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Hiya I have a Makita HR400 and acording to the instructions manual I need a chisel adaptor inorder to use the rotary stop feature. I've tried google and can't find one.
Anyone know the part number I need?
Many thanks for any replys.

Had a look at the owners manual on the Makita website, http://www.makita.com/assets_product/HR ... HR2400.pdf (page 9), and it looks to me like this chisel adaptor is just so that either:-
1) you can hold onto the chuck area of the drill while chiselling,
or,
2) to help keep some of the dust out of the chuck.

You could give Makita a call on 01908 211678 to be sure, but I wouldn't have said the chisel adaptor was esential.

It says it's optional in the manual but it won't work as it is. When I have it on the chisel mode it'll go back and forth but then it starts turning clockwise slowly.
Does it sound like I need a new chuck?

The manual seems a bit vague to me. As Inky says then I wouldn't have thought you'd have needed an "optional accessory" to perform one of its core functions, but it does seem to suggest that you need one to use a chisel or bull point (it does look like it locks onto the body, so perhaps it's to stop it turning from the vibration?).
I'd have thought it would have been in the box if needed, but I've not used it so I don't know.
I'd take it to a Makita dealer, they should be able to advise you if there's a fault (such as the chuck), or be able to get you the chisel adaptor.
The one (and admittedly only) time that I phoned up Makita that’s all they told me to do anyway (hence never phoning again), although at least they can look up the nearest one if you need it.