I was looking at Steve Scherrer’s video about how to install a 70P compass on a Tempest 170.
Two things:
First, he says you need a rubber gasket (part #3750-0000), but that’s the last you hear of it. In the video, you never see it again. Second, he tells you to set up a lubber line and loosen the compass housing, but since the “procrustean” compass recess virtually precludes getting it wrong, it’s not clear why you’d have to fine tune it further.
http://www.youtube.com/watch?v=Byy6PL62DUo&feature=plcp
Why do you need the gasket? And can’t you virtually just mount it, lining it up with the recess lines, and have it come out right every time?
Poly
The video says that the gasket is needed for the poly versions of the Tempest, but they’re mounting it on a composite version in the vid, hence no need for a gasket.
Same as above
but I’ve mounted three into poly kayaks without issue without the neo gasket. The only place I have seen them sold is Campmor.com.
The lubber line is used to line up the front and back lines on the globe of the compass, which you do before you tighten the screws down, which will prevent it from turning.
I used a little sealing good at each hole before I put the screws in, and probably added some on the underside after it was mounted for good measure. If you are concerned about the screws protruding into the front compartment, piece of foam or a bit of rubber will cover the sharp part but be easily removed if needed.
Instructions from yet another p-netter http://www.aquadynology.com/compass_install1.html
I was thinking of simply clipping off
the sharp points of the screws on the inside/bottom side
If I recall…
the gaskets were necessary only for the early roto T-170 models to prevent interior protrusion.
Anytime a sharp screw protrusion cannot be avoided you can install and set the screw, observe the degree of interior protrusion, remove it and Dremel the tip flat. Re-install the screw perhaps adding a touch of an appropriate sealant.
Screws
On the inside, I just cover them with the same sealant goop I use in the screwholes. I also put a line of goop under the main housing to keep the compass from migrating if it works loose while I’m paddlimg (VERY unlikely, but I like the security just the same)
That will work…
…if you can reach them. The screws are pretty hard, so it may take some effort to clip them. I typically put a dab of sealant over the ends after clipping them
that’s what I was thinking of doing
Of course, man proposes, God disposes – those screws may be tougher than I thought! 
I don’t know what to think about that
I would like to believe that the gaskets were only for the early models, but the video was uploaded May 2012; the video lays out everything you will need, and the project (the video) looks new.
Personal preference
I use bolts. Not a fan of screws in poly or composite. Still use caps or a dab of silicone to prevent tearing dry bags.
The newest video…
they’ve uploaded for a skeg cable repair illustrates the task on a skeg they haven’t used in their roto boats for nearly two years now. To verify you could join the Yahoo Tempest Owners Group where Steve Scherrer presides. He;s one of the designers of the Tempest series and could certainly give you the exact poop on the matter.
I’ve been trying to remember what the exact issue with the roto T-170/compass install was and I’m thinking it may have been the way the compass sat in the recess. I distinctly remember a person in the know saying it had been or would be resolved in subsequent production. Don’t know if that actually happened.
I’d just use a gasket if you’re not sure. Can’t see where it would be a difficult item to fabricate on your own using the compass as a template. I don’t see a real downside to using a gasket.
For this application I’d surely agree with wavespinner and use the machine screws/washers/thread-lock nuts. You have access and these would afford a noticeably more secure mounting for a deck implement. The machine screws are easily trimmed to length to avoid having them protrude from the crown of the nut inside the boat.
Kayak decks can take some abuse when doing rescue work and the locked screws also have the advantage of not vibrating loose during transportation and such.
Found it…
The compass recess on the roto T-170 is slightly undersized due to shrinkage after molding. This is why they recommend using a gasket for that model.
LOL 
Similar effect on the kayak as it cools after being removed from the mold.
Probably to a less dramatic degree than the situation our dear George experienced
Rubber Gasket
I have a 2012 Zephyr. I bought a 70P compass but I haven’t yet installed it. Based on that video I purchased a gasket and bolt kit at the same time. Is it fair for me to assume that the gasket won’t do any harm even if it is not necessary?
dry fit
Assuming you’re talking about a plastic Zephyr, try dry fitting the compass first. If you don’t feel the bottomside of the globe contacting the hull, you’re probably safe to skip the gasket.
That being said, I’ve never mounted a compass in any boat, whether a sailboat or a kayak, without some sort of gasket under the mounting rim. No boat surface is perfectly fair, and the gasket helps smooth out some differences between the mounting plate of a fitting and the boat itself.
Overkill
Machine screws and locknuts really aren’t necessary to hold a 70P. There is very little stress on the compass screws and I’ve never seen one loosen for any reason. If you’re applying sealant to the screws as you install them (which you should do), that pretty much guarantees that they’re not going to back out.
Gasket Important
The issue with glass boats, and perhaps plastic ones too, is that the compass recess may be a little undersized, resulting in pressure on the liquid enclosure when the compass is screwed down. I was told that this could cause the compass to lead fluid, if it is deformed enough by the pressure.
I don’t think the gasket can hurt, unless it makes the compass turn too easily and puts the reference lines out of square.
Alan