Sorry....I gotta disagree with much of what has been said here.....
On my '81 .... which I am 99% confident has the factory original hose , master, and reservoir.... and I believe all years are the same.....the clutch master...and clutch reservoir...have 6mm OD plastic fittings. See the pics below for proof....
The brake hose fittings measure about 7.8mm...so I would call that 8mm...although I suppose 7.5mm hose would fit okay....
I would suggest Larry that you measure - like I did - the OD of the nipples on your clutch master and reservoir !!! And go from there...
I am in the process of rebuilding the pedal box with new masters and hoses.
I needed some new clutch hose. By the way...I measured the old clutch hose to be about 45" in length.
I searched around for options and bought some hose from Eurosport UK:
Eurosport (UK) X1/9 Reservoir Hose - 6mm (per 30cm) - Low pressure EPDM hose designed for connecting hydraulic fluid reservoir to master cylinder. Now supplied unbraided in black with reinforced inner. This hose is 6mm internal diameter, suitable for all clutch circuits and 6mm brake reservoir...
www.eurosport-uk.net
Note that they too agree that the clutch hose for all X1/9s is 6mm ID....
The hose I received was very nice quality. Italian made and even stamped on the hose as being for brake fluid. 6mm ID 13mm OD. Black in colour but smooth as opposed to the original hose that had a braided cloth exterior. Which was okay be me...as the braided hose showed the dirt and was difficult to clean. The hose was supple but shows no tendency to kink at all...
I would suggest you get some hose from them
For some strange reason...I only have one X1/9....but bought 9 feet of the hose. Enough for 2 cars.
Larry. If...you are really stuck...or shipping from them is too high or would take too long....I could send you my extra 4 1/2 ft. But what with I paid in shipping, duties, taxes......and then shipping to you ( would be 3 days from here )..that would cost you about maybe $25 US or so. Contact me if you want....
Doug