As I understand it the flying brick engine was externally the same between the LT, K-GT and K-RT up until the GT was redesigned a couple of years ago. Internally I think there are some difference with cams and pistons. So, if you going to put a LT motor into a GT, you're going to be losing some performance. JM2CW
The older K1200GT/RS engine was externally the same, but had higher compression forged pistons, and different cams. It was tuned for more horsepower at higher RPM than the LT engine, so if an LT engine is used in the GT it will not perform as well, but would work just fine.
At least one person tried the reverse in the past, putting the GT/RS engine in the LT, but it was not a positive change. The LT was too heavy, and the lower torque at lower RPM made it a dog at everything except high speed. The LT engine is tuned for much better low RPM torque and a little flatter power curve.
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