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XK120

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  1. A Jaguar XK120 owner..but these use the Smiths bellows sleeve thermostat, we are busy checking to see if the new Moss Europe bellows sleeve thermostat that is spec for Triumph, Healy.,will fit, work in the XK, It "fits" ,that is its outer dimensions fit..but..will the sleeve block the bypass. There is a lot to this..I will summarize as I see some misconceptions. The thermostat's job is to warm the cold coolant to optimal engine operation temperature which is usually in a range of about 20 D., say 65C to 85C. Its job is also to cool hot coolant to that same range,. It does this by the top poppet valve closing the passage to radiator when cold, and the sleeve will be low on the thermostat out of the way of the passae to the block.,called the (radiator) bypass: .it bypasses the radiator,,..so the passage to the engine block direct is now open. Coolant is warmed more rapidly, lessens cold engine wear, the choke will cut out etc. As the coolant warms, the thermostat starts to open and then becomes fully open at a specified temperature..different thermostats that fit and work ,,can have different full open temps. So when open, now the thermostat has "said" warm enough! stop the warming, ! ,.now I need to cool the too warm coolant, and run all coolant thru the radiator, and shut off coolant thru the engine where it would have been warmed too much by now.., so then the radiator cools the coolant..and soon the coolant is a bit too cool..so now the thermostat says,.,oh--too cool..and closes to the radiator while it opens to the engine, It will cycle like this all the time..though on a hot day it will just stay open all the time if the radiator can not make the coolant too cool. If you block off the bypass: ..warm up will be slow, the engine may run too cool all the time, and both are not the best for wear, fuel-air mix, combustion etc. Racers do this..but they run WOT..and have far fewer cold warmups than daily drivers. Once the thermostat is wide open to radiator (and closed to engine ) .,.let's say at 80C, the radiator just cools..if the cooling system is in fine shape..it just stays a bit over and under 80, as the thermostat cycles. A colder temp thermostat will NOT make the car run cooler if it is running at a temp over the thermostat spec, say 85C..the stat is open at 80 anyway, all coolant to the radiator, A lower temp stat may allow the enine to run at say 73C..and a benefit is 7 degrees of "room" on a hot day,.but it won't take lon to reach 80C anyway..the hot day/traffic issue starts in anyway quite quickly and the 7 de won't save you. The overheat issue is not the thermostat, but is the ability of the radiator to cool, and the internal condition of the cooling system. (crud, corrosion, debris, old rust and sludge etc). At this point..realize you enjoy a vintage car, as its vintage temp indicator needle moves to the vintage 100C mark. BTW..bellows sleeve thermostats will self destruct if more than a 4lb, (7lb absolute max maybe which only provides 3 d anyway) radiator cap is used. Do not try to raise boil temp with a 13-15 lb cap. Your old head gasket or core plugs and the thermostat will be your next repairs. And..BTW..50 50 antifreeze provides about 107C boil..(at seal level--each 1000 ft takes away 3 de.) Look us up at Jag-Lovers.com , thanks, Nick
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