Auto DeComp Cam for WR426

17 replies to this topic
  • JBlinky67

Posted May 09, 2005 - 05:59 PM


I want to buy an '02 wr426. Does anyone sell a cam with the auto decompression like the '03 and newer Yamaha's? I don't mind not having an electric start, but I would like to have the auto decomp.

  • JMathis

Posted May 09, 2005 - 06:09 PM


Yes, do a search on Ebay or Google and you will find plenty. I got mine from a Yamaha shop in California from their ebay store. I highly recommend the cam, what a difference it makes in the kick. :)

  • Math

Posted May 10, 2005 - 04:49 AM


Hot Cam sells an autodecomp 426 exhaust cam (fits YZs and WRs).

Otherwise, get a YZ450 cam from e-bay. You'll end up to be YZtimed though.

  • JBlinky67

Posted May 10, 2005 - 06:11 AM


Do you remember what dealer it was? What was your search phrase? All I get is the Hot Cam listings. I guess that's ok, but I want to make sure I see all my options. Which cam has the decomp, the intake or exhaust?

  • Math

Posted May 10, 2005 - 10:15 AM


It's the exhaust.

If you try to look on e-bay, there is not necessarily one for sale right now... have a look frequently (everyday) in the look in ebay motor/motorcycle parts and search for "426"... you'll find them if any available.

  • JBlinky67

Posted May 11, 2005 - 04:07 PM


One more question: Is this a WR450 cam or is it a 426 cam with the decomp added? Is there difference in the power afterwards if I use the WR450 cam? If so, what? Sorry about all the questions, I just really want to get all the info available.

  • JMathis

Posted May 11, 2005 - 04:52 PM


It was North County Yamaha and I did not notice any difference in power, I did it specifically for the ease of starting.

  • Math

Posted May 11, 2005 - 05:32 PM


There is no 426 cam available with the decomp unless you go to the aftermarket like HotCams

All the 450 yammies (WR and YZ) have an autodecomp on the exhaust cam.

Not sure about the difference between the YZ450 and the WR450 exhaust cam in terms of powercurve but the timing is different for sure so there must be one difference in the powercurve.

With the 426s you could swap the exhaust cam from WR timing to YZ timing to WR....

On the other end, if you want to YZ time a WR450, you need to switch to a YZ 450 exhaust cam... I'm not sure to understand why exactly though... something about the electric starter and the positioning of the autodecomp... :)

  • JBlinky67

Posted May 12, 2005 - 11:47 PM


So let me get this straight: If I buy an '03 WR450 cam, I will be able to time it 2 different ways on the 426?

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  • Sylvain

Posted May 13, 2005 - 07:55 AM



I just installed a 2003 YZ 450 exhaust cam in my 2002 wr. I did that to have the auto decomp cam. But you also end up with the YZ timing if you go this way.

If you want wr timing with auto decomp you have to get the 2003 wr 450 exhaust cam.

If you install the 2003 YZ cam and time it like a wr, you will loose the auto decomp feature. Because the postition of the cam plunger will be changed.

With the Hot cam part the timing is neither YZ or WR. It's Hotcam's timing.

I hoped it helped.


  • byggd

Posted May 13, 2005 - 08:41 AM


I have read that the 03 YZ cam works fine on a 01-02 WR426 and it adds auto decomp.

