Pick up a set of six pac pistons, 70 style. I have the later low compression ones ? (In the hole about 1/4) on the six pac rods and internally balanced crank. My question should I have it all rebalanced again? Just for street use and occasional blast. Opinions needed

hi


weigh the two types and that will tell if the pistons need adjusted !

Pic of piston top?

There is only ONE style of 6p piston

Me thinks he is talking about the low compression pistons in his motor now with that low deck height

I don't completely understand the question, but if your swapping out stock low compression pistons and rods for 6pak pistons with the big rods you'll definitely need a re-balance. going from lighter to heavier parts makes balance a little more complicated. weight will have to be added to the crank, or dampner, or converter or maybe all.

Why don’t you explain what you have in the motor now (pistons, rods) and what you just bought.
What damper do you have, what converter, etc.

You’ll get better information that way.

Ok here goes, have only a clue which engine the ones I have came out of. No valve notches, the ones I’m getting do. I have a motor home block (1974 or later?) with what looks like same pistons and big rods. They could also be just regular 70-71 hp pistons-big rods too.
But I’ll do what one suggested, weigh both, how much difference can I get away with?
I know what regular balancing is, I have shapers that turn two pieces of steel 7000 rpm but haven’t delved into the science of engine balancing the reason I ask on here. The assembly I have has been internally balanced before. Enough info?

weight will have to be added to the crank counterweights, or removed from the cheek close to the crankpins.
Choose one: Mallory tungsten @ $$$ per gram, or lots of machine work.

In the OP you state the pistons are 1/4" in the hole. I can't think of any stock combo that would be deeper than around .180" down.

Generally, if an engine is shop-balanced there will be stampings on one of the crank counterweights.

It was a guess but I can measure if you need an exact measurement.
As for stampings how about a piece of mallery

I don’t recall ever seeing any 440 steel crank big rod motor being internally balanced or any cast crank being internally balanced, you mention that in your first post.
But, if what you had running ran without vibration you could match the new piston weight to the old. I will assume the six pack piston is heavier since it has more CD.

I would just weigh the piston sets you have and lighten the new (hopefully heavier) piston to match the old.

Isn’t the blueprint spec on these pistons ABOVE the deck ... .010 or so.

And as far as REAL LOW down the hole .. that has to be the 413 industrial motor.... near 250 ... IIRC.

Then there was a huge dish in the piston AND THEN the huge combustion chamber in the head .. like 100 cc PLUS !

Speed-Pro/TRW L2355F has a compression height of 2.061", so around 0.021" down depending on whats been done.

What's the compression height of the new pistons? I don't know what actual factory six pack pistons have, but it must be close.

How much does balancing cost? Easy for me to spend your money, but I wouldn't think twice about having it balanced, provided that it wouldn't require adding weight and there was a "significant" difference in weight between the pistons.

i can see where putting heavy pistons and rods on a cast crank could be an expensive balance. it's easier to balance for lighter parts than for heavier parts. there may be better alternatives.

Ok got them today, both measured up
SP piston is +- 2.085 comp height with a 10.72 deck that comes up zero deck height
Other on is .110 lower or 1.97 ch
crank is steel and has plugs welded in so internally balanced
Pins on SP are thicker so to adjust the weight I could do some lightweight tapered pins
But, I don’t see any signs of balancing on the rods or pistons so just as well get it all rebalanced again
Now I’m torn between putting the pistons in an already good running 440 or use the 70 block I have with perfect standard cylinder bores
And rework another set of 452s. But then I’ll have two extra 440s and no cars to put them in.
Thanks for your help guys

another question; do you want to run pump gas?

'72-later big rod forged crank 440's were internally balanced because the lighter smog piston compensated for the rod weight.

That would explain the motor home engine I have, straight balancer with a chamfered edge.
I will run 93 gas with the right dist curve, just a pro street tubbed 73 cuda, not a daily driver.

They are REAL HEAVY pistons.... EZ pull weight out of them with big end mill OR EVEN a big drill bit using a good drill press.

I remember getting my balance card back when I used a set of the SP 6pk pistons my first build