I have a David Hollom Arrival IOM and the fin mounting size is M3.5 x 40mm straight slot and I don't like the lack of positive contact when starting to enage the fin to hull (supporting the fin and lead weight) and was looking for a socket head screw but being M3.5 these are thin on the ground as M3.5 is only really the thread of screws used on electrical assessories fronts.

Is this common size or do people use M4 screws? Do people tap out the retainer to M4 etc

 

The tapping size for M4 is 3.3mm so there will be some thread left after you drill it for M4 - not ideal.  You would need to use a drill press, or preferably a lathe, to stop the tap wandering and producing a 'drunken' thread.  

BTW I can find M3.5 socket headed stainless steel screws on eBay.

Hi John949

Many thanks for replying to my post, checked yesterday and several of the IOM sailing looked to use the same thread size and one had the same straight slot screw head and the other made a home made hex head by cutting off the screw head and glueing to nuts to the thread rod.

I will have another look on ebay, I did find a supplier but may PayPal login was playing up so I left.

 

Regards

 

Craig