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Does your powervalve linkage cover leak oil?

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I see a lot of older YZ250 with oil leaking from the rubber portion of the Power Valve linkage cover gasket.  The oil always covers the area around the water pump.

 

A few years ago I figured out that the cause of this problem is wear on the main powervavle shaft and the bore it rides in.  The shaft develops a bunch of play and the tiny seal cannot do its job.

 

The PV shaft is an 8mm diameter steel shaft.  It rides in an aluminum bore about 8mm wide at the left side of the cylinder and only about 2.5mm - 3mm wide at the right side of the cylinder.  Trouble is that right side, where the supporting bore is very shallow sees the most radial loading and stress because the linkage is pushing on the shaft in order to open the valve.  The bore wears out and eventually the seal leaks.  Yamaha engineers probably weren't worried about this issue because the life of the cylinder plating and the life of that shaft-bore are about the same.  When we re-plate our cylinders to extend their service life, we can run into this problem.

In my case, I bought a used cylinder with bad plating and had a 295 big-bore kit built for my newer bike.  The fix  I came up with is simple and is detailed below in photos so that anyone interested can make these modifications, or have a machine shop do it for you. This is a permanent fix, it should last far longer than the original design, and should extend the life of the shaft as well.  I hope it's useful to some of you guys.

 

 

The flanged bronze bushing came from McMaster Carr.  It has 8mm ID/Bore, 11mm OD main, 14mm OD flange:

 

 

IMG_4404_zps296a30f6.jpg

 

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I used a reamer to open up the aluminum shaft bore in the cylinder casting to slightly under 11mm so the bushing will press-fit.

 

 

 

 

IMG_4409_zpsb1275211.jpg

 

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Final assembly, the shaft is supported by a much wider bearing surface using this bushing:

 

 

 

IMG_4419_zps7dec2e92.jpg

Edited by cwtoyota
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Great solution!

 

My bike has unknown hours on the cylinder, and that area was starting to get wallowed out as well. Thought it was pretty cheap of Yamaha to not run at least a bushing in that area, or a much larger bearing surface (really should be at least as thick as the shaft's diameter.  I planned on making a bushing if things ever got that bad, but they never did. And now I'm parting it anyway....

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Do you guys replace that seal with each top end?  I had countless hours on my 2000, now over 100 on my 07 and I don't get a bit of seepage through there.  I replace the seal each top end (40 hrs).

 

CW- Did you maintain the seal or did the bushing take the bore?

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I did seals with mine. Didnt leak, but I just noticed how loose the saft was getting in there.

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Wow, great in depth and logical fix !  I have an issue with the seal/bore fit on my Athena 144 cyl . The seal pops out of the bore very easily. Next top end, i'll have to review this mod and see if it will help my issue.

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I see a lot of older YZ250 with oil leaking from the rubber portion of the Power Valve linkage cover gasket.  The oil always covers the area around the water pump.

 

A few years ago I figured out that the cause of this problem is wear on the main powervavle shaft and the bore it rides in.  The shaft develops a bunch of play and the tiny seal cannot do its job.

 

The PV shaft is an 8mm diameter steel shaft.  It rides in an aluminum bore about 8mm wide at the left side of the cylinder and only about 2.5mm - 3mm wide at the right side of the cylinder.  Trouble is that right side, where the supporting bore is very shallow sees the most radial loading and stress because the linkage is pushing on the shaft in order to open the valve.  The bore wears out and eventually the seal leaks.  Yamaha engineers probably weren't worried about this issue because the life of the cylinder plating and the life of that shaft-bore are about the same.  When we re-plate our cylinders to extend their service life, we can run into this problem.

Great post. Thanks.

 

In your experience, to get say 50 hours out of a new seal, how much free play on the stock alum bearing surface can be tolerated?

 

Also, when the play gets excessive, what's the impact on the PV function?

Edited by numroe

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Great post. Thanks.

 

In your experience, to get say 50 hours out of a new seal, how much free play on the stock alum bearing surface can be tolerated?

 

Also, when the play gets excessive, what's the impact on the PV function?

I can't speak to the life of the bore, as I've never worn one out on my own.  The cylinder I used for my 295 top end was a used core from flea-bay...  It had damaged plating and a worn out pv shaft bore when I sent it to EG for the mods.

My 2002 had a little play when I bought the bike used in 2007.  That was my primary bike for 2.5 years.  It has a lot of play now and it leaks too...  I finally got around to doing this mod that has been on my mind since I did the first top end in that used 2002 bike back in 2007.

As for the impact on PV function, I really doubt it's very much in terms of timing of the valve.  It could cause that steel stopper piece to mangle the round part at the upper end of the aluminum main powervalve.

