First Fin Box Install-Would love a little guidance!

Aloha Everybody.

OK, so I am making a nice little 7’0" egg for a friend. I am going to install a center fin box and Future side boxes for versatility. I have got the Future box installation down pretty smooth, but I’m at a loss for how to get this fin box in there correctly. I’ve been searching around the archives…but I am still kinda clueless with the router and how to get a nice clean hole for the box. I’m currently in the process of trying to make a jig that will work, which is not all that easy for me. I think I have a handle on how to glass it in…but if you could throw some tips my way… :slight_smile:

Specific Questions…

How do you get the square corners for the box hole with a circular router bit? It just doesn’t seem like I am going to get a square shaped hole with a round bit. Am I wrong?

Any tips that have been learned from experience? Hopefully it will help me not make the mistakes that some of you might have already made while learning the process.

Looking forward to any helpful replies.


Gotta resize my pictures…coming soon.

I’m not the pro at this so I won’t give you instructions. Lots of stuff in the archives but you already know that.

Here’s the mistakes I’ve made so far with box install :

Cut too deep…That’s right, went right through to the other side. Big air!! Measure thickness before cutting holes. Modern boards have thin tails.

Cut too big… I didn’t make a jig and my measurements were about 1/8" too big all the way around. Works fine ,added alot of weight to tail filling in extra space around box.

Forgot to wipe down box with Acetone before install… need to remove the mold release agent from the plastic box. Roughing it up a little with sand paper helps too.

Have fun ,

measure twice , cut once


I’ve never used a router, I have none.

I simply drill4 holes, one at each corner. Then I cut with a razorblade or with a metal disk fit on a dremel-like tool. Finally, I remove foam with the razorblade.

I’ve never put a box in a single stringer board, so I had no need of removing wood.

Not a pic from me.

the fin boxes i’ve been using have rounded (radiused) corners at the outside, so the curved hole the router will bite is just what you want.

if you have a jig and a router bit with a bearing at the top simply clamp the jig in place and route out. i suggest routing in 1/4 " increments. that way, you can make sure that your cut is not too deep. you may want, at the final stages to make smaller bit adjustments (1/8th or less), , so that you get a nice flat fit of the box inside the slot. you may have to, because of rocker, cut slightly deeper at one or both ends, or the middle. cut and fit.

if you do not have a bit with a bearing at the top and/or you do not have a jig, you can freehand.

place the box on the board, centered on the stringer and outline with a soft pencil. then run 2 or 3 ( i like 3) layers of masking tape around the outline of the box which you have just drawn. make sure the tape is on the outside of the lines. then carefully route out using the tape as a guide. i think i am using a 3/8s bit, which allows me to route our the foam on each side of the stringer at first and then do sidewise cuts throu the stringer for ease of removal.

if you’re not using a jig, you may want to practice on some scrap foam or busted boards.

i always wipe the box with some acetone before installing. i like to paint a hot coat of resin on the bottom of the box and on the bottom of the slot and then set in place, centered over the stringer. then you can mix up your resin and cabosil or chopped fiber slurry and pour around the box when its ready. after that you can sand and gloss over and you’re good to go. unless you’re a hopeless paranoid like me and want to put a glass patch over the box.

good luck.

Use a small 1/4" bit and the corners will match the corners of the box. Make multiple passes, each one shallow. Your first one should just go through glass, not really into foam much at all or it can chunk out. Cut tight to your box. Its always easier to take some more away than it is to add it back in…

Just to confuse you even more I use a jigsaw but I put a spacer onto the base plate so that the maximum throw of the blade is 1" - 1 1/4".

Drill the 5/16" holes at the corners and join the dots with the jigsaw…

Aloha Guys…

Thanks for the tips so far. I made the jig and practiced on an scrap baord and it seems to work well. The hole may be a tad too wide…but I think it will work.

How many of you actually glass over the finbox? I have the board hotcoated already, and was planning on glassing over the box with at least one layer of 6oz. Is it even necessary? Seems like some do and some don’t so I’m curious if I need to do this step.

Thanks again!


In my opinion, there is no need to glass over the edges of the fin box. After all, the thing is sitting in a bed of foam. A little grip at the top edges won’t help that much. (Maybe a little).

But I do many of my boards with two stringers, 1 1/8" apart. The box sits nicely between the stringers giving it lots of resistance to movement. Very strong. Doug

Howzit OahuSurfer, If the hole seems to wide you can glue some thin wood slats to the side to narrow the hole. Best is to make your jig with adjustable settings so you can use it for more than just one size box. Use a mixture of resin, milled fibers and pigment(optional) for setting the box,it’s much stronger than Q-sel. Not neccessary to cap box with fiberglass. Aloha,Kokua

Not neccessary to cap box with fiberglass. Aloha,Kokua

True, but there are plenty of us who always do.

I’m not in a production setting; .5 square foot of glass & resin isn’t going to cost me anything; I hate spider cracks across the front of finboxes (I surf mainly single fin longboards with big fins that torque a lot); I think it helps insure a water-tight seal around the box…

Kokua’s absoultely right that its not necessary, but if you have the time & inclination, it certainly won’t hurt either :slight_smile:

Howzit Benny, I’ve got a trick for those cracks that works great. After you route out for the box use a pencil or chop stick and punch a hole on both sides of the stringer at both ends of the hole about half way down and about 1/2" deep. when using resin milled fiber mix it acts as an strengthener and since I’ve been doing this I don’t see any cracks any more. Aloha,Kokua

Cool trick! Thanks :slight_smile: