i once heard that the motor was aluminum...is this true?
if the motor is aluminum instead what would be the cost compared to others to get a bore?

 

Same price as a standard bore.

The motor is mostly made form aluminium, but the sleeve is steel. There is no way you can run a piston in an aluminium sleeve. Most ATV’s have aluminium motors, but it does not mean everything is made form it. Some parts are steel, and some are other types of metal.

 

Yeah, my understanding is the sleeve is aluminum with nikisil.

 

I must admit, it’s the 1st time I have heard of an aluminium sleeve. Never to old to learn something. Yes if they coat it with a much harder metal or material, like Nikisil it will hold up. A pure aluminium head will not work. I know on some motors they also chrome the inside of the sleeve, to get the same result as with the nikisil. If they do bore it, you will have to get the sleeve recoated with the nikisil, and it’s not going to be cheap.

 

Not sure about the YFZ but, most aluminum / nik coated cylinders are NOT sleeves. They are castings. Meaning the aluminum is poured into a mold. That mold is for the piston / cylinder and the oil / water passages. The cylinder is then machined and coated with nikasil for hardness. The problem with boreing this type of cylinder is it isn't designed for a thinner wall. If you bore it to put a steel sleeve in it you may not have the supports needed for a sleeve since the jug wasn't originally designed to support a sleeve. You also have to worry about how the head gasket will seal and if there is any movement in the sleeve. . . forget sealing that head gasket.

Is the bore of the 450 the same as that of the 439? You may be better getting a 450 jug and stroking the engine to get more cc's.

IMO the YFZ is pretty pumped in stock form. I don't think this is a good platform for heavy modification.

 

You can probably get a couple extra mm out of the stock jug without a sleeve and getting it niked.

That looks like a good deal! Nice thing is that is an installed price so if there are issues they can't point the finger at you and say you did something wrong.