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Thumper38

Decompression plugs

23 posts in this topic

Are the decompression plugs the same and are they needed for the 05? I had it pop out on my 03 and was wondering if the same would happed on the 05! :cry:

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Yes and Yes. Nothing has changed. Actually, they might have revised the part, but it looks the same, so....

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I am new to the 4 stroke world. I just bought my 05 Yz 450f a few weeks ago. Can someone go into more detail about the decompression plugs. I don't even know what they are much less that they fell out.

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I am new to the 4 stroke world. I just bought my 05 Yz 450f a few weeks ago. Can someone go into more detail about the decompression plugs. I don't even know what they are much less that they fell out.


All YZF cylinder heads except the '06 and later 450 are machined to accept a manual compression release shaft, even though they are not equipped with one any longer. Since '03, they have been shipped with a plug that looks exactly like a shaft seal without a hole in the center of it closing off that bore. It's made of light plastic with a rubber coating. This bore runs straight forward from the right exhaust cam follower and exits on the front of the head facing forward, at about 10 o'clock from the head pipe. You'll see a square lug with a threaded hole that has nothing in it right next to where the plug is.

The trouble with this is that without the decomp shaft in that bore, the plug is exposed to the unbaffled pulsing of crankcase pumping pressures generated by the piston, and can under a variety of circumstances just sort of randomly be ejected, leaving an open passageway in your cam box. Not Good.  Typically, if you experience this, you will find that your breather hose has been blocked or pinched shut, probably by one of the soft metal clips that hold it to the frame near the bottom. Be sure to check.

You can prevent this by installing a Decomp Bore Plug from a supplier on eBay (obviously the cleanest solution), safety wire (it's not the easiest thing to figure out a way to lash down), or use an 18mm steel or brass cup plug (freeze plug, expansion plug), or think of something else. However you fix it, you should do something about it, because eventually you're likely to toss the stock one.


The OEM plug is shown at the bottom of the camshaft/chain page, currently item 15, PN 90338-18064-00.

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I am new to the 4 stroke world. I just bought my 05 Yz 450f a few weeks ago. Can someone go into more detail about the decompression plugs. I don't even know what they are much less that they fell out.

To paraphrase grayracer - there is a rubber plug in the front of your cylinder head that is prone to falling out. Buy a machined one from Thumpertalk that you can bolt into place instead.

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It's made of steel with a rubber coating.

I'm not 100% sure that mine was steel on my '05 unless it's no thicker than a beer can.

I popped a small screwdriver right through it when I pried mine out. I was all set to drill it and run a screw in for a handle but I didn't need to.

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I popped the lid off last night and I see a little sand floating around with the cams. : (

If you ever riding in the sand, it's worth the $22 for the better plug from the TT store.

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I bought a TT plug for my '04 but haven't replaced it yet.The stock one never fell out on me.I did however have the bolt in the side start leaking one time because apparently the threads aren't too great.It leaked a little,but I was worried about the plug falling out more.I talked to a mechanic at North County Yamaha (nobody knows anything about four strokes around here,not YZs anyways) and he told me that you can actually have someone weld both holes shut if you'd rather do so.The bolt would thread tight if you removed the copper washer,so I just took the washer off,and machined off the thickness of the washer of the tapered end of the bolt and no leaks.

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be careful with the little bolt it can strip out really easily, dont overtighten it when you install the new plug and use a little locktite.I got a 90 degree offset geared tool from home depot for like $15 bucks, you can put a self drilling tek screw in the end of it 1/4 head and drill it into the plug and pull it right out, that way it takes about 30 seconds and you dont have to mess with the radiator.The plug was a thin sheet metal with a rubber coating.

B.T.W. the tool I was refering to goes in a drill

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The OEM plug can actually be removed just by prying it out of the head with a very small, like pocket sized screwdriver. They come right out with very little encouragement (that's the problem, isn't it?).

The biggest reason the lock bolts get stripped is that the new bore plug doesn't get inserted to where the locking groove lines up with the bolt nose. If this isn't done, the bolt comes up against the fat part of the plug shaft with only about two threads engaged. Check the alignment visually, and be sure you can run down the bolt until it seats with just your fingers, or very, very little effort at least. And, of course, they are small, so don't over torque.

The best way to install the seal is to put it in place over the plug shaft and use the plug to push it in Like This. That way, it goes in straight, and there's less risk of damaging it when you insert the plug.

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alright this may sound dumb, but can someone tell me where exactly the plug is? Is it in the front pointing forward, or is it in the side above the head pipe?

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sry nvm i just looked at grays link and figured it out, i was just worried about that weird little hole on the side of the head, what is that?

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(what) about that weird little hole on the side of the head, what is that?
That is a drain passage for the spark plug well, which prevents water or oil from standing around the plug should it get in there.

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Auto decomp was required for the electric start introduced on the WR in 2003.

I'm not sure but I think the YZ went to autodecomp the same year.

I believe the plug issue was eliminated when the head was redesigned in '06 or '07.

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Manual decompression went out with the 426. The '03 450 and 250F models were equipped with automatic decompression (actually compression reduction at cranking speeds) to allow the use of electric starters smaller than the engine itself.

The system works by lifting one exhaust valve off its seat during the first 50% of the compression stroke. As soon as the engine starts, a flyweight disengages the system, and the engine runs normally with full compression.

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