Suzuki DF 25 Service Manual page 150

VALVE SPRING
Valve spring free length
Check spring strength by measuring free length.
09900-20101 : Vernier calipers
Valve spring free length
Standard :
IN & EX 47.38 mm (1.865 in.)
Service limit :
IN & EX 45.48 mm (1.791 in.)
If lower than service limit, replace valve spring.
Valve spring preload
Measure valve spring preload.
09900-20101 : Vernier calipers
Valve spring preload
Standard :
IN & EX 193 – 223 N (19.3 – 22.3 kg , 42.5 – 49.2 lbs.)
Servicelimit :
IN & EX 177 N (17.7 kg, 39.0 lbs.)for 37.5 mm
(1.48 in.)
If lower than service limit, replace valve spring.
Valve spring squareness
Use a square and surface plate to check each spring for squareness
(clearance between end of valve spring and square).
09900-20101 : Vernier calipers
Valve spring squareness
Service limit:
IN & EX 2.0 mm (0.08 in.)
If measurement exceeds service limit, replace valve spring.
for 37.5 mm (1.48 in.)
POWER UNIT 7-33