Changing the bob weight after balancing

[msundstrom]

Are there limits to how much you can add or subtract, primarily to the piston weight, after it has been balanced without rebalancing? Example: cut valve reliefs deeper on a bbc @ 7,000 rpm?
Thanks
Mark Sundstrom

[Frontenginedragsters]

Mark:
 You can cut for valve clearance without a problem assuming your only removing .020 or .030" to intake OR exhaust.
That usually only takes off about 2-3 grams. That puts you into "overbalance" and that is RPM friendly.
If you have to cut both [int. & exh] then you better cunsult the guy that balanced it.
Weigh the pistons before and after cutting then call your balance shop.
Often times we have to cut valve pockets to enlarge the diameter as well.
Im sure you know now but its always a good idea to mock-up the motor first before balancing.
Matt

[msundstrom]

Thanks. I'm considering a cam/head change on my spare short block.