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wow, great thread!

I'm currently fixing a worn out 2002 yz250 with the same problem and have the bushes made and ready to install.

 

On a separate note, my own bike is a 2006 yz250 with an oil leak from near the power valve cover. After some detective work, I finallly figured out that my 2006 had a blocked crank case breather.

My leak looked just like this (this is not my post by the way):

 

http://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=video&cd=1&cad=rja&uact=8&ved=0CDIQtwIwAA&url=http%3A%2F%2Fwww.youtube.com%2Fwatch%3Fv%3DHnIjlpTNu3s&ei=Q3EyU5-kIsWC2wW-oYHICA&usg=AFQjCNHbcabB85jkBGSDgd3zU4sZKx-HsA&sig2=qQqqIGftqtrwKTv18FbMmA&bvm=bv.63738703,d.b2I

 

and i bet this guy had a blocked engine breather (the fat black breather hose), and didn't even know it.

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How is there enough space for the seal to be used on top of the new bushing?

When the new bushing was pressed in, does the outer edge of the flange contribute to the press fit surface area? Or is the press fit zone only the 11mm diam surface (the inner 2.5mm width)?

It would be good to use the flexible seal part if possible.

I've studied your pics, and have mine on the bench. But I'm unsure on these two things.

If you had another couple of pics like this one, but with the bushing inserted, then another with the seal in place, then it'd probably make more sense.

Thanks very much for the help.

 

A few years ago I figured out that the cause of this problem is wear on the main powervavle shaft and the bore it rides in.  The shaft develops a bunch of play and the tiny seal cannot do its job.

I used a reamer to open up the aluminum shaft bore in the cylinder casting to slightly under 11mm so the bushing will press-fit.

IMG_4409_zpsb1275211.jpg

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To CW or anyone else who has done this mod:

I think I understand:

- the dimensions of the machined bushing before press fit. Including a 1.5mm long/thick 12mm OD flange.

- the inner original alum hole is reemed out to 10.95 or something to give a press fit.

- The OD of the seal is 12mm to fit into the 12mm recess in the alum hole.

- The seal is 3mm thick.

So if the seal is 3mm thick, and the flange is 1.5mm thick, with the seal installed on top of the bush flange, does this mean the seal to alum surface area is halved? I assume it has to be reduced.

In other words, is the 12mm diam recess in the alum only 3mm deep, in which case the 1.5mm flange consumes half of it, or is there more than 1.5mm of space for the seal to push into, on top of the new bushing?

It's all fine of that's how it works. I'm curious how secure the seal is. And want to be sure I under this before I give the bush specs to a machinist.

Thanks in advance.

PS. I'm not at home this week to look at or measure anything on my cylinder's PV chamber.

Edited by numroe

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The seal is 3mm but the aluminum surface area is still enough that the seal just fits without sticking out. I put the seal in after the bushing but I honestly don't think it is necessary. The oil would have to defy the laws of physics to leak once that bushing is press fit in.

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Thanks. I was thinking about not using the rubber seal.

If the fit is tight enough between the 8mm shaft and the bushing, then I expect much less oil would get past it, than gets past a leaky seal. But if the seal still fits on top, then sure I'll install it anyway.

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I checked my notes and apparently I did have the flange reduced from 1.5 to 1 MM. This is what I gave the machinist.

IMG_20150426_103132.jpg

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Did you hand ream it or use a mill? Also, did you freeze the bushing or just use a press? I'm wondering if this can be done with the head still on the bike. I have a Gorr head and don't want to replace the two-piece gasket if I don't have to.

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I reamed it then froze the bushing and pressed it in. I wouldn't try and do it without removing the cylinder but you could do it without removing the head. Just remove the swing arm pivot bolt and the motor mounts then tilt the motor or just completely remove it to pull the cylinder.

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StillSmoking, thanks. All clear.

ADVR, In my 2004 frame, the cylinder comes off with head installed. But I do need to move the radiators a little. Alum frame I don't know.

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Hey guys sorry to bump this somewhat older thread but I wanted to make one of these pieces for my buddy's yz that leaks pretty bad. I just want to confirm the dimensions I have already defined in a cad program.

Bore ID: 8mm

Total height including flange: 8mm

Width of flange: 14mm

Width of outer bore: 11mm

Flange height: 1.5mm

I was also wondering what type of tolerances you guys had these machined too since it's such a tight spot on the cylinder. If anyone could get back to me that would be awesome. I'm gonna try to send a few designs to the CNC machine in hopes they'll be manufactured well. If anyone wants some of the solid works files I made let me know I can post them or pm them if interested. Thanks again guys!

EDIT: I attached an image with what I have so far for dimensions

Edited by cardi4

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