five7kid

iTrader: (14)

As previously discussed, I'm rebuilding a ZZx shortblock (may be a ZZ, ZZ1, ZZ2, or early ZZ3 - at any rate, it has pink rather than PM rods). The builder that took it as a core saved one of the original pistons in order to be able to compare it to the rebuild pistons that go back in, which are Speed Pro H423NP30 hypereutectics.

He says the old piston weighs 802 grams, and the new ones 680. Now, I don't go around weighing pistons as a matter of course, so I have no clue whether those values make any sense.

He's wondering if that weight difference would cause issues. Normally, he says, he just balances whatever's going together, so he's not sure whether the lighter pistons would be okay with the crank balanced by the factory for the heavier parts.

What thinkest thou?

rx7speed

seems weird to hear you ask a question other then to have someoen else think about something.

but I wouldn't think so as long as all the pistons are balance with one another.
the crank itself should be balanced if spun only by itself right?
I'm thinking by adding weight to the crank wouldn't unbalance an already balanced object as long as all the extra weight you are adding is balanced as well

get what I'm saying?

kind like you take a spinning rod that is balanced
attach two 5lb weights on both sides
as long as both 5 lb weights are balanced to one another you should be good

just don't add a 5lb weight only or a 5lb weight to one side adn a 10lb weight to the other


but don't hold me to it just giving my thought here

Vader

Yes. Dynamic balance.

You have a spare LT1 or 305 crank lying around? It might have counterbalance closer to the lighter piston/rod assemblies, and not require as much time/$$ to balance out once the dummy weights are clamped in place. . That's a fairly significant loss of dynamic mass. If you don't get a dynamic balance, you might have some problems with harmonics.

800+g sounds pretty meaty. Was it a truck engine, or did the machinist forget to strip and clean the old pistons before checking? Pin still in?

Streetiron85

I'm presently dealing with the same problem.
I have a balanced rotating assy and a piston set that I'd like to use which is lighter than the ones that were originally included with the assy.
The balancing shop suggested shopping for a heavier set of wrispins as a possibly more cost effective alternative.
I'm still working on that one.

five7kid

iTrader: (14)

It was a ZZ crate engine. It had been in a pickup, apparently.

I have a 305 crank, but this one is steel. The earlier crates had different pistons than the later, but they were all hypers, so I don't know what was different there (may have been SAE vs. metric groove dimensions).

I wasn't present when he weighed them, so I don't know if it was pin-in or pin-out. Maybe he's just after the extra $125, I don't know. But, I suppose it would be nicer to have it balanced, anyway.

REALPOWER

According to david vizard's "small blocks on a budget" book, he says that balancing is not necessary.

VILeninDM

I was curious so I had to pick up that book.

Vizard says way more than that (p.56-57). To begin with, he said that if you are not balancing rotating assembly, you still need to balance rods, pins and pistons using Comp Cams rod balancer (whatever that is, I can't even picture how that would work). Then do some fancy math (I don't feel like typing all that up right now) and if the number comes close to what the stock crank was balanced at, "it will not be essential to rebalance the crank."

At the end he says, "balancing ... is recommended. But be aware that your motor will be just as reliable and run just as well without balancing."

5-7, feels kinda weird giving you my opinion (usually its the other way around ), but sounds like there is a big difference between those pistons. The good part is that you save 15% of the oscillating mass which will only give you more dynamic power out of this thing, but they are definitely not what was stock in that engine.

If it was me, and I put all that effort into building that thing, I'd would definitely spend extra $125. Now if it was $625 I would have to think about it further.

Streetiron85

Update:

I just got back from weighing my own pistons, and the Silvolite cast outweighed the Speed Pro hypereutectic by over 200g.
It was about 800g to 570g, both with the pin in.
I don't think it will be possible to find a wrist pin heavy enough to offset that weight diff.

five7kid

iTrader: (14)

I don't have Vizard's book. It just has never come up. Most of my rebuilds have been stock or near stock.

