I bought a Bosch Reman starter for my VW at a local parts house. Installed it and discovered it was no good. I had bench run it before the install and it spun fine!
Turns out the end cap that is bolted to the shaft end had been damaged and interfered with proper operation when under load.
A pita to remove, install, remove, install.
Look your purchase over carefully and don't trust a bench test.

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1981 240D 143k 4 spd manual -SOLD

2004 VW Jetta TDI 5 speed 300k -still driven daily