Dec 17 PM, rebuilding the 42UL

Before sending out the Jugs and pistons to get coated, I weighed the pistons so that I could later balance the flywheel ass’y. I use the S&S setup which is a static balance routine. You weigh all the rotating parts, and all the reciprocating parts, and use the weights to determine the weight of the counterweight to be used for balancing. You use all the weight of the rotating parts, crankpin, bearings, and nuts. plus a portion of the weight of the reciprocating parts, conrods, pistons, pins, rings. As the conrods go up and down as well as round and round, you weigh the big end and small end separately to add to the two different weight categories. Here’s some pics of the process:

Here we see the bobweight used in balancing a flywheel weighing in.

In the above pics, you see the setup for weighing objects. Just a digital kitchen scale used for cooking from Wallyworld. The Plate it sits on has been leveled to maximize accuracy for the scales and later when the balancing jig is on it:

When you’ve weighed everything, you balance for all of the rotating weight, (crankpin,bearings, nuts, etc. and a portion of the reciprocating weight. The percentage S&S recommends is 60%. The 1301 Gms shown as the weight of the Bob weight is one half of the balance weight that the above math gives us. That’s because we’re balancing one flywheel at a time with the S&S jig.

Driveside flywheel, unbalanced with temp. weights added

Here you see the drive side flywheel with the bobweight bolted on in the balancing jig. All the chunks of bolts sticking to the rim of the flywheel are temporary weights held by tiny supermagnets to give me an approxomate idea of how much weight will need to be removed 180 degrees opposite to balance the wheel. Weight is removed by drilling holes in the rim of the flywheel, and the bolt ends kinda tell you how many and how deep the hole you drill need to be. Just a rough approximation, but helps get in the ballpark faster.

holes laidout for drilling to remove weight. 2 holes have already been drilled

finished drilled flywheels. Note similar weight was drilled in each flywheel.

As you can see, quite a lot of weight needed removal to get the wheels to balance. You may also have noticed that the weight was removed from around the area of the crankpin. Most flywheels you see drilled the weight is removed from an area 180 degrees from the crankpin. First, let me mention that these are Truett and Osborn 4 1/2” stroker flywheels, and that will figure into the explanation. Also, the motor we’re putting the flywheels in is a big twin flathead. First, flathead parts are heavier than corresponding OHV parts, rods and pistons in particular. Second, when T&O manufacture the flywheels, they are figuring that the wheels are going to have to balance out with parts from an OHV engine. So, they cast in a counterweight into the flywheels that will be appropriate for that application. So the cast in counterweight is lighter than would be needed for a flathead. Secondly, when you order stroker wheels for a flathead, they make them by turning down the circumference of a set of OHV flywheels to the smaller diameter needed for flatheads. This machining ends up cutting off some of the cast in counterweight. The end result is there isn’t enough material left in the cast in counterweight to balance for a flathead motor, so material needs to be removed from around the crankpin to achive balance.

You may need to contemplate on how the machining down the circumference of a flywheel will alter how they will balance. Consider that the flywheels are thicker in the counterweight area than they are near the crankpin side of the wheel. The difference in diameter is 1/4” So, taking off that 1/8” of material from the wide width vs the narrow width is going to be the difference of balancing weight lost off that counterweight.

It’s interesting to see that the amount of weight drilled out of the two flywheels is quite similar. I think this reflects on the quality of the T&O wheels, as it indicates a uniformity of material in the castings. I just trued up these flywheels, they were within .002” when I first threw then in the truing jig. Two well placed hammer blows, and they were within +- .0005” .