It's all down to batches and country of manufacture. The T in UT1 means Thailand. But there is an identical(in principle) engine called the U1 which is made in Japan, which mostly only goes on to OEM equipment and to the States. Generally speaking the quality of castings and machining is better on the Jap engine which means they are more consistant and almost invariably good ish out of the box. However if you get an engine from Thailand, a good batch can be much quicker, but it's a lottery, equally you can get a dog. I've seen engines with sequential engine numbers where exactly half are good and half are bad. A friend who works for Honda and has visited some of the factories confirms that casting are poured from scrap (old engine blocks etc.) and (ports etc,) are hand finished. To add to this what isn't clear is how many parts are shipped between the two countries for assembly, so perhaps the Thailand engine can have some Jap bits and visa versa.
To cap it all off, part numbers are the same and if you order parts, sometimes you will get a jap part and sometimes a Thai part.
On the higher revving 120, this is the difference between dropping valves or not. The Jap colletts are so exactly made they take the heads of valves at high rpm, while the more crudely made Thai ones give the valve a softer landing.
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