Originally Posted by Head2Wind
there was discussion related to this subject some time ago, "should I 'update' a 950 engine with a 990 pump impeller and cover?'.
There was a supposed 'unofficial' view/response from a KTM engineer or something to that effect, that it was NOT recommended to perform this alteration to the cooling system on a 950. As I recall the reasoning was that the cooling passages on the 990 cylinder and head are different. When applying this change to the 950 (the change to the pump increases flow rate) there may be problems/issues related to coolant flow through the head.
I want to share one related experience with the larger water pump.
The temperature with my oem 2006 950 always went forth and back between 4 and 5 bars on the display when idling. When summer days got hot (yes, I know this is not Arizona or Africa) the few single days, I saw occassionally 6 bars on the display to my dissatisfaction.
After replacing the motor with a 2009 990SDR (with 28 instead of 25 mm tube on the pump suction) and with the same temperature sensor that I moved from the 950 to the 990, I have never ever seen more than 4 bars on the display!
The valve clearance on the exhausts in my old 950 changed considerably with need to change the shims one step and sometimes even two each winter service (every approx. 20000 kms = 12.5 kmiles). After two summer seasons with the 990 motor (40000 kms and exactly the same kind of torturing of the motor) I just changed the 4 exhausts shims to one step thinner, but that was a borderline need only.
The 990 (at least my 2009 990) maintains a much more stable temperature than the 2006 950 did. One consequence is that you hardly need to check the valves. I tend to believe I will get much longer life out of my 990 motor than I did with the 950.
Note: Of some reason KTM went back from 28 mm tube to 25 mm some one or two years after 2009. Just wonder why.