Will an '05 yz450f motor fit on an '03?

The motor on my '03 YZ450f recently seized. It busted the piston and cylinder and bent the valves, etc., etc.

I may have a chance to pick-up a motor complete with Carb, CDI Box, and all motor wiring except for the kill switch from an '05 YZ450f for a decent price. Will this fit on my bike? Are there any parts that are not compatible between the two different year bikes?

What are some questions I should ask about this motor before I purchase it?

I am strapped for cash, so I'm looking for every less expensive option than to pay $2000 to fix mine.

Thanks.

it should fit,pretty much the same from 03-05:excuseme: one good question might be if its gaurunteed to run.

u would prob. be better off looking for an 03 motor on ebay.....good luck:ride:

Why? There are no significant differences in the two engines. If you use the '03 CDI and harness, you'll hardy know it;s not an '03.

Why? There are no significant differences in the two engines. If you use the '03 CDI and harness, you'll hardy know it;s not an '03.

that and the 05 has less engine braking due to a faster primary gear ratio than the 03/4:thumbsup:

The primary drive ratio is higher (lower numerically) in the '05 and later to improve the performance of the clutch. To compensate, the later bikes have lower final drive ratios (13 tooth front sprockets vs. 14 on the older bikes). Since this results in the same overall gearing, the engine RPM relative to rear wheel speed is the same, so it has no effect on engine braking.

The primary drive ratio is higher (lower numerically) in the '05 and later to improve the performance of the clutch. To compensate, the later bikes have lower final drive ratios (13 tooth front sprockets vs. 14 on the older bikes). Since this results in the same overall gearing, the engine RPM relative to rear wheel speed is the same, so it has no effect on engine braking.

the clutch spins faster at the same rpm,therefore less engine braking. the secondary gearing had to be changed or the thing woulda been like running a 15t. anyway you look at it this change was done to reduce engine braking and was advertised as such by yamaha :applause:

in fact;from yamahas PIG (product information guide) "all new internal primary ratio to spin the gears faster for reduced engine braking"

Yamaha also advertised that the '04 had increased rotating mass, and except for a bigger rod bearing, that was BS.

Clutch speed has nothing to do with engine braking. Ask yourself this: Do you have more engine braking in third or in second? It's about the speed of the engine, unless the engine is fundamentally different in some way, such as compression ratio, cam timing, wrist pin offset, etc. Idle speed will change it somewhat, too. But the primary ratio won't change it unless the overall gearing, and thus, engine speed changes.

Yamaha also advertised that the '04 had increased rotating mass, and except for a bigger rod bearing, that was BS.

Clutch speed has nothing to do with engine braking. Ask yourself this: Do you have more engine braking in third or in second? It's about the speed of the engine, unless the engine is fundamentally different in some way, such as compression ratio, cam timing, wrist pin offset, etc. Idle speed will change it somewhat, too. But the primary ratio won't change it unless the overall gearing, and thus, engine speed changes.

i thought you were a smart fella there gray but even my unedgmacated arse can figure this one out. ok,(these are just thin air numbers to use as a constant) the 04 motor has a slower primary gear ratio,so the clutch and mainshaft spin at 500 rpm @ 5000 rpm motor speed;lets say output shaft speed in 1st is 125 rpm,2nd 250 rpm,3rd 375 rpm,and 4th 500 rpm. so using 14/50 final gearing mph speeds are say 25,35,45,and 55.

lets say the 05 motor at the same 5000 rpm has 625 rpm clutch speed(25% increase) so in 1st gear the output shaft rpm is now 156.25 and on down the line in 2(212.5),3(468.75),and 4(625).

so to keep the mph close to the same you go to 13/50 on the 05(remember all my #'s are hypothetical,so if 13/50 ain't 25% less than 14/50 oh well)

so in any compared gear at 5000 rpm either bike will be doing the same mph,now when you let off 4th gear on the 04 the internals are spinning 25% slower than that of the 05. let off the 05 in 4th and the stuff is spinning 125rpm faster,my guess is that the inertia of the faster moving gears is gonna produce less percieved engine braking by overcoming the actual compression braking of the motor.:applause:

Your hypotheticals are a bit far off the mark, old friend.

The primary ratio of an '03/'04 YZ450 is 2.818. At any RPM, divide by that, and get your mainshaft (clutch) RPM. For instance, at 5000 rpm, the clutch and main shaft are turning 1774 rpm. In the '05 and later models, the primary ratio was bumped up to 2.652. There is a 6% difference between these two numbers. Taking the same 5000 rpm and reducing it by the ew primary ratio, it comes out 1885 rpm, only 6% faster.

Now, then, the transmission ratios of the '05 and the earlier models were identical, but the '05 was built with a 13/48 final, I believe. That gives you an overall gear ratio (engine to rear wheel) n fourth, since you mentioned that, of 10.282 (2.652 x 1.050 x 3.69 (48/13)). Comparing that to an '04 with a final drive of 14/48, we find that it has an overall ratio in fourth of 10.145. So, stock for stock, in any given gear, at any given speed, the engine in an '05 is actually spinning just over 1% faster. 1%.

