Displacement

    Norton twins have been built in several different displacements with four different bores & three different strokes. In addition, some alternate displacements can be had by substitution of aftermarket parts. Here are some of the more common possible choices with comments on application.

Norton non-unit (big) twin bore & stroke data

ID

Type

Size, CC

Bore, MM

Bore, Inches

Stroke, MM

Stroke, Inches

Rod, Inches

Rod, MM

Rod Ratio

4K
P-speed

5K
P-speed

1  

88

  497

66

2.598

72.6

2.874

6.000

152.4

2.099:1

8,397

10,496

2  

99

  596

68

2.667

82

3.228

5.875

149.2

1.820:1

7,435

9,294

3  

Manxman, 650SS

  646

89

3.504

1.676:1

6,849

8,562

4  

Atlas

  745

73

2.874

5  

Commando

  827

76.9

3.028

6  

Big bore 1

  917

81

3.189

7  

Big bore 2

  952

82.5

3.248

8  

Big bore 3

  964

81

3.189

93.5

3.681

1.596:1

6,520

8,150

9  

Big bore 4

1000

82.5

3.248

10  

Maney destroked

  499

73

2.874

59.6

2.346

2.451:1

10,228

12,785

11  

750 RH2

  747

76.9

3.028

80.4

3.165

1.817:1

7,582

9,478

18  

Schmidt long rod

  745

73

2.874

89

3.504

6.400

162.6

1.826:1

6,849

8,562

12  

Long rod 850

  827

76.9

3.028

6.100

154.9

1.741:1

13  

Long rod 1000

1000

82.5

3.248

93.5

3.681

1.657:1

6,520

8,150

17  

Short stroke 500

  494

68

2.667

68

2.667

5.875

152.4

2.194:1

8,965

11,206

Norton unit (small) twin bore & stroke data

ID

Type

Size, CC

Bore, MM

Bore, Inches

Stroke, MM

Stroke, Inches

Rod, Inches

Rod, MM

Rod Ratio

4K
P-speed

5K
P-speed

14  

Jubilee

  249

60

2.362

44

1.732

13,857

17,321

15  

Navigator

  349

63

2.480

56

2.205

4.3125

109.5

1.956:1

10,886

13,607

16  

Electra

  383

66

2.598

“4K, 5K P-speed”: RPM at 4,000, 5,000 feet per minute mean piston speed

ID:
  1.  Model 88
  2.  Model 99
  3.  
  4.  Atlas, early Commando
  5.  Commando 850
  6., 7.  Big bore
  8., 9.  Big bore, stroker crankshaft
10.  500 special short-stroke racer, 850 bore
11.  750 special short-stroke racer, 850 bore
12.  850, MAP special long rods
13.  Big bore, MAP special long rods
14.  1958-66, rod length unknown
15.  1960-65
16.  1963-65
17.  Billet short-stroke crank, Commando rods

18.  Jim Schmidt long 750 rods and pistons; click this text to see more details.

Cam Timing Adjustment in Small Increments

    The std. Norton twin 48 tooth cam drive gear runs at 50% engine speed off the 24 tooth pinion gear, as shown here. A full rotation of the camshaft is 720 engine degrees. Click the picture for a larger view.
    Each gear tooth gives a 15° change in cam position: 720° ÷ 48 = 15°.
    The two cam sprockets, one each on the drive gear and the cam itself, each have 18 chain teeth, each gives a 20° degree change: 720° ÷ 18 = 40°.

    In addition to these two increments (15° & 40°), the difference between them can also be used by advancing one and retarding the other. Please note that these adjustments advance or retard the intake and exhaust functions simultaneously; changing the relative position of the intake and exhaust lobes requires a cam change. Below is a table of alternate positions and their effects. Calculations are limited to a range of 20° as being of the most practical use.

Cam Timing Increments: Stock Sprockets

To retard the cam

Gear teeth @ 15°

Sprocket teeth @ 40°

To advance the cam

Gear teeth @ 15°

Sprocket teeth @ 40°

-  5°

-  3

+  1

+  5°

+  3

-  1

-10°

+  2

-  1

+10°

-  2

+  1

-15°

-  1

std.

+15°

+  1

std.

-20°

-  4

+  1

+20°

+  4

-  1

Alternate Cam Drive Sprocket Choices
    BritBike Forum contributor “beltdriveman” suggests that much finer increments of camshaft position can be obtained by substituting two 17 tooth engine sprockets and the triplex primary chain from a Triumph Tiger Cub (200cc single cylinder OHV); a half-link must be added to the chain for proper length and tension. The movement per sprocket tooth becomes larger at 42.353°: 720° ÷ 17 = 42.353°.
    In several instances there are more than one combination of gear vs. tooth alternates that will give the same result within less then .5°; in the interest of brevity only the combination with the fewest teeth is displayed. In addition, even more alternate positions can be obtained by use of an offset key or a vernier cam sprocket.

Cam Timing Increments: 17 Tooth Cub Sprockets

To retard the cam

Gear teeth @ 15°

Sprocket teeth @ 42.353°

To advance the cam

Gear teeth @ 15°

Sprocket teeth @ 42.353°

-  1°

-17

+  6

+  1°

+17

-  6

-  2°

+14

-  5

+  2°

-14

+  5

-  3°

-  3

+  1

+  3°

+  3

-  1

-  4°

+11

-  4

+  4°

-11

+  4

-  5°

-  6

+  2

+  5°

+  6

-  2

-  6°

-23

+  8

+  6°

+23

-  8

-  7°

+  8

-  3

+  7°

-  8

+  3

-  8°

-  9

+  3

+  8°

+  9

-  3

-  9°

+22

-  8

+  9°

-22

+  8

-10°

+  5

-  2

+10°

-  5

+  2

-11°

-12

+  4

+11°

-12

+  4

-12°

+  2

-  1

+12°

-  2

+  1

-13°

-15

+  5

+13°

-15

+  5

-14°

+16

-  6

+14°

-16

+  6

-15°

-  1

std.

+15°

+  1

std.

-16°

-18

+  6

+16°

+18

-  6

-17°

+13

-  5

+17°

-13

+  5

-18°

-  4

+  1

+18°

+  4

-  1

-19°

+10

-  4

+19°

-10

+  4

-20°

-  7

+  2

+20°

+  7

-  2

    These scans were (shamelessly) copied without permission from “Norton 850 Hop Up” by Jody Nicholas, appearing in the May 1974 issue of “Cycle World”. Click on each thumbnail for a full-size version on a separate page.

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