found one that needs an engine rebuild, it has a K&N air filter, Maxiss rear tires, Dunlop front tires, full set of spare tires, Moose racing handle bars, DG front bumper, and Nurf bars. what i am most interested in is the engine across the years, do the newer ones have a noticeable increase in power? how hard is it to rebuild these?

 

The '04/05 model years actually have a little harder "hit" compared to the rest. They are not down on power at all, and the engine is about 98% the same throughout the model run. It will pay to do a little research, but it isn't any harder to rebuild than the other 450's(or most four strokes in general.)

A few things to do/make sure of

1) There may be a rubber plug near the top of the engine. It will blow out if overfilled with oil(easy to do if you don't know to run the motor for a bit before checking oil levels) Most have been replaced with an aluminum plug that won't blow out by now though. This was only an issue for '04

2) The charging system was designed to only work at high rpm or under load. As a result the battery may prematurely die. You can remedy this by always running with the lights on, or put a rectifier kit on that simulates the lights on. This is an issue with '04 and some '05 models. My brother has an '04, I have an '05. Both are run without lights or the rectifier kit, we just keep a trickle charger on them between rides(I also put a bigger battery in both though.)

3) Some '04 had a water pump seal issue. I wouldn't expect this to be an issue any more, as the bad ones were evident in the first few hours of use(visible leaking coolant.)

4) You can rebuild it with an '06 plus crank and get it to a true 450. The '04/05 models are 439cc as 440 was the limit for the national mx series when Yamaha was developing the machine. Actually either an '06-09 crank(shorter rod) and piston or I believe an '03-05 WR dirtbike crank(longer rod) and piston will get you to 449cc.

5) Look into the oil mod also if it hasn't already been done. It basically will bring the motor up to the '07 spec where oil is shot at the base of the piston/crank for extra lubrication and cooling. There are several ways to do the mod, I can PM you a link with a good way if you need it.


Good luck if you get it! Both my brother and I race ours, I haven't ever lacked power, just skill and/or courage, lol. They really are great machines, very few issues other than normal wear and tear on either.

 

I have the '04. Scooter hit everything I would mention. The rubber plug at the top. Change it immediately. It's cheap and when it blows your oil goes with it.

I initially thought I had the charging problem but, turned out I was mistaken in thinking I had already changed my battery. My newest battery has lasted 2 years no issues.

I have also replaced the water pump gasket. Easy thing to do. Only done it once luckily.

No experience with the oil mod. Could have been done before I bought the machine but, lots of reports on it.

As for power. Couldn't be happier. It is an awesome machine. I have found mine is much lower geared than other 450's. 2nd gear in mine is like 1st in my bro's 450R. Makes for better hill climbs and a powerful low, mid range.

 

if you could send me the link for the oil mod that would be great. what is all done with the oil mod? can the crank on these 450s be pressed apart to put a new rod on? i was on J&T's (i think that what it was called) website and they say that a full top and bottom rebuild can run over $2K, and i just about spit my coffee on the keyboard. is this true or are those exaggerated numbers to help sell their product? to put a crank and rod out of a 06-09 450 into this 04 is just buy it and drop it in, or are other mods required? are there any noticeable power gains? is it cheaper to buy a brand new 06+ crank and rod from Yamaha or to press the original apart (if possible) and put a new rod on? also just one more question, is the yfz 450 engine the same 450 that is in the dirt bikes?

 

J & T was the link I was going to send you. I was looking at just the oem high pressure oil pump is about $100, so what they charge is well worth the price. If you can do the work yourself, and rebuild before she blows, it won't cost anywhere near 2k. You can get a big bore kit for $5-600.00, and a new hot rods bottom end kit (crank, bearings, gaskets) for $450. You could easily shave a few hundred off that if you just recondition the stock jug. Yes, the '06 crank is a drop in item. If you are looking to rebuild the '04, I would just go with a brand new one now.

Yes, the YFZ motor is very similar to the dirtbike 450, especially the WR. For instance, you can retard the exhaust cam one tooth("cam mod") or just drop the bike cam in the stock location. They are the same grind cam, just a few small differences.