I have to replace pistons in my enigne that I had originally balanced. The same pistons arn't available anymore, but others are. Problem is, the replacements weigh in at 520 grams, and the originals were 597 grams.

Engine balance = 3 things

1/ Rotating balance (crank, flywheel, big rod-end, etc.)

2/ Reciprocating weight (piston, small rod-end, etc.)

3/ Equal balance (all pistons are the same weight, etc.)

Although I kind of understand the above, I don't have a feel for what is an acceptable deviation, and what's not.

So.....how much will changing piston weight throw off my engine balance and how much can I get away with before it becomes significant?

77 grams is a alot it really should be balanced to be right.

 

How much can I get away with?

around 20-30 grams you will see and feel vibrations.

 

20-30 grams? OK, that sort of makes sense. I was curious though. One would think you could get away more with reciprocating weight changes vs rotating balance changes.

I'm being a bit lazy (and cheap). I certainly could buy other weight pistons and have everyting re-balanced. I also have a set of +0.040" pistons that may be more or less the weight I need, but I don't need (want) to re-bore at this time.

If I remember correctly, Ford recommended a balance piston weight at 660 to 666 grams. I'm way off from that.

Thanks Guys !!!

 

Roll the dice

 

OK...I found out a bit about balancing. Basically a whopping 50% of the reciprocating weight (primarily the piston and con-rod small end) and 100% of the free rotating weight (primarily con-rod big end) is used to calculate the bob-weight. The bob-weight is added to the crank for balancing the rotating assembly. Therefore, the weight change of a piston will be significant to the balance. Makes sense Ford once said to limit it to +/- 0.5%

 

520 x .05 = 26 gee seems that was mentioned somewhere...... :thumbup:

 

Nice try OHD, but not quite right.....LOL. 0.5% is 0.005 and that works out to 2.6 grams. HOWEVER, in the end you are most likely on the mark. If 2.6g is allowable, then it is likely that the allowable amount is 1/10 of the significantly bad, or noticeable (shaking) amount, which would then be your 26g. So feel pleased with yourself. I do believe you are on the money! Thanks!

 

With having over 70 grams difference in your pistons, as long as you don't start your engine, you shouldn't feel any vibration at all..... thats over 13% difference. Kinda like havin all your clothes on one side of your washing machine! Better rebalance it.....

 

My 2 cents (for what it's worth)

Around $200 USD to get everything re-balanced. Should be no problem beings the replacements are lighter than the originals, they only have to remove more weight, not add heavy metal.

Or, save the $$$ and put it twards a rebuild IF the engine fly's apart.

I read somewhere (not to sure on the actual numbers) that 70ish grams is equal to something like several hundred pounds on a rotational axis of the like that we have in our motors.

 

520 x .05 = 26 gee seems that was mentioned somewhere...... :thumbup:

1/2 % is .005 :thumbup:

Generaly speaking, overbalance for high rpm operation will run up to 30 grams per bobweight/piston.
That means a heavier bobweight is used, which will leave the crankshaft counterweight heavier than the piston side.

 

Decisions, decisions.....it's stressing me out!!!!......LOL.

I have 3 blocks, two 428 cranks, and 1 set of +40 pistons. Two blocks are 410, and bored +30, but one would have to be bored to +40 to make it fit for use. The other is a "never before bored" 428

The current block......I can't get +30 pistons unless they are so light I need to do a rebalance. I don't need to rebalance if I use the +40s I already have. But it goes against my grain to bore a block with perfectly good cylinders.

Oh.....for interest...here's the weight specs I measured last night. Very confusing, but I assure you they are correct....I did them all twice as it looks like the results are backwards....but they arn't...

Size_______________+30___________+40

Piston______________598___________596
Rings______________ 64.4___________61.3
Snaprings____________2.7____________2.7
Wrist Pin____________151.5_________186.5

So, the +40s should be able to replace my broken +30s as long as I use the +30 wristpins. If I do that, it's only 5 grams difference, plus it's negative, which is probably better than positive.

If I use either of my other two blocks, I will need a new cam and lifters.....or at the very least, new lifters.

If I rebalance my already balanced crank for new lighter +30 pistons, then it will be way out if I eventually want to use it with the +40 pistons or +30 428 pistons, both of which are extremely close to what I have now for balance.

