Assembling an engine is a straightforward gig, but plenty of steps need to be taken prior to stuffing your choice of block with the latest aftermarket goods. One of the more important pre-assembly jobs is to make sure that your rotating assembly is properly balanced and ready to buzz in perfect harmony.
Bear in mind that unless you purchased the crankshaft, rods, and pistons as a matched set from a manufacturer, this is a procedure you don’t want to miss. If you happened to purchase a used rotator, unless the seller is willing to give you an expressed guarantee that it’s drop-in ready, you will be much better off taking it to a machinist and having them check it for you.
As for the cost, most balance jobs price in at around $200 and typically take up to two hours to complete—of course, this is assuming everything checks out clean. If weight has to be added for a perfect balance, you can expect the price and the amount of time it takes to get the job done to go up accordingly.
Follow along as we divulge the complete details on what it takes to balance a rotating assembly. It isn’t a difficult job for the machine shop to tackle, but definitely one that needs to be addressed before assembling your next powerplant.
Rocco Acerrio of A.R.E. Performance...
Rocco Acerrio of A.R.E. Performance & Machine in Simi Valley, California, explained that the bob weight is made up of the two different factors, namely the rotating and reciprocating weight. We started by weighing the big end of the rod, which is part of the rotating weight by using a special fixture to position it correctly. Each rod is then matched to within a 1/2 gram of the lightest rod.
The other component of the...
The other component of the rotating weight is the rod bearing inserts. An additional factor is the estimated oil weight; for most street applications, this is estimated at 4 grams whereas high-performance combinations can vary between 2-4 grams. This is contingent on naturally aspirated and boosted applications.
For the reciprocating weight,...
For the reciprocating weight, we start with the small end of the rod. To ensure that we only get the weight of the small end, the same fixture is used again, along with a similar procedure to match its weight.
Each piston is then placed...
Each piston is then placed on the scale to add its mass to the bob weight. The lightest of the set is the number used on the bob card. The others are then machined to match within a 1/2 gram.
Wristpins can also vary in...
Wristpins can also vary in weight, so they’re weighed individually. The lighter wristpins can be matched with the heavier pistons to help balance the weights. For this reason alone, the wristpins should stay as a set to their matched pistons; a reputable machine shop will always have these labeled accordingly.
The next step will vary, depending...
The next step will vary, depending on your choice of bushed or pressed connecting rods. A bushed rod requires a wristpin lock that also needs to be factored into the reciprocating weight. A pressed-style rod clamps onto the wristpin itself, eliminating the need for any locks to hold it in place.
The entire ring pack, namely...
The entire ring pack, namely the top and second rings, along with the oil rails and expanders are measured as a complete unit. Generally, most rings are matched so well that you only need to weigh a set for one cylinder to get the info needed.
After all the components are...
After all the components are measured and entered, the computer generates the information needed to assemble the bob weights.
Here’s the complete set of...
Here’s the complete set of parts to assemble the bob weight. These vary in thickness of weights, including small washers that weigh as little as a 1/2 gram.
To assemble the bob weight...
To assemble the bob weight to the specs calculated, you need to first assemble the half bob weight as shown on the initial specs at 904.9 grams (Caption 09).
With the bob weight assembly...
With the bob weight assembly sorted, these are then placed on the crankshaft journal. Once complete, both halves equal the full bob weight specs.
After spinning the crankshaft,...
After spinning the crankshaft, the computer starts to calculate the amount of weight and the location of where weight will need to be added or removed. In this instance, a 1-inch drill is used to remove weight.
Once the weight is removed...
Once the weight is removed or added, the machine then spins the crankshaft again to check the tolerance. If the specs are met within a 0.2 ounce/inch, then the job is complete. If not, the computer will tell you where to make the additional adjustments.