TECHNICAL DATA (250cc Models)
Cubic Capacity 247.4cc (15.094 cu.in) Cast iron cylinder head:
Stroke 64.5mm (2.5394”) Valve guide external diameter 0.624/0.623”
Bore 69.876mm (2.751”) Valve guide hole (in cylinder
(Rebore to 0.020” when wear exceeds 0.065” head) diameter 0.623/0.622”
and again to 0.040” after further 0.065” wear)
Aluminium cylinder head:
Compression ratio: Valve guide external diameter 0.6275/0.6270”
Early models: Valve guide hole (in cylinder
250 Clipper 7.5 to 1 head) diameter 0.626/0.625”
Crusader 250 8 to 1
Crusader Sports 8.5 to 1 Cam follower spindle diameter 0.5005/0.4995”
Later models: Cam follower internal diameter 0.5020/0.5010”
250 Clipper, Crusader, Idler pinion spindle diameter 0.05005/0.4995”
Crusader Sports, Trials 8.75 to 1 Idler pinion internal diameter 0.5025/0.5015”
Super 5 9.75 to 1 Contact breaker and oil pump
1963 models onwards: spindle diameter 0.4370/0.4365”
250 Clipper, Crusader, Contact breaker and oil pump
Crusader Sports, Trials, bearing diameter
Super 5, Continental and GT 9 to 1 (after reaming) 0.4377/0.4375”
Piston Diameter Valve clearance with cold engine:
Bottom of skirt (fore and aft) 2.7485/2.7475” Cast iron head Alloy head
2
nd
and 3
rd
lands 2.729/2.726” Inlet 0.004” zero
Top land 2.725/2.722” Exhaust 0.006” 0.002”
Piston Rings Valve spring free length:
Width, Plain rings 0.0635/0.0625” Inner (with steel top collars) 2.
1
/
32
”
Scraper ring 0.156/0.155” Outer (with steel top collars) 2.
3
/
32
”
Radial thickness 0.1215/0.1135” Inner (with alloy top collars) 1.
1
/
2
”
Gap when in unworn cylinder 0.020/0.015” Outer (with alloy top collars) 1.
11
/
16
”
(Renew piston rings when gap exceeds
1
/
16
”) Renew when reduced by 1/8”
Clearance in grooves
Compression ring 0.005/0.003” Valve timing with 0.015” clearance
Scraper ring 0.004/0.002” 250 Clipper and Crusader
Piston boss internal diameter 0.7501/0.7499” Exhaust opens 80 degrees before B.D.C.
Gudgeon pin diameter 0.7501/0.7499” Exhaust closes 30 degrees after T.D.C
Con rod small end diameter 0.7507/0.7505” Inlet opens 50 degrees before T.D.C.
Big end internal diameter 1.8540/1.8535” Inlet closes 60 degrees after B.D.C.
Bearing shell internal diameter 1.7515/1.7505” Crusader Sports, Super 5, Continental,
Crank pin diameter 1.750/1.7495” Trials and GT
Exhaust opens 57 degrees before B.D.C.
Rocker bearing inner diameter, old 0.626/0.625” Exhaust closes 31 degrees after T.D.C.
Rocker spindle diameter, old 0.624/0.6235” Inlet opens 27 degrees before T.D.C.
Rocker bearing inner diameter,new 0.6880/0.6875” Inlet closes 55 degrees after B.D.C.
Rocker spindle diameter, new 0.6871/0.6862”
Camshaft bush external diameter 0.562/0.561”
Inlet valve stem diameter 0.3430/0.3425” Camshaft bush internal diameter 0.5627/0.5625”
Exhaust valve stem diameter 0.3410/0.3405” (After reaming)
Valve guide internal diameter Timing sprocket 14 teeth
(After reaming) 0.3447/0.3437” Camshaft sprocket 28 teeth
Engine sprocket 23 teeth
Contact breaker points gap 0.015” Clutch sprocket 49 teeth
Ignition timing, advanced
3
/
16
”-
7
/
32
” before T.D.C. Final drive sprocket (standard) 17 teeth
Spark plug: Oil pump:
250 Clipper, Crusader, Trials, Feed plunger diameter 0.12475/0.12450”
Champion L7 or L10S Return plunger diameter 0.31225/0.31200”
Crusader Sports, Super 5, Continental and GT Speed ½ engine speed
Champion L5 or L11S stroke 0.375”
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