Harley Flywheel Balancing App
Harley Flywheel Shop BALANCING · TRUEING · TORQUE SPECS — SHOVELHEAD / PANHEAD / KNUCKLEHEAD / SPORTSTER
Flywheel Balancing Calculator
Static balancing the S&S Cycle way. Weigh every part in grams, enter them below, and the calculator returns the bobweight to bolt into each flywheel — plus a suggested shim stack from the S&S balance kit.
Job Info
Reciprocating Parts up & down
Rotating Parts circular
Weigh each rod end with the rod centerline level (pointer at zero). Crankpin end is roughly twice the wrist-pin end.
Balance Factor
S&S recommends 60%. Below ~50% the engine tends to vibrate vertically; raising the factor shifts vibration toward horizontal, which most riders find less noticeable. 60% is the all-around street compromise.
The S&S Formula
Bobweight per flywheel = [ (Balance Factor × Reciprocating Weight) + Rotating Weight ] ÷ 2
| Reciprocating weight = | both pistons (piston + pin + clips + rings) + wrist-pin end of both rods |
|---|---|
| Rotating weight = | crankpin + nuts + bearings + cages + key + locks/screws + crankpin end of both rods |
Drill the heavy spot to remove material. S&S forged steel wheels: holes ≥ ⅛" from the rim, ⅛" apart, leave ⁵⁄₁₆" of material so you don't break through. Cast-iron stock wheels: holes ≥ ¼" from rim, ¼" apart. Never drill deeper than ¼" near the crankpin hole. Magnaflux for cracks if in doubt.
Method and figures per S&S Cycle Balance Kit Instruction Sheet No. 5010. For off-road / racing builds; verify against the current factory manual for your engine.
Flywheel Torque Specs by Year
Pick an engine era. Values are pulled straight from the factory service manuals in this shop, in foot-pounds (ft·lb) unless noted. Heads up: the early manuals don't publish a numeric flywheel torque — they tell you to draw the nuts "very tight" — so those eras list the factory fits & clearances instead.
Timing Marks (S&S flywheels)
| Big Twin | F & R marks = 35° BTDC (front / rear). TF = TDC front. |
|---|---|
| Ironhead Sportster | F & R marks = 40° BTDC. TF = TDC front. |
| V2 (Evo) Sportster | F & R marks = 30° BTDC. TF = TDC front. |
Mark centered in the timing hole = the stated BTDC figure. Mark toward the rear of the hole advances ~5°; mark just leaving the hole retards ~5°. S&S suggests timing big-inch engines at 30–32° BTDC.
Sources: 1948-57 EL/FL Panhead, 1959-69 FL Duo-Glide/Electra-Glide, and 1970-78 FL/FLH Shovelhead factory service manuals (this shop's PDFs); S&S Cycle Instruction Sheets No. 2002 & 5010. Always cross-check the manual for your exact model.
Flywheel Trueing Guide
Trueing removes runout so the mainshafts spin dead-true between centers. Always measure on the mainshaft bearing surfaces — never the flywheel rims.
What you need
| Truing stand or lathe centers | to support both mainshafts between centers |
|---|---|
| Two dial indicators | 0.0001″ or 0.0005″ resolution, on the bearing surfaces |
| Lead / brass / soft hammer | never steel directly on the rim |
| Wedge & pry bar | to spread the wheels when correcting |
Target: S&S trues to 0.0005″ or less on either mainshaft bearing surface. Factory H-D spec of 0.001″ is acceptable but tighter is better.
Procedure
Mount between centers
Place the assembled flywheels on the truing stand with both mainshafts supported between centers. Make sure centers and shafts are clean.
Set the indicators
Position one dial indicator on each mainshaft's bearing surface (the area the main bearings ride on), close to the flywheel. Zero them.
Rotate and read runout
Slowly turn the assembly a full revolution. Note total indicator movement (TIR) and where each high spot falls. Sprocket and pinion sides may differ.
Diagnose the error
Both indicators high at the same point usually means the wheels are out of parallel (one taper sitting deeper). Indicators high 180° apart means the crankpin holes are mis-aligned (offset). Determine which before correcting.
Correct — offset (pin holes not aligned)
Tap the rim of the appropriate wheel toward the low side with a lead hammer to walk the wheels into line. Small, repeated taps. Re-read after each.
Correct — out of parallel (squeeze / spread)
To close a gap, squeeze the rims together at the high point in a press or vise (soft jaws). To open, drive a wedge between the wheels at the correct point. Re-read.
Chase it down
Work the largest error first, recheck both indicators each time, and converge toward ≤ 0.0005″. Expect to alternate between offset and parallel corrections.
Final verify
Confirm both bearing surfaces read at or under spec through a full rotation, and recheck connecting-rod side play (0.015″–0.035″) once trued.
If you must strike the rim, use a lead hammer — a dented rim can make it impossible to recondition the mainshaft tapers later. Don't take runout readings on the rims; rim runout is not a valid indication of mainshaft concentricity.
Trueing guidance per S&S Cycle General Flywheel Instruction Sheet No. 2002. Practice and feel matter — this is a guide, not a substitute for experience or the factory manual.