[MikeField]

I''m assuming an aluminium mast here. Before mounting your cleat, you should make sure that there''s some internal blocking inside the mast at your selected point. If there''s none, I''d be trying very hard to run fastenings right through the mast inside an internal spacer. You need blocking or the spacer to stiffen the mast wall where the load''s going to come on. (And then I''d probably put another cleat on the other side while I was about it, just for good measure.)

Whether or not there is any blocking or access to insert a spacer is of course another question.

A jib halyard takes a good bit of tension (especially if you set up the luff with a downhaul,) and if you simply tied it off to a cleat fastened only into the mast wall, I think you could expect trouble before too long -- either the bottom and then the top fasteners tearing out, or the mast under the top of the cleat crushing in. Or both. (I assume your jib isn''t set flying, but if it is then you lose some mast support and possibly the mast too.)

Sorry to sound pessimistic about this. If there''s no blocking, can you perhaps get access for a spacer via an exit point for internal wiring or main halyard or some such? If not, I''d try to think of some other way to tackle the issue.