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buzzgrizz

decomp mod

31 posts in this topic

I finally decided that I'm going to do the mod after a horrible race I checked the stickey on it but I would like to known what all I am going to need tools included and what else i should replace while I'm in the motor thanks.

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The only specialized tools you'll need is feeler gauges to check valve clearance with, and it's a good idea to have a flywheel puller on hand in case you manage to get the chain looped off the bottom of the crank.

Which cam are you using?

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Only special tools are feeler gauges for checking valve clearance, a small in-lb torque wrench for cam cap bolts(cheap one from Harbor freight works fine for me, I also needed an angle extension to get into tight areas) and a flywheel puller for the cam chain replacement if you do that.

Will need a plug for the hole left after removing the manual decomp (can buy aftermarket, use freeze plug, or keep original mechanism in place)

I would replace cam chain while you have the cams off

Its possible to re-use gaskets, but the cam chain tensioner gasket or left side cover gasket (if you replace cam chain) may need replacement if damaged.

I used a hotcams ex cam since the marks all line up for timing and its about the same cost as a new oem cam anyway (if you use the oem cam you count pins on the cam chain)

You probably will need to adjust valves/shims after installing the new cam

Can’t think of anything else right now.

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Your going to need a 1/4" drive torque wrench for the cam cap bolts and a good set of feelers & flywheel puller, like Gray mentioned.

I would go ahead and replace the cam chain while you have it apart. :banghead:

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And don't forget a mag cover gasket... it seems like they tear pretty easily. :banghead:

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Your going to need a 1/4" drive torque wrench for the cam cap bolts

:banghead: How did I forget that?

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With the Hot Cams , or any other auto decomp cam made specifically for the 400 and 426, remember that the instructions for timing the cams found in the Mod threads DO NOT apply.

That cam times exactly the way a stock one would; line up the marks. The instructions that come with the cam should be followed here, not the ones in the DCM thread.

The intake should be usable. Signs of wear would be a concave dish in any lifter(s) (replace the cam and worn lifters), nose of the cam rounded over on a line parallel to the shaft axis, any visible scoring of the lobe contact surface, and scoring or measurable wear at the shafts.

Also, if the wear at the original exhaust lifters is significant, you should replace them.

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Will need a plug for the hole left after removing the manual decomp (can buy aftermarket, use freeze plug, or keep original mechanism in place)

QUOTE]

Where can I find a plug?

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Get a 18mm freeze plug from NAPA or another autoparts store, its about $1. I used stainless steel, since they didn't have brass, but I don't think it matters. Put red locktight around the lip of the plug and press it in. To press it in you can use a c clamp or big channel locks. I tried those and wasn't happy with the way it seated, so eventually took the right radiator off and used a socket to tap it in flush-I would recommend this method.

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Really there are 3 options: you can leave the decomp mechanism in place at the head for free (although if you do this, make sure to search on here because I think you may have to cut the internal portion of the mechanism off), use 0.69 cent freeze plug or pay for a nice aftermarket plug. I was too cheap to pay for the aftermarket plug:smirk: .

Either way, I would stress that it is really easier to take off the right radiator (or at least loosen to the point you can move it out of the way) so you can pop the original seal out and put the plug in. I tried without doing this and the time spent was more than if I just took off the radiator from the beginning.

Here's pics of the 0.69 cent option:

008.jpg

004.jpg

007.jpg

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  • Search eBay for "decompression plug" under YZ450
  • Use an 18mm "freeze plug" from an auto parts store
  • Use the stock '03-'05 POS (90338-18064-00) (if you duct tape it into the head, it will be at least twice as likely to stay there)
  • Cut the inner half-round tab and the lever off the OEM decomp shaft and use that

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Ideally, you twist and lift them out with your fingers (good luck with that, BTW). Sometimes a magnet works.

Next thing to try is to prudently grab them by the edges with needle nosed pliers.

The lifter bore tends to develop a bit of a burred top edge where it intersects the cam bore, particularly if there is any scoring there. The judicious use of a knife blade can scrape away a minor case of this. Clean up well.

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I use a magnet to suck them out. If you use a magnet, make sure its a strong one so it picks up the shim too. Using a weak magnet is a quick way to lose a shim.

When you install them, put the shim in first with you fingers and then oil the bucket up good and drop it in. Before you install the cam, make sure you can spin the bucket easily with your fingers... if you can't spin the bucket then take it back apart and do it again. Remember to triple check your valve clearances after your done.

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thanks for all the information I just have to wait on parts to get here now and I'm also replacing the water pump which shouldn't be a big deal should it?

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thanks for all the information I just have to wait on parts to get here now and I'm also replacing the water pump which shouldn't be a big deal should it?

Your welcome. Water pumps are easy. Just pay attention to which way the seals come out. There are 2 seals that face different directions. Be sure to check the shaft for wear too, its common for the seal to wear a grove in the shaft causing the leak.

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