New freewheel?
 

Hi,

I've recently changed my chain on my mountain-bike because it was all rusty and I was unable to get it cleaned. Now the following issue has come to light : when riding my chain 'skips' on the 5th and 6th cog of my 7speed freewheel. The chain only skips when riding the biking, when I turn it upside down and engage the drivetrain by spinning the cranks the chain does not slip. The chain only skips on the 5th and 6th cog, not on any of the other cogs.

Does this indicate that the 5th and 6th cog of my freewheel are worn out?
If so, I need to replace those cogs but I guess you can't replace individual cogs so I'd have to buy a new freewheel altogether, correct?
What model should I get? The freewheel I have now is exactly the same as the one described in this thread: Sunrace HG II. It has the following cogs: 13T-15T-17T-19T-21T-24H-28H(7speed).
Would this Shimano freewheel fit : http://www.amazon.co.uk/Shimano-Tour.../dp/B003662HBK ?
For removing my freewheel I would also need a freewheel remover like the Park FR-1, correct?

Regards,
logion

That freewheel will work fine and the FR-1 is the tool you need.....

New chains skipping on used freewheels or cassettes is very common. It would have been helpful if you had checked the old chain for stretch. A stretched chain is a predictor for worn sprockets.

Another factor that would indicate it's a new chain/worn sprocket issue is if the skipping was on the sprockets you tend to use the most.

The MTB has been stationary for quite some time but my guess is that I indeed drove the 5th/6th cog most of the time while commuting a couple of years back.
Thanks for the info, it might have seen obvious but I wanted to avoid unnecessary spending; money is tight atm.