I've never seen it, but from what I understand the rod scale weighs each end at the same time - basically two scales side-by-side. You want to lessen the big ends to match the rod with the lightest big end, and when they are all equal, take meat off the small end of the heavier rods until they match the lightest rod (edited - original late-night post was convoluted). The big end plays a bigger part in the rotating balance than the small end.

I went through all this math about 21 years ago in college, but haven't applied it since.

Might make sense just to go through with the balancing.

REALPOWER

I knew there was more to it, i just didnt have the book at my apartment. That was the line I was thinking about. THanks for the clarification.

contactpatch

Clue me in.
57, are you trying to save the expense of an internal balance,
and just have it balanced with the HB and flexplate weights?
.
re internal balance, I would think it would be straightforward
for a machine shop to balance 'lighter' pistons by
removing weight from the counterweights.

tilstad

This is what I'm thinking also. When balancing is mentioned, most of the times it is refered to the "expensive mallory" wheight applied. In your case, no such thing, only removing material from the counter wheights.

On another note, my impression is that a good balanced rotating assembly, and then preferably an internal balance, would make an engine rev high alot safer, due to less harmionics to destroy your seup.

anesthes

iTrader: (12)

I think you'd be crazy not to balance it.

Now, as far as numbers go. I know l98 pistons weigh around 600 grams each, plus about 134 grams for the pin.

Are you using a 1pc gm steel crank?

We just balanced my setup, and the H beam rods, and SRP pistons came out to being a little over 80 grams less than the original setup. (crank looks like swiss cheese now. heh).

Anyway.. Balance it. Make sure he does it with a stock flywheel attached. (since one pc rear flywheels havea small counter weight)

-- Joe

Confuzed1

iTrader: (4)

Short and sweet 5-7...balance it. Seems to me the lighter pistons might change the harmonics and put the "critical" speed in a RPM range you don't want it in. 125 bucks will give you a cheap peace of mind IMO...

five7kid

iTrader: (14)

The question is whether to spend the extra money for an internal dynamic balance.

In a previous thread "Pink vs. PM", I discussed the rod weight - 604 to 608 grams w/bolts/nuts, no bearings/pins. The pistons, although he didn't give me 8 numbers, apparently are all very close (Speed Pro matched, sealed set). I suppose I could have looked this up before: The Federal-Mogul website lists the pistons as 536 grams (their similar forged pistons are 565 grams) - so if his scale is accurate, that means the pins must be 144 grams.

The crank is a factory ZZ crate steel one-piece seal unit. He would balance it with a stock flywheel attached. In fact, he asked me to bring mine down, only problem is it's attached to the 305 right now. Between the two of us, we need to come up with one (he mostly does older two-piece stuff, or the customer has had a flexplate available).

anesthes

iTrader: (12)

Well it sounds like you know what you need to do. Just a matter of getting it down there.

Flexplates at autozone have the same counterweight, and are about 40 bux. buy it, balance, and return it.

-- Joe

five7kid

iTrader: (14)

Yeah, I suppose so.

Another case of "might as well" project cost inflation.

REALPOWER

That seems to be the theme of my project.

five7kid

iTrader: (14)

Additional details and current status:

Piston weights quoted by the builder were with pins. The old piston had been through the cleaning tank and rings removed. He had another 350 piston laying around that hadn't been cleaned thoroughly, it weighed 770 grams with pin. He took the pin out of one of the new pistons and weighed it - 536 grams, right at FM/SP's website stated weight. Pin was 144g.

Bottom line - we're going to balance it. Zoned-out has a Pioneer FRA160 flexplate for $44.99 - I'll get Checker to price-match it for 5% less. Given my thread about "issues" removing the plate from the 305, might as well get a new one (here we go again...). I'll get that to him, we'll get this show on the road.

Next - TH700 front pump change-out...

Vader

Well, really, Mr. T. Did you honestly expect anything different? Why should you be so special? Always add about 30% for those unforseen items and "As long as I'm there" costs.

A cam swap is really never just the $400 for the kit , gasket, and seal.

