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If you replace the stock rods and pistons with Jim's lightweight kit, and don't rebalance the crankshaft, the BF only changes by a few percentage points. When I did the calcs using the average weights I've measured over the years for stock parts, compared to the weights of one of Jim's 750 kits I bought quite a while ago, the BF only changed from the stock 52% to 56%. The weights of the stock parts varied some over the years, and so might Jim's kit, so that's not a precise calculation, but probably accurate within 1% or so. Probably not enough to notice. That's actually kind of convenient, because you can get the lower force from the lighter reciprocating weights without the bother of re-balancing. On the other hand, if you were doing this to a Commando engine, and then mounting it in a featherbed frame, I'd think you would want to re-balance the engine to a higher BF anyway. But then that's what this thread is trying to sort out, isn't it?

Ken

<blockquote data-quote="lcrken" data-source="post: 295885" data-attributes="member: 1258">If you replace the stock rods and pistons with Jim&#039;s lightweight kit, and don&#039;t rebalance the crankshaft, the BF only changes by a few percentage points.&nbsp; When I did the calcs using the average weights I&#039;ve measured over the years for stock parts, compared to the weights of one of Jim&#039;s 750 kits I bought quite a while ago, the BF only changed from the stock 52% to 56%.&nbsp; The weights of the stock parts varied some over the years, and so might Jim&#039;s kit, so that&#039;s not a precise calculation, but probably accurate within 1% or so.&nbsp; Probably not enough to notice.&nbsp; That&#039;s actually kind of convenient, because you can get the lower force from the lighter reciprocating weights without the bother of re-balancing.&nbsp; On the other hand, if you were doing this to a Commando engine, and then mounting it in a featherbed frame, I&#039;d think you would want to re-balance the engine to a higher BF anyway.&nbsp; But then that&#039;s what this thread is trying to sort out, isn&#039;t it?Ken</blockquote>

[QUOTE="lcrken, post: 295885, member: 1258"] If you replace the stock rods and pistons with Jim's lightweight kit, and don't rebalance the crankshaft, the BF only changes by a few percentage points. When I did the calcs using the average weights I've measured over the years for stock parts, compared to the weights of one of Jim's 750 kits I bought quite a while ago, the BF only changed from the stock 52% to 56%. The weights of the stock parts varied some over the years, and so might Jim's kit, so that's not a precise calculation, but probably accurate within 1% or so. Probably not enough to notice. That's actually kind of convenient, because you can get the lower force from the lighter reciprocating weights without the bother of re-balancing. On the other hand, if you were doing this to a Commando engine, and then mounting it in a featherbed frame, I'd think you would want to re-balance the engine to a higher BF anyway. But then that's what this thread is trying to sort out, isn't it? Ken [/QUOTE]

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