Then there's my rusty +30 block....very nice shape....has the extra webbing. Should make a strong engine.

And my original bore 428......I want to save that for a rainy day....or maybe it's starting to drizzle out?.....hmmmm

Then there's that other 428 crank......can't remember....it was either an A or a B, but regardless, from some sort of Cobra-Jet. It needs to be ground...rusty journals. I hope it's straight.

Current crank......bad story.....I bought it ground 20-20...tags still on it. Took it to my Machine Shop and they found taper along the journals......new grind to 30-30. First block cracked and antifreeze took out the crank bearings. Crank now at 40-40, and that's it....

Anyway...not sure if I am asking anything or not, but it helps for me to write it all down like this. Comments are most welcome though.

 

Decisions, decisions.....it's stressing me out!!!!......LOL.

I have 3 blocks, two 428 cranks, and 1 set of +40 pistons. Two blocks are 410, and bored +30, but one would have to be bored to +40 to make it fit for use. The other is a "never before bored" 428

The current block......I can't get +30 pistons unless they are so light I need to do a rebalance. I don't need to rebalance if I use the +40s I already have. But it goes against my grain to bore a block with perfectly good cylinders.

Oh.....for interest...here's the weight specs I measured last night. Very confusing, but I assure you they are correct....I did them all twice as it looks like the results are backwards....but they arn't...

Size_______________+30___________+40

Piston______________598___________596
Rings______________ 64.4___________61.3
Snaprings____________2.7____________2.7
Wrist Pin____________151.5_________186.5

So, the +40s should be able to replace my broken +30s as long as I use the +30 wristpins. If I do that, it's only 5 grams difference, plus it's negative, which is probably better than positive.

If I use either of my other two blocks, I will need a new cam and lifters.....or at the very least, new lifters.

If I rebalance my already balanced crank for new lighter +30 pistons, then it will be way out if I eventually want to use it with the +40 pistons or +30 428 pistons, both of which are extremely close to what I have now for balance.

Then there's my rusty +30 block....very nice shape....has the extra webbing. Should make a strong engine.

And my original bore 428......I want to save that for a rainy day....or maybe it's starting to drizzle out?.....hmmmm

Then there's that other 428 crank......can't remember....it was either an A or a B, but regardless, from some sort of Cobra-Jet. It needs to be ground...rusty journals. I hope it's straight.

Current crank......bad story.....I bought it ground 20-20...tags still on it. Took it to my Machine Shop and they found taper along the journals......new grind to 30-30. First block cracked and antifreeze took out the crank bearings. Crank now at 40-40, and that's it....

Anyway...not sure if I am asking anything or not, but it helps for me to write it all down like this. Comments are most welcome though.

If you would have posted this info in your first post, it would have made the answer to your question much simpler! Swap wrist pins and GIT'ER DUN!

 

In How to build max performance David Vizzard states that being overballanced can actually be an advantage, and that in highly stressed engines this is done deliberatley. "as the piston reaches BDC the extra crank weight helps to counter some of the downward pressure loads." He says you can use about 50 grams before there is a dectable vibration, and even with more it will be less vibration than a ballanced 4 cylinder.

That book is a great read. It taught me how to ballance my 327 to within 1 gm variation for the cost of a digital scale and a homemade jig. (The book shows the commercial variation of the jig).

One thing I would add to your 3 steps to ballancing is that you must weigh the small ends and big ends of your rods seperatley, (thats what the jig is for).

If it were me I would get such a jig and scale and get all your weights equally matched and then use the heavy wrist pins as mentioned.

Man.. I wish my 327 pistons were that light!!!

 

Well.....I didn't know all the details when I first posted.....i.e. the weights of the +40 pistons I already have.

Those "replacements" that were too light I mentioned, ... were what I could find available, but havn't purchased yet.

I hate boring out a block with perfectly good cylinders, so I have been tempted to buy the light +30 pistons, rebalance the crank (which is already a 40-40 grind) and run it till it drops. Subsequent to that, use my other 428 crank and other parts.

However, I think you are correct. Makes sense to re-bore and go with the +40s for now.

 

OH...while I was typing, another post came through!!!

I have some books by Dave Vizzard, but not that one....I must look it up. Thanks!

