So, an interscrew has two components;
a machine screw
a captive threaded, splined bush
The engineers name for a bolt. Machine screws have a threaded shaft and a head. Nowadays threads are all metric, so M4 = 4mm shaft diameter etc. Heads can be categorised by their shape and by the way you grip them to tighten or loosen. Common shapes are;
Pan head (PH) - a head with a flat underside, largely parallel sides and a flat or shallow raised top. When describing length (eg 12mm), this is measured from the end of the machine screw to the lower surface of the pan head. Fits into a straight drilled hole.
Countersunk (CSK) - the standard countersunk head has a flat top and is what you'll pick up in a DIY store. The head has a V shape and is designed so that the flat is flush with the panel surface. When describing length, this is measured to the widest edge of the head, which logically should be flush with the panel. Of course the hole into which it is being placed also needs to be countersunk.
Raised Countersunk (RCSK) - like a standard countersunk, but with a raised or domed head. To my eyes this is often better to look at than a standard countersunk screw.
Common grip types are;
Slotted (SL) - a single slot across the head top. It's important to ensure your screwdriver is the correct width and thickness - it should have no looseness as if it does you will likely destroy the head at worst and make it look a mess at best.
Cross-head (CR) - the slots run across perpendicularly. Sometimes referred to as a Phillips head. The crosshead is the one most commonly used now as it provides roughly double the grip of the slotted, though perhaps in not quite such an aesthetically pleasing package.
Hex (HEX) - to take an Allen key - neat, more secure (as slotted/cross head tools are more common) but way to easy to 'round' the inner edges.
If you do 'round' a HEX try a Torq bit too.
Of the above, cross-head can take more torque, but slotted look better, particularly when the direction of the slot on all the heads line up.
The photo (from left, shows RCSK, HEX CSK, PH and CSK machine screws in 6/9mm bushes.
The bush is like a splined nut where the thread hasn't been taken all the way through and where the shaft ends in a 'hat' type arrangement; neat to look at.
The hole into which the bush will locate is normally either the same (if metal) or slightly smaller (if a softer material eg wood) so that the splines will grip. Note that as the top normally has no other means to grip (see above) this friction fit is the only mechanism for stopping the 'nut' turning. Bushes come in metric sizes only, typically M4 or M5. The splined shaft typically comes in 6mm and 9mm lengths, measured from the underside of the 'hat' to the end of the threaded splined shaft. The bush can usually accommodate a screw shaft to a depth of 1mm below the shaft length, so 5mm for a 6mm shaft.
Materials and threads
Early interscrews, as commonly found on Westerly yachts were made from chromed brass in good 'ol imperial sizes ie 4BA and 2BA. 4BA is ~3.5mm, so M4 is the replacement. 2BA is ~4.7mm so M5 is the replacement. All metric interscrews are stainless steel.
I've made this a 'main' item as there is quite a large difference between the old imperial and new stainless steel variants. Those used in Westerly yachts are typically 4BA with a bush diameter of ~5.0mm and often no splines. Our new bushes of M4/M5 6/9mm all have a shaft diameter of ~6.7mm so quite significantly larger; take note.
Getting the correct size.
We stock a range of Machine screws, generally in lengths of 6, 8, 10, 12, 16, 20 and 25mm length. As mentioned above the bushes come in 6mm and 9mm, measured to the panel surface (ie underside of 'hat'). Bush 'hat' heads are 9.60mm diameter with a 'hat' edge thickness of ~1.2mm rising to 1.75mm in the centre of the dome.
M4 machine screw heads vary in diameter but from our M4 stock the figures are: CSK/RCSK 7.3mm, PH ~7.8mm and HEX (4mm slot) ~ 10.0mm and for M5; CSK/RCSK ~9.0mm, PH ~9.9mm. We don't have any M5 hex to measure, but these can be sourced.