Does a balanced kit need rebalancing if changing out pistons?

[fairlaniac]

Buying a balanced 4.25 stroker kit that was a 445 w/4.08 bore. I'm buying it all except pistons as I'll need to get 4.26 bore pistons. Can any of the previous balancing be saved? New and never installed.

Thanks,
Doug

[gt350hr]

    Doug,
        I deal with this every day and the general feeling is plus or minus 10 grams ( on the overall set) is not a problem. The old standby 50% balance ( rotating to reciprocating) factor is still the most common but many over or under balance based on rpm and power factors. This "can" move the "harmonic" point of a given engine out of the operating rpm range and keep it from failing prematurely. The idea is to keep the engine out of that rpm or in it as little time as possible. It IS important that the replacement set all be within a gram or two of each other. You don't want a ten gram variance in the "set".  A sure sign you have a balance OR harmonic problem is little things like valve cover bolts that won't stay tight , etc.
    Randy

[fairlaniac]

    Doug,
        I deal with this every day and the general feeling is plus or minus 10 grams ( on the overall set) is not a problem. The old standby 50% balance ( rotating to reciprocating) factor is still the most common but many over or under balance based on rpm and power factors. This "can" move the "harmonic" point of a given engine out of the operating rpm range and keep it from failing prematurely. The idea is to keep the engine out of that rpm or in it as little time as possible. It IS important that the replacement set all be within a gram or two of each other. You don't want a ten gram variance in the "set".  A sure sign you have a balance OR harmonic problem is little things like valve cover bolts that won't stay tight , etc.
    Randy

Thanks Randy! I am not sure what you mean "ten gram variance in the set"? From lightest to heaviest?

Thanks!
Doug

[Falcon67]

Also - depending on weight - the piston mfg should be able to get close to what the older pistons weigh.  I considered this when I bought my 3.85 stroker for the 351C.  If I get lucky and are able to buy a 4.125 bore block, I should be able to order pistons for a 4.125 at the same or nearly the same weight as the 4.030s. 

[gt350hr]

    Doug,
        I deal with this every day and the general feeling is plus or minus 10 grams ( on the overall set) is not a problem. The old standby 50% balance ( rotating to reciprocating) factor is still the most common but many over or under balance based on rpm and power factors. This "can" move the "harmonic" point of a given engine out of the operating rpm range and keep it from failing prematurely. The idea is to keep the engine out of that rpm or in it as little time as possible. It IS important that the replacement set all be within a gram or two of each other. You don't want a ten gram variance in the "set".  A sure sign you have a balance OR harmonic problem is little things like valve cover bolts that won't stay tight , etc.
    Randy

Thanks Randy! I am not sure what you mean "ten gram variance in the set"? From lightest to heaviest?

Thanks!
Doug

   Yes exactly. + or - 1 gram in a set of 8 is an everyday standard at many manufacturers.

[gt350hr]

Also - depending on weight - the piston mfg should be able to get close to what the older pistons weigh.  I considered this when I bought my 3.85 stroker for the 351C.  If I get lucky and are able to buy a 4.125 bore block, I should be able to order pistons for a 4.125 at the same or nearly the same weight as the 4.030s.

    Chris ,
       Any time you increase bore size it becomes difficult to match weight. ESPECIALLY if you are switching manufacturers as forgings vary greatly from company to company. Some parts are so "near net" that weigh can not be matched when increasing bore size .
     Randy