

Funny story from my local local shop.
Had one bare double hump head there to replace one valve guide
(because of bent valve) when done the shop owner said bring the valves and other head in because I would like to touch them $$$
Lapped them all in with and had good results

 

The machine shop said they would weigh the pistons. So it should be in balance. No worries. A lot of people don’t understand that the factory balances all engines. But in a slightly different way than a machine shop does. But like said , it’s usually close enough! I alway kept a high school science class type of balance scales around to compare piston and rod weights.






Bones

 

Yes, it should be fine. But it might not. I get the go fast or parts swapped motors balanced, leave the stock rebuilds alone.

 

I"d be looking for another machine shop !!!

^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Squirrel"s advice above is excellent (as always)

Oldmics

 

I used to have a lady friend that was a working girl and she wanted money to touch my stuff.

 

My speed shop ComTech was for many years the best damn balance shop in south Florida,I used SW set up.,and did many championship race cars an boats.
****If your not going to have it balanced by a good shop;
And your putting new pistons in,an or new rods,or flywheel,or clutch,there are things you must check.
The new piston an pin must be same weight as those your removing{even after the valve eyes are cut}. The rods are not just checked for over all,;they are checked for big end match,an that must be done with a jig ,by hanging smallend/pin hole from off scale to jig. after that the pin/small end is matched by cutting only that end,as total rod matched ,all needs to be same{{ both big end an small end=total/and that all needs to be same as stock rods your replacing. {with in 3 gm.s of each other is good}
Flywheels total lbs dose not matter balance wise,but many flywheels are not just balanced by its self{ like a wheel is} but some must be matched to it's crankshaft,as there is counter balance built in to the design<you can not used any flywheel that is not designed for that crankshaft. Clutch pressure plates can be off,but most new replacements are balanced from there factory. Front dampers/pulley also can be a big mistake if not matched to factory engine design an size.
Only if your using all stock parts to the vary engine your rebuilding. If you do not check like listed,you may fine out to late,it shakes bad when running !
Hope that helps anyone in doubt about what can an can't be done,with out having engine done at a good shop.

 

I'm a retired machinist, industrial and automotive so I take my engines pretty seriously, not critiquing your guy but I'd be wary of a shop using the term "it SHOULD be fine".
It probably will be but if it's not and it ends up NOT fine and you don't get it in writing, one of you will forget that conversation and it won't be you.

 

Let me clarify about the shop I use, they build some good engines, but send out for balancing as they don't have the equipment to do it fully, the turn around time was quite a bit (I think they said in the 6-8 week range), when I spoke to them about it, I wasn't real clear on what I was doing, they may have assumed just rebuilding with jobber parts, in fact I didn't talk to them in the shop, it was in passing in the grocery store. The last time I spoke with them when I picked up my block, I mentioned I might bring it back and have it gone through later due to some unexpected $$ coming my way. He may have assumed just a factory .030 rebuild.

I was asking for clarification here, i/e if it should be re-balanced, I'd find another shop to do the work instead of bringing parts from one place then to another. That said, I don't have a relationship with another shop, and as mentioned before they make money via labor. I'm just trying to get informed based on what route I take.

 

Guys, maybe some of you have not been a line mechanic and have only been associated with speed shops. In every shop and dealership in the country , that services regular cars, parts are interchanged every day with out consideration of weight. Maybe not so much now, due to engines lasting so much longer. But in the old days, in most shops, you bought a set of over size pistons to fit the bore, resized the rods( making them slightly lighter) and put everything together, figuring the piston manufacturers build the oversized piston the same weight. And sent the car down the road with a happy customer! I never saw a regular family car engine ever “ balanced “ in the 50 years I worked on vehicles, both on the line and off the line. Don’t get me wrong, I’m not against balancing, it’s a good thing. Just not really necessary in a run of the mill engine. If you are going to spin that engine 7000+, it had better be balanced! Just my experiences!



Bones

 

In Boneyard’s comment above, the key is assuming that the replacement pistons are reasonably close to the same weight as the ones being replaced. In a line mechanic/factory parts situation it’s a reasonable assumption. If you’re doing an independent rebuild you have the time and opportunity to at least compare the weight of the old pistons with the new ones. If there is a significant difference I would consider having the assembly rebalanced. It’s very true that many rebuilt engines go together without additional balancing and run fine, I’ve done my share, but for any car I really care about and plan on keeping for years it’s money well spent in my book.


Sent from my iPad using The H.A.M.B. mobile app

 

If you have ever balanced a reciprocating assembly,,,it can be confusing .
There is a form you can look at on the internet and it will explain it somewhat.
It is not an exact science,,,,and can be somewhat of a black art .
If the pistons are lighter than stock,,,it works out a little better .
But,,,,the engine balance also goes by rpm too,,,,,the sweet spot that is .
Most racing engines are balanced for high rpm,,,since they pretty much live there .
And since the piston weight is only calculated at half value,,,it doesn’t have as bad of an effect on the overall balance .
Example,,,,if the new pistons are 20 grams different weight,,,the assembly only sees 10 grams
difference in the balance .
I always have all my pieces already balanced at home,,,and then I just give the shop the bobweight that is needed and they take it from there .

Also,,,,,like the man said,,,,,almost completely unnecessary in a stock type engine .
Unless,,,it is a mile different from factory in weight,,,(grams ) .

