2 stroke vs. 4 stroke rebuild

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dlholder21

Member
Dec 24, 2008
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What is the price difference between a 2 stroke and a 4 stroke rebuild? What all is envolved with a 4 stroke rebuild?
thnx
 

IndyMX

Crash Test Dummy
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Jul 18, 2006
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Amo, IN
Depends on what you are talking about.. Top end? Whole engine?

Top end on a smoker usually is a piston, wrist pin, rings, measure the bore and possibly have the cylinder replated. Plus all associated gaskets.

Top end on a thumper you would do all that, plus possibly replacing the valves, springs, seats, valve guides..

As for cost, a basic top end on a thumper isn't that much more than a smoker, so long as you aren't messing with the valves. However, the valves are pretty pricey to replace.

Then there is the labor cost. If you aren't familiar with the workings of a thumper, and aren't the type that learns quick, you are best off letting someone with experience do the work. $$$$$

Having said that, and having almost completely rebuilding my 450 on my own, I'm not one bit unhappy about having to do it. The smile that thing puts on my face versus the 125 I have, more than makes up for the hassle of the work.
 
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sharky243

Member
Dec 14, 2008
246
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My 03' yz450f has probably close to 400 hours on it. I've replaced the piston twice, but the valves, guides, seats, and crank are all origonal. It is due for valves now but the bike has been very good, it won me a lot of races !!! 4 strokes can be a bit more costly to re-build, more so if you let it blow up, but if you maintain them, they will last a long time. Once you get past the valve train, the 4-strokes are basically the same as a 2-stroke. The valve train is pretty simple aswell. I did ride an older KX500 once after I re-built it for the owner. All I can say is WOW !!! It takes some balls to be able to hang on to a 500cc 2-stroke.
 
M

Matt90GT

Member
May 3, 2002
1,517
1
Depends on the motor really.

If you get into a late model motor that has power valves, you have just as many parts as a 4 stroke that can break or need replaced.

If you go say late model CR450 vs CR250R, if you had to replace everything from the crank up you are probably about the same cost. Most of the time, the power valves dont need replaced. That cannot be said for the valves of the 4 stroke. Plus you need new cam chain and adjuster there.
 
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motometal

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Sep 3, 2001
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Matt90GT said:
If you go say late model CR450 vs CR250R, if you had to replace everything from the crank up you are probably about the same cost.
Click to expand...

How would you conclude that, when the four stroke has more, and in some cases more expensive parts?

I'm not counting the power valve stuff on the two stroke because honestly in my experience the most you would have to do is clean it with rare exceptions. Intake and exhaust valves open and close many times more often than power valves cycle.
 
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Matt90GT

Member
May 3, 2002
1,517
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have you ever seen a 90s KX250 cylinder? they have a TON Of power valve parts. And each part is $50. Sounds like a 4 stroke! haha. And the parts seize and then start stripping the gears off.

Like I said if you were to replace EVERYTHING from the crank up, it would be close the same amount.
 

IndyMX

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CR450 is a two stroke.. note the lack of an F
 
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sharky243

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Dec 14, 2008
246
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If all you had to do was change the piston and rings, the cost is not much more than a 2-stroke. If you have to start changing valves, thats where extra cost comes in to play. Take the yz450f's for example, If you were talking OEM valves, each valve is $75.00+, and there are 5 of them. That doesn't include re-cutting the valve seats if needed either. Valve guides, there's another expence but they rarely need to be changed.
 

Patman

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Dec 26, 1999
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sharky243 said:
My 03' yz450f has probably close to 400 hours on it. I've replaced the piston twice, but the valves, guides, seats, and crank are all origonal..
Click to expand...
:think: Maybe you should visit Las Vegas.
 
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motometal

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Sep 3, 2001
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Indy....sorry, you lost me on the CR450 two stroke thing. Huh?



Regarding the valvetrain, last I heard the experts recommended changing the valve springs at regular intervals...which makes sense considering how many cycles they go through. The springs aren't too expensive considering the consequences of their failure.
 
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sharky243

Member
Dec 14, 2008
246
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Are you trying to say that I'm lucky Patman ? that can't be farther from the truth. LOL !!! Eveyone I know that has the same bike as I do say the samething. The 03' YZ450F's are vertualy bullet proof as long as you maintain them, and use good oil. Riders that don't oil the air filters properly, tend to have more valve issues because of dirt ingestion. Thats like sand blasting the inside of your combustion chamber. :yikes:. Something I've always done though is replace the valve springs whenever the top end was apart.
 

whenfoxforks-ruled

Old MX Racer
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Oct 19, 2006
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Merrillville,Indiana
The crf450 is comparable to the cr500, yeah the 450f is a little smaller? But that f would cost a ton more to rebuild!!!!!!!!!!!!
 

IndyMX

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Here's a pic on a CR450R 2 stroke

http://seanano.org/vehicles/pastvehicles/cr450r/images/09252001/right.jpg
 
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