The ratio was changed to reduce the load on the clutch plates. It has no effect on engine braking.

So, is it true that if I use my '03 CDI box and harness with the '05 motor, I will not have to change my sprockets? Did they make any positive changes from 03 to 05 CDI'd that would make me want to use the 05 CDI?

No, the increase in the primary drive ratio on the '05 means that you'll need to adjust your final drive (rear chain & sprockets) by 6% to match up with the gearing you had before. Going from a 14 to a 13 tooth front (the '5 engine may actually still have one on it) will do that almost exactly.

Using the '03 CDI should sharpen the somewhat soft performance of the '05 somewhat, however.

Great, thanks for all your help.

Just to summarize...you would advise using my '03 CDI box with the new '05 engine and I should put a 13 tooth front sprocket.

How do you mean the '05 is softer that the '03?

The '05 doesn't have the aggressive low end hit that the '03 does. However, the camshafts and flywheel weight are the same in both bikes. Most of the change in character is a result of remapping the ignition.

Your hypotheticals are a bit far off the mark, old friend.

The primary ratio of an '03/'04 YZ450 is 2.818. At any RPM, divide by that, and get your mainshaft (clutch) RPM. For instance, at 5000 rpm, the clutch and main shaft are turning 1774 rpm. In the '05 and later models, the primary ratio was bumped up to 2.652. There is a 6% difference between these two numbers. Taking the same 5000 rpm and reducing it by the ew primary ratio, it comes out 1885 rpm, only 6% faster.

Now, then, the transmission ratios of the '05 and the earlier models were identical, but the '05 was built with a 13/48 final, I believe. That gives you an overall gear ratio (engine to rear wheel) n fourth, since you mentioned that, of 10.282 (2.652 x 1.050 x 3.69 (48/13)). Comparing that to an '04 with a final drive of 14/48, we find that it has an overall ratio in fourth of 10.145. So, stock for stock, in any given gear, at any given speed, the engine in an '05 is actually spinning just over 1% faster. 1%.

The ratio was changed to reduce the load on the clutch plates. It has no effect on engine braking.

well hypotheticals are just that,how close they are is irrelevant. 25%....6% whatever i didn't feel like wasting the time to research the exact #'s.

and i'm gonna venture that the rotating mass involved equals more than a 1% reduction in engine braking. but you know it all:banghead:

btw; nowhere in any of the literature i researched before buying my 05,or since then has stated the primary drive ratio's were changed for clutch performance. it did however mention reduced engine braking several times....hmmmm:excuseme:

oh and if the final drive only changed 1% and the primary ratio 6% then the 05 will be geared taller therefore lowering the rpms at a compared mph (04/05)...stock for stock less engine braking?

oh and if the final drive only changed 1% and the primary ratio 6% then the 05 will be geared taller therefore lowering the rpms at a compared mph (04/05)...stock for stock less engine braking?

The '04 final drive ratio using a 48/14 is 3.43. The final drive ratio of an '05 using 48/13 is 3.69. That is a difference of 5%, not 1%. Comparing the overall final ratios is where the 1% comes in. The '05 has a 6% change in primary ratio compensated for by a 5% change in final ratio, for a net difference between the two bikes of 1%.

Clutch inertia has very little effect on overall engine inertia because the speed is so much lower than than the crank, and the clutch, in attempting to drive the crank, is at a 2.5-3:1 disadvantage. And then, to change it only 6%? Yes, theoretically, it adds some small amount flywheel effect (roughly 1/5 of the 6% increase in clutch inertia), but I seriously doubt it will be noticeable. Let me also point out that going from a stock to an 8 ounce flywheel (directly on the crank) makes almost no difference in engine braking above 3000 rpm, anyway.

the final drive is not in question in the 1st place,quite simply if your getting the same mph(+-1%) by the changes in final gearing and the transmission internals are spinning faster in the 05....when you let off the gas there absolutely has to be less resistance due to the fact there is less reduction to overcome between the wheel and crank:foul:

Since, during engine braking, the rear wheel is driving the crank, the only reduction that matters is is the overall reduction between the rear wheel and the engine. As we have have established that there is only a 1% difference between the two, with the '05 actually having the greater overall reduction, how does that square your statement?

You'll get a bigger change in engine braking by raising your idle than by a compensated change in primary drive ratio.

The relative speed of the internals will make a big difference (in regards to engine braking) do to "moment of inertia"... :applause:

Just a personal observation...from the pants feel for the two bikes I cannot feel any difference in engine braking between 04 and 05. I own the 04 and my riding partner owns the 05 but mind you this isn't with stock gearing. You guys got WAY too technical for me to understand lol.

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