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84rm250

12.1 vs 13.1 piston for my yz426?

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I'm rebuilding the top end on my 02 426.

Would it be worth it to buy the 13.1 piston or should i stay with the 12.1? How much reliability would i loose with the 13.1 if any?

I just rebuilt the head with KibbleWhite SS valves so that part should be good.

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IF the bike came with titanium valves, only go with titanium valves...

SS could loose control and destroy your motor.

they are heavier and will put more ft. pounds on pressure

I cant remember the numbers off the top of my head but

titanium-50ish pounds

SS- 100-125 pounds

Steel 150 and higher

Honduh's come stock with steel, and thats why alot of people go with SS valves, lighter and puts less stress on the motor

Keep titanium if you want a trusty bike

They may have to be changed more often but its worth it

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I dont beleave my 02 came with titanium valves. Just steel and two were broke when i got the bike.

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IF the bike came with titanium valves, only go with titanium valves...

SS could loose control and destroy your motor.

they are heavier and will put more ft. pounds on pressure

I cant remember the numbers off the top of my head but

titanium-50ish pounds

SS- 100-125 pounds

Steel 150 and higher

Honduh's come stock with steel, and thats why alot of people go with SS valves, lighter and puts less stress on the motor

Keep titanium if you want a trusty bike

They may have to be changed more often but its worth it

I'm not sure this is entirely true. Search on here about SS valves.

http://www.thumpertalk.com/forum/showthread.php?t=742475&highlight=ss+valves

The 2000 YZ426 has SS valves, so you can use SS valves in the 2001/2002 as long as the 2000 springs and retainers are used.

SS advantage is they are cheap. Ti advantage is they are light. Due to laws of physics lighter will accelerate faster with equal force, so you can have a theoretically higher/faster revving engine. I probably wouldn't notice the difference to justify the cost of replacing Ti valves. I have ridden a 2000 426 and notice no difference, so will probably go with SS if/when I replace mine.

As far as piston, go with OEM , its more reliable. I wouldn't go with the high compression unless you feel the 426 is down on power:lol:

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The '01-'02 models were built with Ti valves. The '00 came with the same SS valves the YZ400 had, and there is no reason these cannot be used in the '01-'02 head.

HOWEVER, because the SS valves are heavier, you YOU MUST USE THE CORRECT VALVE SPRINGS from the '00 model along with the valves.

There will not be a noticeable performance disadvantage, and one will not really last longer than the other, but you will notice the price difference. The SS valves are about 1/5 the cost of Ti. If it were my engine, I'd use them for that reason alone.

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... you will notice the price difference. The SS valves are about 1/5 the cost of Ti. If it were my engine, I'd use them for that reason alone.

But Gray you must see the huge weight advantage in a race! :thumbsup:

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If it were my engine, I'd use them for that reason alone.

This is the reason I went with the SS valves. So I do need diffrent valve springs other then my stock 02s to run these SS? Should I buy the kibblewhite spring set? Or is there a cheaper way?

As for the piston, I think I will stay with the stock 12.1 for now. I wanna keep the bike as gas pump friendly as possible, 91 is as high of octane I can get around here unless I hit up the airport for ab gas.

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This is the reason I went with the SS valves. So I do need diffrent valve springs other then my stock 02s to run these SS? Should I buy the kibblewhite spring set? Or is there a cheaper way?

As for the piston, I think I will stay with the stock 12.1 for now. I wanna keep the bike as gas pump friendly as possible, 91 is as high of octane I can get around here unless I hit up the airport for ab gas.

Use the OEM springs for a 2000 model. They should work fine.

Most of the time, maybe unless you operate at or near sea level a lot, you can run pump 91 octane with 13:1 OK. Don't use AvGas in a motorcycle. It's just the wrong stuff. I think you're wise to stay with the OEM piston unless you're racing a lot, and loosing races because you lack power.

