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You are basically SOL. That hub is one of the stupid ones that Shimano made that will only fit a 10 speed cassette. One option I have heard mentioned is to grind, file or have a machine shop turn down the splines on that freehub to fit a 9 speed cassette.
 

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7800 rear hub was made for 10 speed only....Why did you buy it??
The new 7850 hub went back to the small splines (will work with 7, 8, 9, &10 speed hyperglide cassettes.)

PS.. I'll buy your old wheel for $30.
 

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It was advertized as 9,10 speed. If I can get the freehub body off, I can take it to work and get it machined to the smaller splinie level, then just use a spacer to make it 9 speed. Looks like thats the case. Is there any way to just replace the old freehub with a new one that is compatible?

Mr. Grumpy way to go for the impulse sale. I couldn't let it go for that. Its a 36 hole wheel with triple butted spokes, hand built by the mechanic's mechanic, its dang sweet.
 

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Clyde250 said:
It was advertized as 9,10 speed. If I can get the freehub body off, I can take it to work and get it machined to the smaller splinie level, then just use a spacer to make it 9 speed. Looks like thats the case. Is there any way to just replace the old freehub with a new one that is compatible?

.
Just an FYI, the 9 speed cassette is actually wider than the 10 speed cassette so you wouldn't need a spacer.. I've never measured a 7800 freehub body so I don't know if it's the same width as other Shimano freehubs.. All other Shimano freehubs need a spacer when using a 10 speed cassette.

and then you still have the spline issue to deal with... My vote is get a proper hub
 

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spacing....

As already mentioned, the 10 speed spline is actually shorter than the 9 speed model, so the splines would have to be turned down and about 1mm removed from the left shoulder. The material removed from the left shoulder might have to be done with a narrow grooving tool, to insure thefirst cog seats properly. In other words, the bottom of each spline must also be lengthened.

There is one poster on the Serotta website who says he's had this done and it works, but my opinion is that you;d have to find a real generous machine shop to do this cheaply.
 

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Clyde250 said:
It was advertized as 9,10 speed. If I can get the freehub body off, I can take it to work and get it machined to the smaller splinie level, then just use a spacer to make it 9 speed. Looks like thats the case. Is there any way to just replace the old freehub with a new one that is compatible?
There is nothing 9spd compatible that will fit that hub. If you have cheap/free machine shop at work, you can take it off and get it turned/milled. Refer to Shimano online exploded parts diagram. You will need an 8mm Allen key to remove the freehub once the axle is out. I would say that you would want to take a 9spd cassette, as well as one of your 9 speed hubs as a sample for the machinist.
 

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If it was advertized as 9/10 speed, return it. They were dead wrong.
 

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Did you take a real 9spd for him to compare to? The left flange needs to move in about 1mm.

The lower flanges will notch from the steel cogs. This is the problem Shimano was trying to solve with higher flanges. But it happens on other mfgs hubs that use Al. You just twist a stuck cog backwards to free it from the notch.

The Ti body on the 9spd Dura Ace and 10spd Dura Ace 7850 hubs are the best solution to that problem.
 
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