Auto Decompression Upgrade
YZ400/426F & WR400/426F — 2000-2002 models
Auto Decompression Upgrade
YZ400/426F & WR400/426F — 2000-2002 models
This is a modification to allow the 2000-2002 year models of YZ and WR 400/426 Yamaha off-road motorcycles to be fitted with a
2003 YZ exhaust cam in order to get the benefit of the auto decompression feature from the 2003 models. This modification
seems to be suitable for both Steel (2000) and Titanium valved (2001-2002) model heads
Parts Required
Basically, a YZ450 exhaust cam (part no. 5TA-12180-00-00) and a selection of shims to set the valve clearances correctly.
You will also need access to the workshop manual and the correct tools sufficient to change the exhaust cam and set the valve
clearances. The most advanced tool you will need beside the usual spanners and sockets sufficient to change cams will basically
be a good “inch lb” torque wrench, and metric offset feeler gauges.
Installation is not very complicated. I will assume that the reader has removed/installed stock cams from their bike before.
1. Install exhaust cam in the same way you would install your old cam. Set the cam in the head so the top mark points straight
up, then rotate it forward (counterclockwise looking at the camchain side) one tooth. The intake cam marks will be slightly
retarded from what you are used to, don't worry about that. Note: If you install the exhaust cam with the timing mark
pointing straight up (which looks correct) the engine will not start.
2. Measure and record the valve clearance. The clearance will be too large by approx .1-.2 mm.
3. Use the valve clearance chart to determine what size the new shims should be (approx .1-.2 mm thicker)
4. Install shims and time cam. Refer to the pictures for setting cam timing. The 450 cam has two timing marks on it which won't
line up well with anything on the 426 head. (Figure 1) Check that the timing is correct by looking at the right side of the
cams. (Figure 2) They should look symetrical when installed properly.
5. DO NOT rotate the engine until you have checked the cam timing twice with the tensioner installed. Then roll the engine
over WITH A SOCKET on the crankshaft-SLOWLY!
6. Double check all your torque settings and finish reassembly of your bike. Start your engine and allow it to reach operating
temperature. Shut off and change the oil. You should be right to go, but take it easy for the first few laps, the new power
may surprise (and delight) you... First reports are a less severe midrange hit, more everywhere else... “deceptively fast”
Please note: Until there is sufficent feedback and many more miles on a number of bikes, this cam upgrade modification is only
to be recommended as something that you should try at your own risk. The 450 cam does have some minor differences compared
to the standard 400/426 cam, maily the base circle of the lobes, and the cam-chain sproket dimensions.
If you do this modification, please share your experiences and tips by posting to:
Source document is located at: For changes to this document, e-mail:
This PDF file is provided as an information service to interested parties. This modification is not endorsed by Yamaha Motor Corporation, or, and may void your warranty if installed in your
vehicle. Vehicles with such modifications may be used only on a closed course. ... and all that other stuff you never bother reading anyway but we should put in to protect everyone’s butts...
Figure 1. Left (clutch) side of bike, 450 cam installed properly -
do not rely on the dots or other marks as per the manual for
setting the cam timing correctly - see Figure 2 for details.
Figure 2. Right side view of head, note the symmetrical end positions on
intake and exhaust valves. This picture shows the correctly (YZ) timed
cam positions - do not use the conventional cam chain cog marks.
○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○
1 2
Exhaust Intake
Exhaust Intake
Auto Decompression Upgrade
YZ400/426F & WR400/426F — 2000-2002 models
Dyno Runs
These readings should be taken with a grain of salt. There are many flaws in the measurement and comparison. What is interesting
and useable info is the new cam in a fresh motor. Compared to the power curve of the older motor, you can see it has a
broader spread of power, with less of a mid-range hit, and a lot more ‘bottom end’.
1. Stock runs were made at 85 degrees with higher humidity, modified runs were made at 58 degrees with lower humidity.
2. Stock readings were on a significantly worn engine, and mod readings are a fresh cylinder, rings, valve job, etc.
3. Stock readings were on stock cast ports, and mod readings were with modified intake and exhaust ports.
4. The peak numbers are probably lower than actual output due to knobby tire slipping on the dyno drum (but the relative
readings should still be a good comparison).
Figure 1. Horsepower. What is significant is the lesser output at
about 7,500 to about 8,500rpm. This is the mellowed-out midrange.
Also, referring to just the new (red) cam, you can see there is more
HP showing both earlier and later in the rev range. Nice!
Figure 2. Torque. What Yamaha seem to have managed is more
torque in the rev range below 7,000 rpm, them basically about the
same torque characteristics as previous models, so it would seem to
have more ‘pull’ down low, better suited to the 4 speed gearbox.
This modification has been on trial in a number of bikes for a few months now... It seems to be reliable and the minor differences
seem to have no effect on wear, valve train noise, or anything else significant. A number of bikes have many hundreds of miles on
their cams, and all the owners are raving about the smoother power and the easier starting. People doing this mod with a YZ cam
and using WR cam timing may find the auto-decopression feature not as significant as it has been reported by those with YZ
timed bikes.
If you choose to remove the manual decomp hardware, then you can get the small blanking plug from the dealer to suit the hole
left in the head.
Please do not overtighten the bearing and journal cap bolts.. these require very small torque, something like 7.2 ft/lbs... don’t
hook into those with your 1/2” breaker bar...
Be careful out riding, that front wheel will come up on you even easier... But who ever minded that huh?
A great companion modification to this is the JamesNOW! Look at for the 2 page instruction
sheet and summary, including cutting templates and photos. This modificaton has had much positive feedback, esp. for low-end
pulling power and the extra crispness below half throttle.
Also now for 98-99 models
It has been reported to the forums at Thumpertalk that this mod is also suitable for 98 and 99 models of the WR400 and YZ 400.
The same fitment issues apply - you will probably need bigger shims, and perhaps for the bikes that have seen a few miles, you
may want to take the opportunity to freshen up the whole head, and check the health of your valves, their seats, the seals,
springs, retainers, bearings and journal surfaces, the cam chain and the tensioner, as well as the wear that may be apparent on
the intake cam lobes...


