J.A.P. nomenclature, as much as been said so far about these engines. The symbols and number were stamped on top of the valve-chest cover or on the top of the crankcase adjacent, and comprised two groups usually arranged like a large fraction. The numerator gave brief particulars of the engine followed by a /. After the / the letter denoted the year manufacture. The denominator would give the number in figures followed also by a / and after the /, the symbol denoting anything special. Considering first the symbols, they are as follow:
O = Overhead valve.
T = Twin cylinders.
R = Racing.
S = Sports-short stroke only.
C = Sports.
W = Water-cooled.
Z = Dry-sump lubrication.
Y = Double exhaust port.
V = 175 c.c. s.v. 60x62 mm.
Z = 175 c.c. s.v. 55 x 73 mm.
N = 200 c.c. s.v. 55x83 mm.
B = 250 c.c. t.v. 64.5 x 76 mm.
A = 300 c.c. s.v. 70 x 78 mm.
I = 350 c.c. s.v. 70 x 90 mm.
K = 500 c.c. s.v. 85.7 x 85 mm.
L = 550 c.c. s.v. 85.7 x 95 mm.
U = 600 c.c. s.v. 85.7 x l04 mm.
GT = 680 c.c. Twin, s.v. 50°. 70 x 88 mm.
MT = 750 c.c. Twin, s.v. 50°. 70x97 mm.
KT = 980 c.c. Twin, s.v. 50°. 85.7 x 85 mm.
KTCY = 980 c.c. Twin, s.v. 50°. 8/%. 85.7 x 85 mm.
LT = 1100 c.c. Twin, s.v. 60°. 85.7x95 mm.
DT = 1323 c.c. Twin, s.v. 60°. 90 x l04 mm. Used on rail trolleys and
target-towing trolleys. These were 1100 c.c. Twin 60° and aircooled.
O was always used to denote an o.h.v. engine and we have:
HO = 175c.c. ohv 53x78 mm
TO = 200 c.c. o.h.v. 57x78 mm
PO = 250 c.c. o.h.v. 62 :5 x80 mm.
YTO = 500 c.c. o.h.v. Twin. 60 x 88 mm.
POR = 250c.c. Racing (up to 1933). 62.5x80 mm.
NOR = 250 c.c. Racing (1950 onwards). 65.5 x 74 mm.
BOR = 250 c.c. T.T. Racing (1934 and l935). 64.5 x 76 mm.
SOC = 350 c.c. Sports. Short stroke. 74x 80 mm.
SOR = 350 c.c. Racing. Short stroke. 70 x 80 mm.
IOR = 350 c.c. Racing. Long stroke. 70 x 90 mm.
KOC = 500 c.c. Sports. Shortened stroke. 85.7 x 85 mm.
KOR = 400 c.c. Racing. Shortened stroke. 85.7 x 85 mm
J0R = 500 c.c. Racing. Long stroke. 80 x 99 mm.
TOR = 400 c.c. Racing twin. 62.5 x 80 mm.
KTOR = 980 c.c. o.h.v. Racing. 8/50. 85.7 x 85 mm.
JTOR = 998 c.c. o.h.v. Racing. 8/45. 80 x 99 mm.
GTO = 680 c.c. o.h.v. 70 x 88 mm. (674 c.c.)
KTO = 980 c.c. o.h.v. 8/45. 85.7 x 85 mm.
JTO = 8/50 (touring version of JTOR). 80 x 99 mm.
For speedway and sprint engines the following were employed:
SOS = 340 c.c. o.h.v. Grass track. 74 x 80 mm.
JOS = 500 c.c. 80x99 mm.
JTOS = 80/8 o.h.v. Twin. Sprint. 80 x 99 mm. A few 8/80's were made with dry sump, symbols JTOSZ.
W/W = 1,100 c.c. o.h.v. Mark I Racing engine. Lubrication by gear pump. So much for what the symbols mean before the first /. After the / the symbol or letters tells us the year of manufacture according to the following code:
Now let us see how far we have got. An engine with the numerator LTZ/O would mean an 1.100 c.c. s.v. dry-sump twin built in 1938. With KTORW/Z the engine would be a 1933 980 c.c. racing twin, water-cooled. The reader can now tell at a glance what type of engine he has and its age. Considering the denominator, this could bc written as a simp1e number thus: 12345. It could have after it a / and after the / another symbol. These were known as deviation symbols covering a specific modification during a production year that ran from 1 September to 31 August following. A list of deviation symbols is as follows:
A = High lift cam.
B = Modified gearcover and exhaust lifter for rear drive 300 c.c. And 350 c.c. 'Roadster'.
C = Crankcase with inclined platform, Cologne. L Mke. Two stroke.
D = Cast iron piston, flat top fitted to standard engine.
E = Cast iron piston, dished top, fitted to standard engine.
F = Aluminium piston domed top, fitted to standard engine.
G =Gear spindle, ball bearing when fitted as alternative to phosphor bronze.
H = Pulley spindle, roller bearing when fitted as alternative to phosphor bronze or ball bearing.
J = Roller-type camlevers and exhaust lifter to suit 500 and 600 c.c. s.v.
K = Special chainline requirement for pulley spindle.
L = O/T flywheel and screw-on sprocket, flange type, for two strokes.
M = Crankcase without oilbox.
N = Magneto sprocket for type NA, M.L. magneto. Two strokes.
O = Cylinder No. 2 type.
P = Cylinder No. 4 type.
R = Cylinder No. 3 type.
S = Double-row roller big-end bearing. 250 and 350 c.c. o.h.v. racing engines.
T = New-type crankcase and rotary valve. 500 and 600 c.c. s.v. singles.
U = Modifications for new tappet centres. 350 c.c. s.v. roadster.
V = Reduced centre connecting rods giving lower-compression engine.
Y = 1924 pattern crankcase converted to rotary valve.
* = Refer to engine record card.
These symbols were used over a period from 1921 to 1925.
After this period S or * after the / in the numerator would mean the customer's specification only and would have one more parts not standard, the * denoting a specially prepared engine for record attempts or rider's requirements and specially tuned.
TIPS
Gerth-Owe tog tidigare emot beställningar på plåtdetaljer men det blev för mycket. Det är bättre att du ringer själv. Vi har lånat ut fram och bakskärm och ett fabriksnytt kedjeskydd som vi fann 1980 i fabrikslokalen. Märkligt. Här är adressen:
HÖRBY KAROSSERIVERKSTAD
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