2017 Edit: There is an updated version of this article >>here<<.
If you need to replace the motor on your Dyson DC04, there are two different types. You need to know which one yours is to be sure you buy the correct one. They are not directly interchangeable without a minor modification.
If your machine is a clutchless lime green and grey model, it *should* have an Ametek motor fitted to it as standard.
Most other models have a clutch. If your machine is fitted with a clutch, it *should* have a YDK motor fitted to it as standard.
If you are confused about the differences between Dyson DC04 models, there is a guide here: Dyson DC04 Model Index
That said, if the machine has ever been stripped down and rebuilt in the past, it is possible that someone else fitted the other type, depending on what they had handy at the time. Therefore, it is always better to strip your machine down to ascertain which motor is fitted to your Dyson DC04.
Below is a picture showing you the two different types of motor:
Below is a picture showing you the two different types of top motor rubber:
Remember, these motors are not directly interchangeable, so if buying a motor only, be sure you get the correct one.
However, as more machines with YDK motors were made, it is easier — and usually cheaper — to source a new YDK motor than it is an Ametek. There is an easy way around fitting a YDK motor to your machine where previously there was an Ametek fitted. Look again at the pictures above. You see there are two motor rubbers and a plastic retaining ring there? Although they look pretty similar, they actually differ.
If your machine had an Ametek motor fitted, you can fit a YDK type motor with ease. Just be sure that you get the YDK rubbers and the plastic retaining ring to go with it and you are good to go. No other modifications are required. You simply fit the YDK motor together with the replacement fan-case seal, top rubber and plastic retaining ring when you reassemble.
We do not recommend taking the budget route and buying a used DC04 motor from somewhere like eBay. Although you may think you are getting good value at first, this is where a lot of unscrupulous sellers unload a lot of their reject or tired rubbish. For sure, you will get a cheap one there, but after the crucifying fees the seller must pay in eBay and Paypal charges, the net price they receive after postage will be very small. This does not incentivise them to send you a quality item. Additionally, used DC04 motors usually have a very short service life left as they do wear out comparatively faster than some other Dyson models. Buying a used one is a false economy. We recommend you buy a new one.
If you need a new motor for your Dyson DC04, source it from a Dyson specialist. This will ensure you get a quality item from a trusted source at a great price. Below is a link to who we recommend. You can order a YDK motor for £35 including UK delivery. If your machine had an Ametek fitted, they will even throw you the extra rubbers and plastic ring in for free!
Edit 2017: Read where to buy the adaptor kit on >>this post<<