144 reliability

I have a few buddies with them. They seem to be very reliable (just like a stock 125) but the pistons cost a little more than a stock 125.

One buddy is a mid-pack B class rider. He get's 15 hours out of a piston, same as he did with the 125.
His is an Athena kit on a 2012 YZ125. Another buddy is a 30+ C rider who has an Athena kit and goes a lot longer between top ends.

The B rider had his stock top end bored out and ported by Eric Gorr when he was at Millennium and it lasted about 2 hours before the power valve that they assembled came apart and hit the piston. He talked to them and they didn't make it right, so he bought the Athena kit. Also as a result, I refuse to buy pistons & gaskets for my 295 YZ kit from them so it sits on my shelf.

I think Derek of Harris Performance is a nice guy and seems to know his stuff. I've never done business with him, nor have I ridden his work, but after having a few conversations with him on Thumper Talk, I would probably try him. He seems like a guy who would stand behind his work unlike Team Gorr / Millenium.

Out of the box, the athena kit is a little too peaky for my taste. It's more like a KTM power band without the meaty mid-range torque feel that a stock YZ engine has. I think maybe milling the Athena head, then playing with timing & jetting would bring in a little more mid range torque. For sure it's a lot stronger than a stock 125.

You didn't say what brand you've got, but all of my experience with 144's has been with YZs if that wasn't obvious.

Can't say for the KX, but my KTM is a factory 144 so it's just as reliable as a 125.
IMO if you wanna go that large, you should bore and stroke. Just make sure you go with a quality brand.

My buddy had a race prepped KX144 that ran on race gas. The fastest KX I've ever tried, it had all the power down low that you usually don't find on a 125. And the bike was really reliable. It was built on the stock cylinder with a VHM head, they also tried the Athena but it wasn't that great. Poor finish and the porting on the 144 cylinder from Athena is a joke.

He also had a KX144 that was built on the stock cylinder + head and ran on pump gas. It also had the HGS 144 system + Vforce3. He changed the piston after 30h, it never broke down. Really reliable.

YZ144 here. My .02. Take it for what you will.

My experience has been extremely reliable and I have been very pleased with my custom build. I have not ridden a faster 125/144. Under a basic maintenance schedule (not even as frequently as stock MFG recommendations), I have been through 10 rings, 5 pistons/pin/bearing and 2 cranks with out one single hick-up, not one scratch in the cylinder, no failures of any kind and this bike is ridden hard. I would strongly suggest a custom build rather than bolt on or pump gas bore and port of your cylinder and head. I had mine built for use with C12 but I have run it on VP110, TORCO 110 and Sunoco 110, oils have been synthetics. Torco GP-7, Bell ray H1R, Royal Purple synthetic. results at teardown have been identical will all of the above. $70 can of race gas and $15-$20 bottle of oil is pricey but I am very happy with it ride it very fast and you'd be hard pressed to ride a faster or more reliable 144 either.

If you want it to be really easy with still using pump gas, you will most likely wind up with power characteristics that will be narrow and flatten out on top.

If you want to go to 144, I'm assuming that you want more power. If you really want an engine that will make significantly more power and not be a reliability headache, go the extra couple hundred bucks and have a custom build done to the power characteristics that you want,. They will have you take some measurements of your engine assembled and then you send them your cylinder, head, cases and new 144 top end. For probably $750 all in, you will have a bike that you will be very happy with.