Weighing up the parts
Balancing your rotating assembly is critical, not only to ensuring you
have a reliable race engine that doesn’t shake to pieces, but also to
unlocking hidden horsepower and rpm. Balancing the piston and rod
assemblies is work that can be carried out in the home workshop with
minimal tools, if you have the patience. You’ll need scales that read
down to a tenth of a gram. We purchased a set of cheap digital scales
— seriously, these things only cost $12 and the accuracy was fine.
Most aftermarket pistons will arrive with a weight tolerance within
a gram of one another. Our Wiseco pistons came in at 337grams each,
ranging from 337.1 to 337.8. Each time you weigh, make sure you repeat
it two or three times to ensure you’re getting an accurate readout.
Next, weigh the wrist pins. In our case, these weighed between
73.1 grams and 73.3 grams. Add to these the weight of a ring pack
— in our case 7.1 grams — and the wrist pin lock pair weighing 1.9
grams. It’s this combination that you want matching to a tenth of a
gram. Before getting the grinder out, we played around by adding the
heaviest pin to the lightest piston, and so on, to even everything out as
much as possible. Once you have these four combined weights, you
can work out how much meat will need to be shaved, with the goal of
matching the lightest combo — which in our case was 411.0 grams.
We chose to chamfer the inside edge of the piston skirt, as
removing small amounts from there will not affect the strength. A die
grinder, a drill with carbide bit, or a sharp deburring tool will do the
trick. Please note: you need to take equal amounts off each side of the
piston, and take only small amounts at a time to avoid over lightening.
Weight can be removed from the inside of the wrist pin if you need to
remove a large amount of weight. If you choose this technique, a die
grinder is the best course of action.
Weighing the rod is a much more complicated process, as you
need to make up a jig in order to suspend the rod so that you can weigh
each end independently. You can purchase a rig, make something
pretty fancy or, like us, use a few pieces of wood to make a temporary
rig; all can offer you accurate results. The rig needs to allow the rod
to slide on either way round, and hold the rod horizontal with no bind
while also preventing it from twisting and then preloading the scale. We
didn’t redesign the wheel here, but copied what’s available off the shelf.
The main thing to note is that the rod should rotate freely at both
ends to avoid preloading the scale. Some rigs use chain to suspend
the rod, but it’s not as accurate as the swinging rod style. As with the
piston weights, you need to repeat the measurement two or three times
to ensure its accuracy. To shave weight off a rod, you need to be careful
where you are taking weight from. On the journal end, you shave via
linisher evenly from both sides. For the small pin end, linish from the
outside radius as evenly as possible.
elle
(Elle)
#1