Motor Classic: Stroker Clearance and Flywheel Balance

This is the 6th in a series of posts on a flathead big twin engine build that I am doing in my shop. The series starts with Stroker Flathead Engine Build , and last post was Relieving the Flathead . Once the rods are rebuilt, and the cylinders are bored it is time to mock up the engine and do some clearancing for the stroker. Because of the extra stroke, the relationship between the rods and the cylinder spigots, cases and even the bottom of the piston skirts has changed and all of those clearances need to be checked.

The first thing that needs to be addressed is to check that the male rod does not "bottom out" against the inside of the female rod when the pistons are at bottom dead center. Now for standard big twin rod lengths, S&S provides a measurement. With the rods on the crankpin you simply measure with a caliper between the wrist pin bushings at their nearest point. On a flathead with it's longer rods it is a little more complicated, but can be accomplished by checking the measurement between the wrist pin bushings as per the S&S method, and then comparing to the measurement taken from wrist pin to wrist pin with the pistons and cylinders mocked up as in the center picture below. Any extra clearance needed is accomplished by careful grinding on the inside "webbing" of the female rod. (the mock up shown below is done by assembling the crankpin and and rods on one flywheel half and installing in a case half)

Other areas that need to be checked for a minimum of .060" clearance are; piston to piston at BDC as shown in the center picture above, piston to flywheel also shown in the center picture, and rod to cylinder spigot and crankcase as shown in the other two pictures above. Note that it is possible to check these clearances with the flywheels fully assembled, but it is much easier to see the piston to flywheel when performed with only one half.

Once all stroker clearances are verified, the flywheels can be balanced. In this particular case, though, I felt it would be worthwhile to lighten the pistons as much as practical in order to lessen the stress on the antique connecting rods. Lightening pistons can be a real art (or science) in itself, and there are businesses that specialize in that sort of thing. The key is to avoid weakening the piston or distorting it. I have lightened quite a number of pistons over my years of drag racing, and so felt comfortable in doing the work myself. Yes, the pistons undoubtedly would have come out lighter if I had sent them to a specialty shop, (mostly due to my being conservative) but the extra expense was not in the budget.

With the pistons as light as I was going to make them, it was time to balance the flywheels. Most of the shops like mine that perform flywheel balancing use basically the same static balance method. Some may try to tell you that static balancing is antiquated and the real way to balance flywheels is dynamically. They may be wrong. The people over at Darkhorse Crankworks (respected experts in the field of Harley flywheel balance) have written an interesting piece on the subject that you can view here: http://www.darkhorsecrankworks.com/pdfs/nospin.pdf

Static balancing of Harley flywheels is a process of weighing the individual pieces (crankpin, bearings, retainers, rods, pistons, rings, wrist pins, etc.) computing a balance bob weight from those figures and employing gravity to tell you when you have achieved a "balance". The actual modification is done by drilling holes in the perimeter of the flywheel. I put the word balance in quotes because on a Harley engine with its v twin configuration, what you are really striving for is the best compromise for the least vibration in the rpm range where the engine will spend the majority of its time. You will often hear the term "balance factor" used in relation to Harley flywheels. This is the % of the total reciprocating weight that is added to the rotating weight to give the bob weight. My personal rule of thumb is to use a 60% factor for light flywheels such as S&S (where both flywheel halves are about the same size) and 55% for old style OEM flywheels (where the left flywheel half is considerably heavier) Last time I counted (8 or 9 years ago) I had balanced well over 100 sets of flywheels using this method without any negative feedback.