The W198 engine employs a rather unique thermostat arrangement.
Following a failure of the original unit and driven by the insanely high price quoted by Mercedes-Benz for replacement, it seemed appropriate to examine the cooling water thermostat with a view to a functional rebuild.
As originally constructed, the thermostat housing consisted of a front and rear portion encompassing a standard Behr Thomson thermostat unit and fastened together utilizing a rolled crimp formed on the flange of the rear portion. A short session on the grinding wheel removed this crimp and allowed separation of the components.
Perhaps most time consuming part of this project was the sourcing of fasteners small enough to fit the remaining flange areas so as to hold the housing parts together. Radio Shack, an old favorite for unusual parts, was able to provide machine screws and nuts (Part Nos. 64-3010 and 64-3017) in the 2-56 size that fit perfectly. Using a template to assure uniform spacing, eight holes were drilled through both flanges with a size 44 bit and smoothed with a file to provide a proper fit. The replacement thermostat itself is an off the shelf item in the standard 54mm diameter with a 180 degree opening value as available from any auto parts supplier. Notches cut in the perimeter of the thermostat using an abrasive wheel on a Dremel Tool provide adequate clearance for the fastening screws.
Following liberal application of sealant to both sides of the thermostat, the two portions of the housing are brought together utilizing eight 2-56 screws and nuts. After application of paint to restore original appearance, the completed assembly is ready for reinstallation in the car and more years of trouble free service.