The primary problem was that the gear was incorrectly shrinked on the flywheel (!) and consequently
the starter-cog was consumed abnormally (work performed by a Swiss "specialist" by the way).
After having replaced the clutch and the flywheel during the complete overhaul, sometimes the starter didn' t engage on the gear...
Problem solved with the new part delivered by Wood Auto (many thanks to "tlracer" for the tip).
Cheers
