[Intro]
[Overall gearing ratio to output shaft:
1st gear = 67:18 * 39:14 = 10.37:1
2nd gear = 58:27 * 39:14 = 5.98:1
Pedals = 11:17 = 0.65:1]
Rebuild Parts
- Bearings and Bushings [1]
- Seals [7]
- Crankshaft
- Clutch side: 17 × 40 x 7
- Magneto side: 17 × 40 x 7
- Main Shaft (none)
- Drive Shaft
- Magneto side: 20 × 42 x 7
- Pedal Shaft
- Clutch side: 16 x 24 x 4
- Magneto side: 16 x 24 x 4
- Crankshaft
- [note: pedal shaft seals are difficult to find; 16x24x5 seals [8] will work.]
Combustion End
39mm Stroke
Top End
49cc Stock
Piston ported
Bore diameter = 40.xmm
Head gasket recessed into head
Threaded exhaust = 35.2mm x xxx.xxmm x xxx.xxmm long
Internal exhaust dia. = 25.2mm
Square bolt pattern = 7.2mm holes spaced at 47.6mm
Machined height (without skirt) = 61.3mm
Intake diameter = 22.6mm
Two intake bolts = xxx.xxmm x x.xxmm spaced at 40.0mm
Port Timing (assuming 2.0mm deck height):
- Exhaust
- Height to deck: 30.2mm
- Duration: 139 deg
- Transfer
- Height to deck: 34.1mm
- Duration: 110 deg
- Blowdown
- Duration: 14.5 deg
- Pct. of Transfer: 13.1%
- Intake
- Height from bottom to deck: 58.2mm
- Piston height: 46.2mm
- Duration: 112 deg
Port Map:
68cc Olympia Kit
Piston ported
Bore diameter = 46.xmm
Compatible with stock head
Compatible with stock exhaust
Stock exhaust compatible but restricted
Head gasket recessed into head (same OD as stock, smaller ID)
Threaded exhaust = xx.xmm x xxx.xxmm x xxx.xxmm long
Internal exhaust dia. = xx.xmm
Square bolt pattern = x.xmm holes spaced at xx.xmm
Machined height (without skirt) = xx.xmm
Intake diameter = xx.xmm
Two intake bolts = xxx.xxmm x x.xxmm spaced at xx.xmm
Port Timing (assuming 0.5mm deck height):
- Exhaust
- Height to deck: 24.9mm
- Duration: 163 deg
- Transfer
- Height to deck: 31.8mm
- Duration: 117 deg
- Blowdown
- Duration: 23.5 deg
- Pct. of Transfer: 20.1%
- Intake
- Height from bottom to deck: 58.2mm
- Piston height: 41.5mm
- Duration: 149 deg
Port Map:
70cc Polini Kit
[no info]
Bottom End
- Connecting Rod
- Center-to-center length: 90mm (est. -- to be confirmed)
- Small end ID: xx.xmm
- Big end ID: xx.xmm
- Small end OD: ~xx.xmm
- Big end OD: ~xx.xmm
- Overall length: xxx.xmm
Clutch Side
Magneto Side
CEV 6939 Flywheel
6V AC system with two coils (ignition and lighting)
Spins clockwise
[Puller dimensions]
3-wire Magneto:
- Black - Ignition through switch
- Blue - Ignition ground through brake light
- Yellow - Lights coil to head, tail, speedo, horn
- [note: hardwire ignition by grounding blue wire]
- [note: hardwire ignition by grounding blue wire]
Stock timing: 1.9mm = 23 deg. BTDC
Stock points gap: 0.014" to 0.018"
Stock plug gap: 0.020" to 0.024"
Stock plug (30MPH): Bosch W5A, NGK B7HS, Champion L-82
Internals
Output shaft is splined; available sprockets are 10T, 13T (stock), and 17T Rear sprocket (get bolt pattern): 22T, 24T, 26T, 30T (stock), 32T, and 34T available