Dobro Dosli,
make shure that the Startermotor is clean, fresh Grease in the Bearings and that the Carbonlenght is long enouth. Even the Cuppercummotator shut be clean and circular - the gaps between the cupperdivision shut be free from Coaldust - toothbrush (!).
IN ANY CASE: the 10mm narrow nut under the + connector ALLWAYS conter in Tight also in Untightdirection! Othwise the Cupperwiring cut be twistet or worste case: ripp off. I also recommend a ~7mm Minuscable direct between the Battery to the rearesidehousing of the Starter - connectet to 8mm upper Bold. All Powerconectors should be cleaned with a brassbrush!
Then you'll find here the Treath about the startergear: make shure that "The Clip" need 7lb (ore +) to "break free" ... the "Bendixwheel" should be free of "Paradontosis"

The Relay himself cut even get some Maintanance: you unsolder the little cables, open the Housing and polish the Cupperstuff inside : youll find erosion !
Greetz Heri