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More than accelleration of mass, Ti valves are to make sure they can close fast enough to not hit the piston at 10+ grand. When they do hit the piston (which happens a lot with new gen bikes) you lose almost the entire motor. I've seen more than a few CRF/YZs that lost everything inthe top end ,the crank and cases, even the trans main shaft from one of these high rpm explosions. 13-1 will give about 10% more power with 10% more heat.

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Increasing the compression from 12.3:1 to 13:1 will produce about 1-1.5 HP at peak, which is closer to 3%. The heat increase is nowhere near that great either. Most of the power increase with higher compression tends to come at the lower end of the RPM scale.

Valve float is a very rare problem with modern MX bikes, regardless of valve material, although using Ti does make it even less likely. Most of the failures you mention are the result of valves breaking or chains jumping time, not valve float.

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Increasing the compression from 12.3:1 to 13:1 will produce about 1-1.5 HP at peak, which is closer to 3%. The heat increase is nowhere near that great either. Most of the power increase with higher compression tends to come at the lower end of the RPM scale.

Valve float is a very rare problem with modern MX bikes, regardless of valve material, although using Ti does make it even less likely. Most of the failures you mention are the result of valves breaking or chains jumping time, not valve float.

3% would make 13-1 even less worth while. Valves break from the stress of repeated opening/closing cycles. The stress is increased exponentially when the rate of acceleration /decelleration is increased linear. I am amazed at what these modern engines can survive but when they do hit their limit it's usually catastrophic failure. Ti valves also help reduce harmonic vibration in the entire engine. Cam chains will jump time from valve float. The ultra short stroke on these motors reduces rod loads at ridiculous rpm but not enough to keep them from parting company when limits are exceeded. All these motors are on the edge stock. They are designed for high maintenance racing. That means frequent inspection and parts replacement. If you're good with that put as much compression in as a gas engine can take.

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Valves break from the stress of repeated opening/closing cycles. The stress is increased exponentially when the rate of acceleration /decelleration is increased linear. I am amazed at what these modern engines can survive but when they do hit their limit it's usually catastrophic failure.
You are missing the point. Certainly, from the perspective of someone your age who can remember how things were over 30 years ago, the modern 4-stroke as built today seems to operate at an almost incomprehensible level, there is no doubt of that. However, the issue of valve float has not been a significant problem for a good many years now, due mostly to spring metallurgy and rev limiters, but also in part to the use of ultra lightweight titanium valves. There are, nevertheless, a number of extremely high performance engines running loose on the planet still using steel or stainless steel valves, and they also do not typically have any problem with valve float. The exception is "mechanical over-rev", brought on by downshifting too early, for which there is no preventative device in place on any motorcycle I'm aware of (several F1 cars do have this under control, however).

Fatigue as a result of repetitive cycling is also an issue, and is aggravated by valve guide and seat wear.

But your contention that the cam chain is placed under greater stress as a result of increased power output is simply incorrect. None of the valve train bears any part of the engine load. What loads the cam chain is the load of rotating the cams. True, accelerating the cams themselves from a low speed to a higher one does place a load on the chain, and the greater that acceleration is, the greater the load, but there again, the cams are accelerated the quickest in the lower gears, or in neutral, or as the wheel looses contact with the ground under power, irrespective of the amount of power produced. Saying that more powerful engines load the cam chain more because they accelerate more quickly in really reaching for ways to split hairs.

Cam chains also experience little or no stress due to valve float. For those unclear as to what valve float is, it is the condition that occurs when the inertia of the valve opening exceeds the resistance of the spring pressure seeking to close it, and it fails to stay in contact with the cam lobe on the closing side. This causes several problems, but no stress on the chain whatever.

Things that add to the load on the chain by increasing the force required to turn the cams are increasing the speed at which the engine operates, either absolutely or on average, increasing the lift or lift rate of the cams, heavier valves and the required heavier springs ( a point in favor of Ti valves ), and the complete or partial seizure of the cams. Increases in the output torque of the engine are simply not transferred to the chain in any way.

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wiil the stainless steel valves work in a 2001 wr426f? i bought a valve and spring kit for a 2000 yz426f and was wondering if they will work for a 2001 wr426f?

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