BBC piston weight change

[rjpower]

I have a BBC 461 in the shop that I did 10 years ago and some how the guy grenaded a piston .
The replacements are 27 grams lighter .Is this enough of a change to effect balance ?? 6K Max on RPM.

[mag2555]

Find out how much the pins weigh in the new pistons and then look for replacements that are heavier.

The cost of 8 new pins might save the guy the cost of another balance job.

[BillK]

rj,
What pistons are they ? I have had some luck in the past finding single older pistons on E-bay. Also like mag said you can try to find a heavier pin.

[Old School]

From what I am reading you only need one piston. The .150 wall pins weigh roughly 152 grams. The .180 wall pins weigh around 180 grams. Replace the .150 pin with the new piston with a .180 assuming all the other pistons have .150 pins.

Another way to look at it. I have a customer that insist I overbalance his engines 1 percent. The bobweights are around 2300 grams. One percent would be 23 grams. He has never complained about his engines shaking.

I don't think you would be able to tell the lighter piston was in the engine. I would still put a heavier pin in and get it closer.

[mag2555]

I would still like to know how one piston went south if it was not from a valve dropping!

[Walter R. Malik]

I have a BBC 461 in the shop that I did 10 years ago and some how the guy grenaded a piston .
The replacements are 27 grams lighter .Is this enough of a change to effect balance ?? 6K Max on RPM.

Use a heavier pin ...

[PackardV8]

From what I am reading you only need one piston.

I read it he's replacing all eight pistons and asking is rebalance necessary.

Another way to look at it. I have a customer that insist I overbalance his engines 1 percent. The bobweights are around 2300 grams. One percent would be 23 grams. He has never complained about his engines shaking.

Agree, rebalance is not absolutely necessary. Choosing 50% of recip is the general rule, but a couple of percent (or more) either way is common practice for certain applications.

[rcull]

Just a thought, what about using the current pin (you have it) and putting a 27 gram slug in it. Perhaps a light fit with some Loctite?

[Walter R. Malik]

I have a BBC 461 in the shop that I did 10 years ago and some how the guy grenaded a piston .
The replacements are 27 grams lighter .Is this enough of a change to effect balance ?? 6K Max on RPM.

Use a heavier pin ...

I missed that ... I thought you were only replacing One.

Unless the engine was already "overbalanced" I would not worry about it but, you could get 8 heavier pins if it bothers you.

[rjpower]

Thanks for the replies. yes I'm changing all 8 pistons to forged. It had KB 207's in it and is a strong runner
but it sits for long periods ( bad Gas?) and I get ZERO feedback from this guy. Says It just blew up. Yeah right.
I did the install and tuning and there are no signs of detonation. BUT the piston that failed completely crumbled,
The pieces were like gravel. Then of course the pin (press pin) punched a big bole in block. I do not think this thing
has 1500 miles on it. The rest of the parts looked like new. It only bent the 2 valves in bad cylinder.
Thanks RJ

[BillK]

Thanks for the replies. yes I'm changing all 8 pistons to forged. It had KB 207's in it and is a strong runner
but it sits for long periods ( bad Gas?) and I get ZERO feedback from this guy. Says It just blew up. Yeah right.
I did the install and tuning and there are no signs of detonation. BUT the piston that failed completely crumbled,
The pieces were like gravel. Then of course the pin (press pin) punched a big bole in block. I do not think this thing
has 1500 miles on it. The rest of the parts looked like new. It only bent the 2 valves in bad cylinder.
Thanks RJ

Is the engine fuel injected ? I did a Mercruiser 454 recently that had one burnt up piston and it turned out to be from a bad injector. Maybe a vacuum leak to that cylinder somehow ?

[PackardV8]

Says It just blew up. Yeah right. . . . there are no signs of detonation. . . the piston that failed completely crumbled, The pieces were like gravel. Thanks RJ

IIRC, the KB207 is hypereutectic and it doesn't take a licking and keep on ticking. Even momentary detonation from stale gas might have been enough.