Looked at the starter on our Tier 3 sisu MF6499 recently. Its been occasionally grinding teeth when cold as the starter tries to engage with the ring gear, it also seemed to be not picking up and firing properly on a couple of mornings during the 1st start of the day, both symptoms only seemed to happen when its cold, once up and away it seemed better.
I removed the geared up type starter and could see that the armature pinion shaft had a grey sheen from the remains of the grease which had dried out like a sort of teflon coating. It just made the pinion stick rather than sliding up and down the shaft as it should, I could also see where the pinion had ground away flywheel teeth in the 3 positions where the flywheel would stop eventually as pistons came up to compression.
The starter required simply stripping down into its major components so I could get the armature out, clean off the film of dried grease and then reassemble it with a wee bit of lubrication- that had it working well again, now it was time to look at the ring gear. Using a pointed 1/4" carbide arbour in the air die grinder it took about 10 min to do the 3 groups of teeth putting a slight slope back on the teeth which matches up with the pinion teeth so they slide past one another and slide into mesh easily.
Then stick the starter back on and see how it goes. Starting was back to the original non grinding affair, turn key and bingo it was up and running, as for the half hearted picking up of revs- there was no sign of that yet.