Need FCS finbox repair help

Hey all!

First time poster here. Thanks for the endless amounts of knowledge and experience provided here, it’s been a real help. I’ve had trouble finding situations that similar to mine so I thought I’d just ask.

I had a 3 FCS x2 boxes (one fin and another half) get crushed and paid a guy to fix em for me. He replaced 3 of the 8 on a quad fish i have and a year and a half later the repair is doing baaaaad. The three boxes just wobbled all over. I resolved to learn to fix em myself! I have a fair bit of epoxy and fiberglass experience from boats and whatever else.

I started with the 2 that were a full fin. Basically i Just cut em out to see what the hell was going on, and ordered some fin boxes and q-cell. The plugs are holding firmly to a ball of hard crap that lost its grip of the surrounding material, so were pretty loose in there. Seems like the board is polysterene, and it is indeed epoxy.

Wondering what the best course of action is. I guess I’m wondering if I should try to replace foam as it’s kind of a big void, if i should just q-cell it all and cloth and cut new holes, or i should just try to q-cell these plug balls back in, and glass them in proper.

There’s probably an eigth of an inch of foam material missing around the ball- between the ball and the socket here, wondering if that would work out or what.

The other plug seems to be holding up much better, but does wiggle a bit. I’ll probably end up drilling holes and syringing filler into there. Sound fine?

Help is greatly appreciated.


Hi -

It looks like some sort of high density foam insert around those plugs.  The plugs themselves are firmly attached to those foam balls?  Is that correct?  If so, you might just epoxy them back in place.  I’d feather around the top of the opening and cap it with a bit of fiberglass as well.  

Have you tried the ‘Fusion’ plugs?  They are the ones with 2 plugs embedded in a high density foam block.  If you can get those and have access to a router and jig, that would be a better way to fix.  Again, cap with fiberglass around top seam.

I’m not trying to play favorites and not saying these are the best price… just an easy link:


It looks strange.

There is white EPS with the beads showing on the surface. But I don´t know what is going on with the brown/grey foam and foamballs.

It think it could be burnt/melted EPS.

Because instead of beeds, there are voids in form of the beads visible all over the surface of the balls and the hole they were sitting in.

And there seems to be unharmed white EPS beads on the ground of the right hole.

Anyway: I can´t see an H-pattern. Without that, X2 will never hold up fine.

See diagramm 4a/ 4b

I would replace a whole block of foam. That brown foam (melted foam) looks to suspicious for me.

I´d put in high densitiy EPS or other high densitiy foam you can get.

If you have a router, routing out a block cavity and glueing in a new foam block is easy and fast.

Then you can insert new X2 plugs properly with the H-pattern.

But if you have a oruter, I would do it the way John suggest: Insert FCS fusion plugs. 

A jig out of wood is no big deal or if you have a steady hand, you can even route the Fusion cavity free hand.


I think that darker area and the part around the plugs show thermal damage. You’ll need to cut that out. If you choose to use resin and filler, you should cut away the area that is damaged so the new hole is wider where it is deeper to help hold it in. But adding that much resin will probably cause another thermal problem.

I would get some high density foam like Divinycell or 6lb poly foam thick enough to fill in the hole. You only need a piece that is big enough to cover the hole(s). Route a small rectangle in the foam where the plugs came out then glue the foam in with epoxy resin. Sand the foam down and feather out the glass around it, then make a glass patch over that. Check out the videos of the ding repair this guy does to see how he does the feathering then adding the layers of glass. Once you’ve got the initial glass repair, you can install the FCS plugs the same way as you would in a new board. They should be solid.

I would cut it the bad plug and fix it right. The stresses on side fins will break the plug loose. Been there and done that, including thermal meltdown on fin boxes. Luckily you have a solid colored board, so you might be able to match the color and hide the repairs.

Cut out the foam and replace it with high density foam and a Fusion box.  No future problemas if you do that.  Yes I would say the dark color is a result of thermal or “exotherm”.  It may also be just the simple fact that the repair was done with resin that is not meant for surfboards.  Marine resins like West Systems are naturally brown and get darker with age.  You can draw and cut out a peanut sized pattern that will allow you to replace the damaged foam.  Glue the HD foam in with Slow Epoxy and then rout or razor cut the traced Fusion plug into the HD foam.  Glue in the Fusion box with a brushed coat of Slow Epoxy and then laminate a couple of football shaped layers of 4 oz. over the box.   I suppose if you can’t access HD Foam you could use a piece of Polyurethane.   

Another trick I learned somewhere (here?) is to sandwich a couple of thinner sheets of high density foam with fiberglass layers between for the inserts.  When those are installed in the damaged area, it leaves a horizontal plain or two of fiberglass within the inserts to further stabilize the plugs. Hope that mkaes sense.  

Ambrose Curry has shown little wooden strips inserted around FCS plugs like spokes to help stabilize the plugs.

Single FCS plugs are OK if installed correctly but there are better ways…  even they have moved on from the original design.

Thanks everyone!

Yeah I definitely think something weird happened to make the brown colored stuff. Seems like I’m gonna replace the foam and either put these x2 plugs in properly, or put a fusion in.

In terms of foam: I already have a bunch of high density ethafoam, which is a closed cell PE. A good deal denser than sponge or a wavestorm. Would this work well? If there’s an area of this whole thing I’m most unfamiliar with it’s the foam. The area I’d be replacing goes all the way to the deck, so probably a 2"x2"x4" area. As was mentioned, I’d carve it out, epoxy foam in, shave and feather in the surrounding cloth, build up some new cloth, and install the fin system as described elsewhere…

Also it seems like x2 plugs can be put it with just a hole saw and a forstener bit, more or less, while the fusions seem to really want you to route it. Someone mentioned being able to just carve out the fusion plug space with a razer, and I’m wondering how realistic that is. Ultimately the I think there’s plenty of plug install info on here already so maybe I’ll just tap into the wealth of threads and videos once I sort out the foam problem.

I would check that your epoxy and new foam are compatible first. Mix a little epoxy and glue some scrap EPS and a stir stick to the new foam and do some destructive testing and cross sections. .After that, if all good, keep the fits tight to avoid exotherm and hard blobs of epoxy in the system.