Damon

Been there, done that. I am surprised by the weight of the original ZZ4 pistons/pins (if they are, in fact, original). Considerably heavier than most of the "stock" 350 pistons/pins I have pulled out (which are usually around 725g with pins). I suspect you'll find that the "ZZ-4" pistons use a wrist pin that is VERY beefy (thick walls). This thicker wrist pin may not account for the entire ~100g weight difference of the piston/pin combination, but I bet it accounts for a big chunk of it.

Stock O.E. wrist pins in a passenger car small block Chevy are usually right around 140-145g. So what you've actually measured with your new Speed Pro hyper slugs/pins seems to pass the sanity test. 535/145g for the piston/pin is pretty common for an aftermarket high performance hypereutectic slug/pin combo. For comparison, check out the similar Keith Black KB106 flattop 350 hyper pistons here...

http://kb-silvolite.com/performance....tails&P_id=156

Weights look familiar?? Yeah, I thought they might.

I have measured the weight of the "thick wall" pins used with some piston sets and seen them up in the 200g+ range. That's a BIG difference just in the pin. Remember that wrist pins are made of steel so it doesn't take much of a difference in wall thickness to add a LOT of weight.

To the question at hand..... if you are TRULY dropping 100+g of weight off the piston/pin you should rebalance. I regularly do 20-30g piston weight difference without batting an eye or rebalancing (~700g stock vs. 680 for KB hyper slugs/pins). But 100g+ is really getting pretty far out there. Fortunately you're going lighter, which means drilling the crank counterweights (cheap) vs. going heavier which means adding mallory metal (expensive).

It is my hope that this post will put your mind more at ease, not cause further stress. Let me know how I did.

Red Devil

Always something. 30% may be too small as well.

Have you thought about a converter? Doesn't seem like you'll need a vig, but a nice S10 one may fit nicely. Maybe a 'vette servo too?


Did I mention... about time. Again.

five7kid

iTrader: (14)

Yep, those values look similiar. Must be on the right track. You done good.

The converter stalls at 2300 with the 305 in the sig. I'm assuming with more cubes, more lift, and less duration that should be close (3000 would work better with the 305 as it is). Vette servo, et al installed, just obtained a tranny that I hope is in better shape into which all this can go - hopefully, the 2-3 WOT shift will act properly now.

Checker matched & deducted 5% for the flexplate, should be in tomorrow. Get that down to him over the weekend, machine work will be probably done sometime next week.

NHRA Division 5 awards banquet next weekend, start cleaning and assembly the week after that. The builder that did the 396 heads takes the month of January off, I want him to go over the heads when the engine comes out, so that would be late January at the earliest. I want to have as much ready to go when the engine comes out as possible.

Jet-Hot the headers - "no time like the present..."

Might be running by March - I hope.

anesthes

iTrader: (12)

Don't like manuals?

Are you decking the block or anything while it's there? (for quench)

Ugg. I'm soo lazy. My block is on the stand and theres about 20 boxes of parts on the work bench. Guess I should start assembling this weekend.

-- Joe

contactpatch

I calculate something like a delta of
10.8 ounce-inches needed to be changed, on both
ends of the engine, with an external balance.
Is that a lot?
.
.
numbers, per throw
122 gram weight diff, times two pistons
50 percent balance factor {all recip}
1.75 inch, half-stroke
--> 213.5 gram-inches
.
....combine two 90 degree throws, multiply by 1.414
--> 302 grams
.
28 grams per ounce
-->10.8 ounce-inches
...........................
When 400s came from the factory,
how much 'imbalance' was built into the flexplate?

five7kid

iTrader: (14)

The block is being decked for clean-up, more than for quench. Not sure how much will be removed.

I love manuals, but the cost of getting the drivetrain capable of withstanding this engine would be.... well, there we go again.

My 16 year old has taken an interest in this project. I'm sure dormancy won't be an issue. He's got it in his head that he should be able to race the Camaro while I race the '57.

REALPOWER

Speaking of dorms, I hauled a 305 block up to my dorm room last year. Yep, it was a great door stop.

Air_Adam

COOL!!! That actually sounds like fun... I wish I coulda raced with my dad, but he quit many moons ago

five7kid

iTrader: (14)

Give him a reason to get back in it, then.