There is a basic prefix number for many early castings, both iron and aluminum, to identify the type of part used on the engines of H-D twins 1930-51. After this approximate date the actual part number appears with five numbers, then the year, as 23945-56.
    All cylinders begin with “120-”, followed by the first year of use, with rear cylinders adding an extra number after the year, most frequently a “1”. The front cylinders are even numbers (none or 2), the rear cylinder are odd numbers (1 or 3). For example the 1936-73 45 front cylinder is 120-36 (yes, it's used on the 1936 RL), and 120-361 rear.
    Where there are more than one new cylinder introduced in the same year, an extra number is added after the year to both the front and rear, beginning after the earlier number. For example the 1936 VL cylinders are 120-362 front and 120-363 rear.
    Please note that in many instances different and non-interchangeable parts are made from the same basic casting, so this is not an absolute guide to substitution, only to recognition. This is helpful where an actual measurement is not possible - such as internet photographs.

Tappet block casting numbers: “9-”

Model

VIN

Year

Comment

Front

Rear

Panhead

EL, FL

1948-52

aluminum

9-483

9-482

Cylinder casting numbers: “120-”

Model

VIN

Year

Comment

Front

Rear

45

D, DL

1929

T manifold, 7 bolts

120-29

120-291

1930-31

120-302

120-303

R, RL
(G uses the R engine)

1932-35

120-32

120-321

1936

Y manifold, 8 bolts

120-36

120-361

W, WL, G

1937-73

WLD, WLDR, WR

1939-52

120-39

120-391

V

V, VC, VL

1930-33

T manifold, 8 bolts

120-30

120-301

VLD, TNT

1933

V manifold, 8 bolts

120-33

120-331

V, VC, VL, VLD, VLH, VHS

1934-35

1936

V manifold, 9 bolts

120-362

120-363

U

UH, ULH

1937-41

120-37

120-371

U, UL

1937

1938-48

120-38

120-381

Flywheel casting numbers: “150-”

Model

VIN

Stroke

Year

Left

Right

45

D

3.8125”

1929-31

150-29

R

1932-36

150-32

W

1937-73

V

V, VC, VL, VLD 74

4.000”

1930-36

150-30

150-301

VLH, VHS 80

4.250”

1935-36

150-352

150-353

U

U, UL, UH, ULH

4.28125”

1937-48

150-374

150-372

Knucklehead

E, EL

3.500”

1936-39

150-353

150-35

1940-47

150-403

150-40

F, FL

3.96875”

1940-47

Panhead

E, EL

3.500”

1948-52

F, FL

3.96875”

1948-*

Cam cover casting numbers: “97-” or “131-”

Model

VIN

Year

Number

Comment

45

D

1929-31

Oil pump

  97-29

R

1932-33

Early oil pump

  97-32

1934-36

Late oil pump

  97-34

W, G

1937-57

8 fins; feed oil pump

131-372

WLD

1939-*

8 fins; feed oil pump; magneto drive idler post

131-39

WR

1950-52

5 partial fins; feed oil pump; magneto access cover

131-50

V

V, VC, VL, VLD, VLH, VHS

1930-33

Early oil pump

  97-30

1934-36

Late oil pump

  97-34

U

U, UL, UH, ULH

1937-39

Smooth; feed oil pump

131-37

1940-48

9 fins; feed oil pump

131-40

Crankcase casting numbers: “112-”

Model

VIN

Year

Comment

Left

Right

45

D, DL

1929-31

Left race: blind; right race: bushing

112-29

112-291

R, RL

1932-36

112-322

112-323

W, WL, G

1937-38

Races: blind

112-37

112-371

1939-73

Races: snap ring

112-39

112-391

V

V, VC, VL, VLD, VLH, VHS

1930-36

Left race: blind; right race: bushing

112-30H

112-30F

See these Victory Library booklets