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 Interesting crankshaft balancing

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Posts : 242
Join date : 2020-04-21
Location : Andover, Minnesota

Interesting crankshaft balancing Empty
PostSubject: Interesting crankshaft balancing   Interesting crankshaft balancing Icon_minitimeSat Mar 13, 2021 9:06 pm

On the current engine build I was going to KB120 pistons and Scat rods as the pistons are design and material I wanted and the stock rods were  trash.  From what I had read in a lot of places, balancing would be tough and require a bunch of mallory.  55g per piston weight difference as the pistons are generic SBC units weight for gen1 engines, not lightened like the gen 2.  If I needed a journal grind and mallory it was less expensive to just get a new gen 1 weight crank.   Well, crank has been backordered for weeks, and stock crank was in remarkable shape and near high limit like I prefer on the street with stock volume and pressure pump.  I took it to the shop and they agreed, just a polish and would weight as needed.  As it turned out, no mallory at all, and he got the weight by basically just filling the original balance holes with iron plugs, so not much cost at all.

They way they did it was a shock to me, but this is a high end race engine shop with a perfect reputation and everything they have done for me has always been exact. They press in a plug of the right weight and tig it with cast iron specific rod.  I had ALWAYS heard you never, every, weld on a crank except to spray the journals.  He said they have been doing it for years without failures and in 7000+ racing engines.  He was working time and materials, not full engine on quote, so there no financial gain for them by doing it, and they are straight shooters besides, so no reason not to believe him.

I was actually surprised at the labor costs for the work they did for this block.  Cam bearings, deck to zero (9.001) and parallel off of mains, bore .030 hone to select fit clearance to pistons by hole and finish to match rings specification, so pistons are hole specific although the KBs look to be very consistent so not much difference, I think. Rods individually balanced to match on both ends so interchangeable in holes if I want to tweek piston height and actual length vary a bit, neutral balance new damper.  Polish journals.  About $750. As a fill in small job, it was weeks wait to get to it, plus crank was backordered anyway, but the actual work was done in a day or less.

Bottom line is that if your stock crank is good, there really is not a major penalty to going with higher quality but heavier pistons that are better suited to your build.  Scat crank would still probably best benefit is your original needs journal grind and polish, though, even though the difference is much smaller than it would have been if mallory was needed.

Now on to my most hated part of it all....port location and size optimizing for intake to heads and heads to exhaust manifolds.Sad

on edit
I looked around a little and it does seem to be now common way for quality work and way faster, easier, convenient to do. Learned something today, which is always good. I assume the old fill with weld still not be desirable or just poured lead.
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