Can't seem to bleed inner fork chamber

My 06 YZ450 has a cracked free piston (big surprise I know). So I ordered a set of aluminum pistons from Smart Performance. But I can't seem to get it to bleed out. I have followed both the Yamaha directions (which I have done before lots of times without incident) and the directions on the Smart Performance site. The results are the same. When I pump the rod up and down without the valve in place, no air. So Install the valve, push the rod in and it will extend by itself once or twice and then no more. In fact it the rod develops a negative pressure (pulling itself up a bit).


Do I have he wrong piston? The new unit only has one o-ring, while the factory piston has two.... Or am I missing something?


Once the piston has been vented, the top O-ring doesn't do anything but drag.  They are therefore omitted from the design, and should be removed when drilling the OEM plastic pistons.


Are you using one of the SPI ICS spring sets?  If not, it isn't too unusual to have the negative pressure condition, as there is no preload on some of the OEM ICS springs.  Your problem could be leaky rod seals, but bleed by the method I describe in the link below, and roll with that for a while.  See how it works.

As far as I know I have the stock springs. I will try your method when I get home. After I posted last night I went ahead and put the stock piston back in and it does not have the same problem. I also noticed with the stock piston that I have to push a little (feels like I am compressing the spring) to get it down far enough to start the threads. With the vented piston it more or less drops into place without much pushing. 


Also just so I am clear, if the rod does not extend by itself most of the way after bleeding and putting the valve back in, then I still have an issue. But if it extends most of the way, but has a little bit of negative pressure at the end then I should be good?



Do your new pistons also have a seal in the center for the shaft they run on?

I don't think so

That would be your problem.   Carefully switch out the seals from your old pistons and install them in the new.

I just got home and looked, there is an o ring in there. But it looks pretty recessed. I'm going to try your method of bleeding the air right now.

That method is not "mine" so much as it is an elaboration on Dave's (SPI) method.  Just fleshed out a bit is all.

Turns out it was defective pistons, or more accurately something with the seals. I finally got the replacements and they work fine

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