Tommy

 

In racing the term "Over balance" is used quite often. As long as the piston/rod assy weighs less the it was originaly calculated for, you are good. Ive gone thru numerous piston/rod assy in my engine and all with the same crank and the original balance without issue. Most race cranks are over balanced for this issue, although I would think NASCAR is a little more picky because of the length of the race and their engines are one and done and not rebuilt- just sold off after 3 years

 

This is actually directed more to the "unwashed" John Q Public crowd and not those that seek out high quality performance products, FORGED pistons as one example.
In this new age of choosing components strictly by who sells them the cheapest (read that as the Walmart mentality) the level of consistency in balance and size for example is an "anybody's guess" situation anymore.
Assuming things "should work fine" sure can lead to more serious concerns down the road, meaning unexpected $$$ to be spent to rectify given purchases or chosen service providers.

 

If they don't touch the rods, the only thing that changes the total bob weight will be the pistons. Weigh an old piston and compare the weight to the new one and figure out the percent difference and you will know how close you are to the factory balance. This is an over simplification, but it will you an idea.

 

What are you building? A V8 or straight inline 4 or 6 ? It makes a difference V needs balancing. Straight will counterbalance itself.

 

Toxic, I know what you said is true, as I’ve been told that by several learned people and I understand the “ counter balance” as one piston goes up and one goes down. But why when you have a v shaped engine does this not work? They still go up and down at the same( opposite) angle. Maybe I’m dense, but I just can wrap my brain around this! Could you explain? Thanks






Bones

 

Why not buy a cheap digital scale and balance the piston/rings (after file fitting them). Then make a simple rod balancer and balance the rods too. While not perfect, you can learn something beneficial and it definitely will be better than doing nothing. A scale should cost less than $30. Cobbling up a rod balancer can be done cheaply too.
Basically something to support one end of the rod while the other end sets on the scale. Little effort and not much money. Heres one I made out of some kind of old camera stand or something I bought off Craigslist or Ebay. Works pretty good.

 

To balance a straight 6, 4, or 8 you do spin the crank but you do not need Bob weights. You just match the weight of your rotating and reciprocating weight.

To balance a regular v8 you need Bob weights. To calculate bob weights you use 50% reciprocating and 100% rotating. Plus I used to add 2 or 3 grams for carbon and oil cling.

Rotating weight is the big end of the rod and the rod bearing. Reciprocating weight is the piston, pin, rings and rod small end.

Balanced engines are great! I recommend it.

 

Ballancing can get a little complex, I have the equipment and do it regularly .
With the new age of affordable electronics it is easier and more accurate than ever.
Not to get too deep in harmonics and frequency balancing makes everything work together without trying to pull itself apart .
Rule at thumb is 10 grams different in a part change you need to rebalance. The force that is generated on parts when imbalance is there is staggering,they will work if you don't balance them but they will live longer and happier if you do.
Balancing is focused to a specific rpm through math,you can raise that target rpm or lower it by changing the percentage of the balance factor. 50% is pretty much the automotive standard,race and higher rpm then this changes.
So at that exact rpm your engine is balanced,above and below is a compromise from that point. How much,depends on how close and exact the parts are to each other.
if a balanced engine has had a piston or rod change and all things remain the same with math you can figure the balance factor if it went up or down,i.e. 48%or up to 53% so the target rpm moves up or down. It's balanced somewhere as long as it all the same weights with the replacements heavier or lighter.
When weighed out and blueprinted not much,different piston weights to each other and rod weights can make a bad situation worse.
It's hard to determine oil cling to the piston on a running engine so some stuff uses common sense to add and subtract when calculating a bob weight .
Older factory balance jobs leave a lot to be desired.they can be as high as 100 grams off. Motors didn't live as long,newer stuff,ls base are pretty spot on.
I won't even get into poor machine shops that fill a crank full of holes just to have harmonics destroy everything that rotates,I have seen some really nice expensive parts destroyed by shops who say they can balance engines.
Hope this helps you understand it better, I get 320.00 to balance a rotating assembly inc flywheel and balancer. Heavy metal and counterweight machine work adds to the costs.
It's money well spent when you understand what the benefits are.

 

And we're both in the same town, albeit 3500 miles apart!

 

What are the chances,didn't even notice. Now that's funny shit right there.

 

The thing about balancing an engines pistons,rods,and rings is that someone doing it at home has plenty of time to get everything as close to perfect as possible. A professional is going to get it as close as possible in the alloted time he has to do it. For the most part thats good enough.
Myself, I would rather take my time and get them all as good as I possibly can, then take them to a shop and have the crank balanced to them. You need to file fit the rings and assign them to a piston and weigh the completed Piston/ring/pin assembly and match all their weights. A machine shop is capable of doing that, but they can only spend a certain amount of time doing so. This is not a knock on them, because many do good work and the product they put out works OK.

 

Last engine I assembled was balanced , 3 pistons &all the rods showed signs of material removal, none on the crank , all aftermarket parts I'm betting it was worth it but not having to change the crank weight was encouraging ....

 

It boils down to (your) dollars/reserved shop time, you step up to the window and tell them how much and how long, a line of communication with the shop owner discussing your wishes goes a long way in both partys being happy with the outcome.