Posted August 06, 2005 - 09:55 PM


What's the difference between the OEM 2003 wr450 exhaust cam and the yz450 exhaust cam, besides the $50.00 difference. I have my case split for a gear rebuild and thinking about putting in the autodecomp. in while it's apart.

wr450 = part # 5TJ-12180-10-00
yz450 = part # 5TJ-12180-10-00



Posted August 06, 2005 - 09:58 PM


sorry screwed up on pasteing the part #'s

wr450 = part # 5TJ-12180-10-00
yz450 = part # 5TA-12180-00-00


  • Hamish

Posted August 07, 2005 - 02:11 AM


What's the difference between the OEM 2003 wr450 exhaust cam and the yz450 exhaust cam, besides the $50.00 difference. I have my case split for a gear rebuild and thinking about putting in the autodecomp. in while it's apart.

wr450 = part # 5TJ-12180-10-00
yz450 = part # 5TJ-12180-10-00


apparantly, the WR cam will give you better bottom end.
I have some headwork including the floors of my inlet ports built up, and run a YZ450 cam. My bike pulls like a tractor right off the bottom, even when you are in the wrong gear. I would go with the YZ cam. Be warned, you bike will be faster when you fit the 450 cam, but it will feel slower when compared to a YZ timed 426.

  • Sylvain

Posted August 07, 2005 - 06:46 AM


apparantly, the WR cam will give you better bottom end.
I have some headwork including the floors of my inlet ports built up, and run a YZ450 cam. My bike pulls like a tractor right off the bottom, even when you are in the wrong gear. I would go with the YZ cam. Be warned, you bike will be faster when you fit the 450 cam, but it will feel slower when compared to a YZ timed 426.

Agreed. I have the YZ450 cam in my wr and love it. I run a 15 teeth counter sprocket and there is still more torque than I need even for tight section. And as a bonus I find it's easier to maintain the front wheel up because of the longer pull. :D


Posted August 07, 2005 - 02:47 PM


Right now my 01 426 has the yz timing (cam rolled 1 tooth) would this have to be put back to stock timing when I put in the yz450 cam or can I leave it advanced and put the yz450 cam in.

I also bought the vertex piston to put in my bike since I'm doing the bottom mite as well do the top, does anyone have an opinion about the Italian vertex piston. I was told that theses are the OEM pistons in the KTM.

  • calvin71

Posted August 15, 2005 - 09:23 PM


Did you install your self on the cam and how hard was it? i have a 426 2000 do you have a price. and so if i get this cam i dont have to use the decomp lever when starting?

  • Dirt_rebel

Posted August 15, 2005 - 09:54 PM


i have the yz450f autodecomp cam on my 98wr n i love it!
bike now pulls endlessly!! best mod i ever done so far..
it's a must have mod for the older wrs'


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