Unscrew old, screw in new. This was exactly the same as the one I had, the GRAY ST4000MK2 autopilot. Only thing I had to do was drill holes for it to fit on my wheel per number of spokes, but even the inside is already pre-set to accept the drilling. Simple.
Ridiculously expensive from Raymarine, especially since my old motor was fine, and I needed the plastic, but at least the replacement was exact.
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ST4000 MK II
October 25, 2012
Reviewer:
Bill Coxe
from New Bern, NC
Went searching for this upgrade after the original went belly-up. Hodges had the best price and it shipped almost immediately, although it was listed as a back-ordered item. It works great and the service was excellent.
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Raymarine MkII wheel drive unit
September 26, 2012
Reviewer:
Anonymous Person
Good piece of gear, much heavier scantlings than the Mark I, but it is NOT a direct replacement. For bulkhead mounting, the hole for the motor is larger and not on center for the MK I motor. Therefore a little joiner work is required. Otherwise, great improvement.
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st4000 wheeldrive replacement unit
May 22, 2012
Reviewer:
RITZ
from Beaufort,NC
The new st4000mk2 drive unit I purchased from Hodges Marine was a perfect fit with my old st4000 compass and control components, nothing needed adjusting.
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review of replacement wheel drive unit
April 1, 2012
Reviewer:
Anonymous Person
from Southport, NC
I have installed the Raymarine unit onto the old Autohelm autopilot and although I have not taken the boat out of the slip with it, it seems to work perfectly as i put it through some test procedures.
Perfect Replacement 
Raymarine MkII wheel drive unit