I have a thermostat and housing that is larger than one usually found on a 235. Several other thermostat housings that I have removed from 235 engines have thermostat openings of about 1.9" and take a thermostat with a flange that measures 2.1" O.D. This housing has a larger opening of 2.25" and it accepts a thermostat with a flange O.D. of 2.5".The lower housing (which bolts to the block) has a casting number of 3764748, and the accompanying upper housing has a barely legible casting number of 3764749(?). The outlet neck is sized for a radiator hose diameter of 1.5" rather than the 1.25" found on the other 235 housings I have. I suspect that this larger thermostat and housing is intended to fit a 261 engine. The parts books that I own do not show different parts numbers for 235 verses 261 thermostat housings. I am mystified and would appreciate some words of wisdom from our Chevy gurus on this.
