stupid tap/hose question

I want to stop my lawn from turning into straw in the hot weather.
There's a threaded water tap in the back wall of my house. I thought it would be pretty straightforward to put a hosepipe on there, so I bought a Hozelock 21mm/26.5mm threaded tap connector. I thought the tap was 21mm, but the connector seems to be slightly too small. The threads look like they're compatible, but try as I might, I can't screw the connector on. I've even tried filing its thread down a bit with a pen-knife, but no joy.
Is there a trick to fitting the connector that I've missed, or do I have some sort of bizarre non-standard 22mm tap? Would a universal Hozelock connector solve this problem, or is there something else I should try?
Any advice much appreciated.
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I'm amazed if these are described in metric dimensions, because they are almost certainly British Standard Pipe thread sizes (see below). The most common size for hosepipe connections is 3/4" BSP - although some are 1/2" BSP, or even 5/8" BSP. [These are the pipe bore sizes, not the external size of the thread. 3/4" BSP is 1.041" (26.44mm) externally, 5/8" BSP is 0.902" (22.91mm) and 1/2" BSP is 0.825" (20.96mm)]
Get a connector marked with the appropriate BSP size, and you'll be ok.
Roger
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Roger Mills wrote:

One word of warning though; those plastic Hozelok fittings can't tolerate the water freezing - they split. I always used to forget to drain the hose at the onset of winter and ended up replacing some/most/all of the fittings the following spring (after getting soaked turning the water on :-) ).
I've just replaced all mine with some brass ones I found at the local Garden Centre - Screwfix also sell them
http://tinyurl.com/iu27

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I had a further thought about your tap. Are you sure that it *is* a proper tap intended for hosepipe use, and not just an in-line tap with a compression fitting for connecting a rigid pipe to its outlet?
The threads used by compression fittings are *fairly similar* to BSP - but they ain't the same!
It this is what you've got, you'll need to use a normal compression nut and olive to join a short length of pipe to it - with a male BSP fitting of the appropriate size on the other end of the pipe, to take your hose fitting. Maybe a visit to your local friendly plumbers merchant is called for.
Roger
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