THERMOSTAT REPLACEMENT / MODIFICATION
This modification allows one to replace the original thermostat with a modern type.
The thermostat sleeves dimensions and structure are as follows:
1⅝" long, x 2-5/16" external diameter, (or such that is a snug fit into your thermostat housing). Internal diameter, 1-15/16" one end, and 2-1/8", plus .010" the other end. The step should be situated about ⅝" from the narrow end.
1" long, minus the thickness of the thermostat body flange, x 2-1/8" diameter, minus .005" (or slightly greater) to allow a snug clearance fit into the outer sleeve, x
Both sleeves when fitted together with the thermostat in place must be the same length so that when they are fitted into the thermostat housing the ends are flush with the face of the thermostat housing. It can be a bit fiddly to get to this stage but it must be done, otherwise you will have a loose sleeve flopping about in there.
When you have both sleeves the same length with the thermostat in place carefully drill a hole through both sleeves, 7/32" in diameter, directly under the bolt (3/16"W.) that locates the original type thermostat in place, so that when the 3/16" bolt is screwed into place it locates both sleeves into position and stops them moving about.
DO NOT DRILL ANY HOLE THAT WILL FIT OVER THE BYPASS. Make sure your thermostat has a relief hole in the body flange. If it doesn't, drill a 3/16" hole in the flange, being careful not to interfere with the thermostat flange, and make sure it is not obstructed by the sleeves.
See technical drawing below