I will second the information from both of the other replies. The clearance recommended for the B-18 is 0.016 to 0.018. I also recommend the Haynes manual for 120 series. My experience with valve clearances has been that it is deffinately better to error on the side of too lose. Within reason this just results in more valve noise. I have had a problem with valve adjustments getting tighter in a B-18 I owned several years ago. Of course this doesn't make sense but it happened and the consequences can be bad. I thik it may have been due to the valve seats wearing away as the head was pretty worn. In any case I had a valve job on the head and the problem was cured. About the SU carbs Haynes also makes a manual devoted to them. This may be of use to your mechanic in proper tuning. They are often troublesome as most of them are worn out, having leaky throttle shafts.

Best of luck, Dave Starr