Drill or not drill free piston on 09 yz250f.

[pirogue]

I'm changing oil in my forks, I weight 190#s w/o gear ride mx tracks, do I drill the piston as per Smart Performance or no.

Thanks

[mog]

why wouldnt you?

[harrperf]

the better question is why SHOULD you.

Is it better to have that rise in cartridge pressure near the end of the stroke or not...

[DaveJ]

the better question is why SHOULD you.

Is it better to have that rise in cartridge pressure near the end of the stroke or not...

This is actually an interesting question because if you don't drill them, then the third chamber remains closed or isolated. The pressure build-up within that chamber is constant and rather minimal.

Open up the free piston and thereby remove the third chamber, you then expose the air pressure of the lower chamber onto the backside of the free piston, which by comparison, is a lot...and certainly a lot more force than the IC spring is producing at that point...assuming the lower oil level is within range to make a difference. However, in this scenario the real piston is the rod. ?

But the issue with drilling the free piston is more about making sure you keep them from cracking. Yes, I know....drilled free pistons still crack, but not a new drilled free piston.

And the factory bikes run a vented piston, by the way.

[455]

I'm seriously thinking on replacing my drilled free pistons and going it back to stock.

I didn't feel any performance gain with drilling them, if anything it may have added mid-stroke harshness and less bottom out control? I say that due to the oil volume change with the drilled pistons.

Only one way to find out, I'll be calling Dave and placing an order at some point.

[harrperf]

This is actually an interesting question because if you don't drill them, then the third chamber remains closed or isolated. The pressure build-up within that chamber is constant and rather minimal.

Open up the free piston and thereby remove the third chamber, you then expose the air pressure of the lower chamber onto the backside of the free piston, which by comparison, is a lot...and certainly a lot more force than the IC spring is producing at that point...assuming the lower oil level is within range to make a difference. However, in this scenario the real piston is the rod. ?

But the issue with drilling the free piston is more about making sure you keep them from cracking. Yes, I know....drilled free pistons still crack, but not a new drilled free piston.

And the factory bikes run a vented piston, by the way.

eh...some do!

Honda and ktm??

doubt it.

[cj_wai]

the new metal replacement pistons are vented.they also have no top o-ring anymore.

[DaveJ]

eh...some do!

Honda and ktm??

doubt it.

I saw the inside of the forks on Chad's YZ, back in the day, and they were metal and they were drilled.

Not sure about the rest.

But if anyone hasn't noticed yet, the 2010 CRF250R forks, SHOWAs with KYB internals, have a bleed or vent screw for the back side of the free piston. So it would not surprise me if some of the works stuff has this chamber isolated.

Coming soon - free piston sub tanks! ?

[pirogue]

Well they are drilled now, not sure if I ride fast enough to tell right now:ride: Not sure if I bled the first fork IC right or not. left the bleed hole open on the top cap due to motion pro bleeders and a lil shot of air weeped in while bleeding cartrige(in the early stage).

Thanks.

[mog]

I'm seriously thinking on replacing my drilled free pistons and going it back to stock.

I didn't feel any performance gain with drilling them, if anything it may have added mid-stroke harshness and less bottom out control? I say that due to the oil volume change with the drilled pistons.

Only one way to find out, I'll be calling Dave and placing an order at some point.

I would think it hard to get more harshness and less bottoming control at the same time, i think its all in your head ?

I have drilled them and felt no real difference at all.....

[grayracer513]

Well they are drilled now, not sure if I ride fast enough to tell right now:ride: Not sure if I bled the first fork IC right or not. left the bleed hole open on the top cap due to motion pro bleeders and a lil shot of air weeped in while bleeding cartrige(in the early stage).

Thanks.

The air bleed hole in the cap does not vent to the interior of the cartridge. The passage leads to the outer chamber only.

[455]

I would think it hard to get more harshness and less bottoming control at the same time, i think its all in your head

I have drilled them and felt no real difference at all.....

Sure it can do both. By drilling you have effectively lowered your oil level volume from where it was before the drill. Then because it is no longer isolated you have the added outer chamber pressures and it's influence on the back side of the free piston during mid-stroke.

I'll test it so I dont have to guess.......?

[pirogue]

The air bleed hole in the cap does not vent to the interior of the cartridge. The passage leads to the outer chamber only.

sounds great!

thanks again:thumbsup:

[mog]

I have tested back to back with and without, the difference is marginal at most, 5cc of oil either way would throw out the feeling more IMO.I am not sure with a few small holes or one big one, pressure will transfer fast enough for it to matter even.

Like i said its theoretical not actual, or its all in your head not in your riding ?

[MRW]

The Factory Connection and Technical Touch forks I have seen were not drilled. Just making a note:ride:

[mog]

strange as ross told me to drill them....

[Jusa]

The Factory Connection and Technical Touch forks I have seen were not drilled. Just making a note:ride:

Details, please.

[455]

strange as ross told me to drill them....

You should know by now to always listen to me!

?

[DaveJ]

The Factory Connection and Technical Touch forks I have seen were not drilled. Just making a note:ride:

I know that ENZO drills there, and I'm almost certain that Factory Connection does.

In either case, I think we all know that they crack and I think we now know why they crack, so I can't think of why you would not want to drill (vent) them.

[harrperf]

Often I see ICS pistons with fluid trapped in them. These are pistons that have NOT been drilled yet.

Now obviously the seal is leaking to some extent.

That said, when this occurs, I think it's MORE likely that the piston gets filled enough such that it hydrolocks to an extent...and thus causes breakage.

Obviously drilling them "fixes" this.

But I still don't feel it is a performance improvement in any manner.