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Can I wade in on this one ? The thermostats for the K series engines are all 52mm diameter, across the top, where they fit into the "thermostat housing/water outlet". However, there are several different lengths, or depth, dependent on manufacture. I have one on the desk here, exactly the same as your first pic, with 25mm, below the "mounting" point. I have another that is only 20mm in this dimension. When the thermostat is mounted, there is heaps of room below the thermostat. (see pic below with 25mm thermostat inserted) This allows approx 18mm for the back of the thermostat to move down, when fully opened. That is more than enough, as the thermostat valve movement from fully closed, to fully opened is about 10-12mm at most. Your second pic of the Gates thermostat with the 35mm dimension, had me puzzled. Why would they design a thermostat with a temperature below 80 deg. In this case 78 deg C. The answer was fascinating. I discovered that that thermostat was used on early 90s Daihatsu Applause 1600cc engines. When I Googled Daihatsu thermostats, the often question was, I can't find my thermostat. It appears it is at the back of the engine, not the front, up high, & controls the cooled coolant, rather than hot water to the top of the radiator, as in the K series. "On the Daihatsu Applause 16 valve engine the thermostat is NOT located under the alloy housing that attaches the top radiator hose to the head. It is located at the back of the engine block, at the opposite end of the engine, just above where the alternator is situated. Three 10mm bolts hold the thermostat housing to the block. The bottom radiator hose is attached to the housing. All you need to do is disconnect the water temp sensor wire, undo the three bolts & push it toward the alternator. Remove the thermostat from the block replacing it with a new one. A rubber gasket fits around the outside of the thermostat & I used a little silicone on the gasket to hold the thermostat while you re-install the housing. Don't forget to reconnect the temp sender wire & fill with coolant." Obviously cooled coolant after it passes out of the radiator could well be 10 deg C lower than operating temperature. So if the 35mm thermostat is 10mm longer/deeper, than the 25mm one, then it protrudes further down into the chamber below the thermostat. So instead of 18mm of travel, that only leaves 8mm, with this thermostat. It might be borderline, as I don't know what the travel is the Daihatsu thermostat. Apparently, this is where it is fitted< & you can see the special housing is quite deep. But like Dave & Si, I see little need for lowering the thermostat opening temperature. Engines, are designed to operate generally at temperatures between 80 - 95 deg C. The thermostat is designed to do two things. 1. Get the coolant temperature up to operating temperature, just as quickly as possible, from a cold start, as this is the period where most wear occurs in an engine. 2. To control the coolant temperature in a band of say 4-5 deg C. So a 82 deg C stat, might open @ 82, & close @ 78 deg C. Even if you are in the desert, where the ambient & coolant temperature are 40 deg C, at a cold start, that is only half the temperature that the engine needs to be at. In outback Australia, where it is very hot, I know there are lots of people who remove the thermostat all together. There is no evidence, to my knowledge, that removing the thermostat, will increase coolant flow. The reason they remove them, is purely that water supplies are poor, and thermostats reliability is poor, & they don't want a siezed motor. Many motoring organisations, years ago, recommended replacing your thermostat yearly. However, in these days of modern high tech coolants, & more reliable themostats, there is no need for that. I don't think you are providing additional safety, by lowering the coolant temperature. If you are going to generate more heat, as a result of modified or operating conditions, of the engine, and it overheats, then you simply need a larger or more efficient radiator. The usual first step is to go from a 2 core to 3 core, as that requires no "bigger" radiator mods. Cheers Banjo