 

What's the big deal?
Generally balancing only runs about $ 120-150.

 

The BIG DEAL is this:

If I rebalance by hard-to-get 428 crank for new-technology light-weight pistons, then it will be no good for my +40 pistons (which weigh the same as the broken +30s) in the eventual need for a re-bore. It will also be no good for any 428 pistons (which also run about the same as my broken +_30s).

So, why not balance it again (to these new light pistons)? Just ticks me off as I will eventually need to do a really major (expensive, mallory metal) rebalance to a well used 40-40 crank when I need to go to +40 or larger.

Anyway, the fact is 428 cranks are hard to get. A little easier now as SCAT makes them, although I've no experience with any of their products. So, one doesn't jsut do the first thing htat comes to mind.

Similarly, one just doesn't bore an FE block with wild abandon either. They just arn't that common anymore.

There are days I wish I was a SBC guy...so easy...so cheap........sigh....LOL.

 

I am not sure where the Mallory idea came from.
If a crank has been lightened for certain pistons, and then you want to install heavier pistons later, all you do is weld up the drilled holes until the crank comes into balance.

Also if a lot of additional weight is necessary to a crank that has balance holes in it, then lead is added to the existing holes, capped by a steel cap which is then welded into the crank.

If the +.040 pistons were already too heavy, then the intermediate balance would be irrelevant, wouldn't it.
FE 428 balance can be effected by altering the eccentric weight if necessary.
I personally have run the balance machine when balancing lots of engines.

 

If you don't rebalance you are just asking for trouble. You may wipe out the bearings in short order, which, at a minimum is going to cost you new bearings. You may also find or experience further damage due to the unbalance/imbalance. JMO imp:

 

ScotF: Yes, you are right. I never thought about just filling in the drilled hoels with "weld", so I suppose it's not a big deal to get the crank re-balanced. The +40 pistons should work (without rebalancing) as they weigh within 5 grams of the existing broken +30 pistons.

Stroke: Your comments are well taken. In fact, the purpose of my post was to try to establish some limits to acceptable balancing. I've already had this engine balanced, so if all I am doing is replacing pistons, rings, snap rings and wrist pins, I can weigh them and compare to my existing parts. Then the question comes, how far out can I be from the original numbers and still have a reasonably well balanced engine? Well, from what I can see, Ford called for +/_ 0.5%, which is roughly 5 grams total difference. At least one response to this thread has stated 20-30 grams difference can be felt. So the "acceptable" amount seems to be coming out.

Thank-you everyone for your comments, information and opinions. I appreciate them all.

 

I have to replace pistons in my enigne that I had originally balanced. The same pistons arn't available anymore, but others are. Problem is, the replacements weigh in at 520 grams, and the originals were 597 grams.

Engine balance = 3 things

1/ Rotating balance (crank, flywheel, big rod-end, etc.)

2/ Reciprocating weight (piston, small rod-end, etc.)

3/ Equal balance (all pistons are the same weight, etc.)

Although I kind of understand the above, I don't have a feel for what is an acceptable deviation, and what's not.

So.....how much will changing piston weight throw off my engine balance and how much can I get away with before it becomes significant?

Reading thru this I have a couple comments:

First, over balance, this is acceptable in a high RPM engine around 6000 RPM and up it smooths the engine a bit and some people report seeing a little extra power. Below 6000 the engine will shake like crazy.

Second, is amount. Typically a couple grams is about the widest tolerance. This is less about how much out of balance will it take for you to feel and more about how much it's trying to make the crank find an instant center that's not on the main bearing centerline. These loads are reacted by the bearing webs into the engine proper. They are an alternating loads which is seen as using up the structure's fatigue life, rather like bending a paper clip back and forth till it breaks. The engine has a lot of mass, it can take quite a bit of out of balance before you feel it, but the bearing bulkheads and crank see it all the time.

You're in a position where the new pistons are lighter, you can go a couple different directions:

A) Find a pin heavier by the amount needed, thus avoiding re-balancing.

B) Re-balance, the lighter piston will result in metal removal from the crank counter balances, this is a lot cheaper process than if weight had to be added. The lighter crank assembly takes load off the bearings and will accelerate quicker.

Bogie