Cockpit Brace Installation

by Wally Montalvo

Early Miatas didn't come with threaded holes for the later model cockpit braces.

Parts needed:
(2) 12mm STEEL HARDENED bolts, >2.5" in length.
(2) 12mm STEEL HARDENED nuts, non-aircraft, of suitable pitch to match the bolts.
(2) 12mm I.D. wave lock washers
(1) 1.125" wood/metal hole maker.
(2) 1.125" polished metal hole plugs. If you can find them in plastic, all the better.
(1) Exacto Knife
(1) Cockpit brace from a later model year.
optional: 3/8-1/2" thick slightly larger than a quarter-dollar space washers
(1) empty one gallon plastic jug.

If you "peek" under the assembly which houses the seat belt bolt you'll notice a hole which can accept the 12mm bolt.

Using the smallest possible drill bit, after estimating where the center would be, drill holes in the plastic cover until you find the very exact center. Leave the drill bit inserted in that hole.

Change over to the 1", 1.1.25" or 1.25" hole maker. Remove the small drill bit and gingerly cut a hole in the exact same spot. Do not use force as you are dilling through plastic.

Repeat for the other side. Insert cockpit brace to verify that the holes are centered correctly.
(Look down into the cockpit tubes.) Now would be the time to insert the 12mm bolt into the hole. It doesn't fit, does it? You will need to enlarge the hole. With the brace still on use an appropriate sized drill bit to enlarge the hole. Once the bolt drops in easily put aside the cock pit brace.

If you think that you'll ever be removing the plastic assembly which houses the seat belt mechanism then you need to extend the hole backwards using the Exacto knife. Miata has a removal "tab" for the seat belt bolt. You want to make clean parallel line cuts.

You could remove the door sill plates to access the seat belt mechanism, remove the seat belt bolt and the seat belt mechanism, reach inside to install the nuts for the cock pit brace.

Or you could drill a rear access hole for the new nuts. I chose the latter.

I estimated an inch below the top of the plate which the bottom of the cockpit brace tube sits upon and on axis with the center of the hole through which the bolt will be inserted. If you insert a small ruler into the new cut out it will tell you the height to the top of the plastic when it hits the bottom of the plate (do not insert the ruler into the hole since it will measure some 8").

If necessary use a piece of thin copper wire to measure to the top of the plastic (from the bottom of the plate), make a right angle bend, extend it backwards until it clears the rear, bend again; remove, measure the short end, transfer the length to the long end, mark, add an inch, re-insert back into the cavity, mark where you will be drilling a rear access hole.

Important: remove the screw/nuts securing the side of the plastic housing. Insert a shim such that when you are drilling a hole it, the chuck, will not gouge the plastic.

Lift up the convertible top such that you can put the gallon jug underneath (outside the car) and prop it up while you drill.

Having located the new hole position (see above), drill a 1.125" hole. If the metal "slug" falls inside you'll need to fish it out with a magnet. It'll probably rattle around until it falls into the plastic "catch" net of the seat belt mechanism, it may even cause seat belt binding or cause it to tear. You must remove this old metal slug at all cost! Now, you could remove the seat belt bolt and jam a plastic bag into the cavity so that the slug doesn't fall into the seat belt mechanism. Then fishing it out with a magnet is a breeze.

For a "clean" install, install the 3/8-1/2" thick washer which is 1.5" in diameter over the mounting hole (Miata sells a square spacer, part number unknown, to do the same thing. You may need to 'square' the 'round' washer accordingly.) Install the cock pit brace, sitting over the spacer. You may want to put some sticky glue onto the washer bottom so that it doesn't move around.) Insert the wave washer onto the new bolt. Thread the bolt into the hole and through the bottom spacer at the same time.

Balance the nut on your finger tip. Insert your finger into the rear access hole. Move the bolt down until you can thread it onto the nut. Finger tighten the bolt (you won't be able to turn the nut, so turn the bolt until it strikes the wave washer.) Repeat for the other side. The wave washer should apply enough force that it will be very easy to tighten. Tighten until the wave washer is flat, then add an eight of a turn.

The side hole will probably not be exactly 1.125" so you may need to "open" outwards the metal plug fingers. Plug up the new rear access hole.

If you used the 1/2" washer the cock pit plastic end caps should fit perfectly. If not, you'll need to use a rasp to file down 1/4" of the plastic tang which fits into the cockpit brace hole. Once you know that it fits perfectly drop a dab of silicon on the top of the bolt, install the end piece. Removing this end cap should be no harder than removing the un-filed end cap - it'll be just as hard to remove without breaking. The "fingers" have to be pinched from both ends, regardless.

Without the 3/8" spacer the top of the cock pit brace plastic cover will hit the seat belt plastic cover. Gingerly remove the seat belt cover and remove 1/4" of plastic from the front until the first bend. Reinstall the cover and check for striking as you rotate the cover.

Time: 20 minutes to remove the door sills, seat belt covers, seat belt mechanisms, seat belt bolts.

Time to make the rear access holes and top plastic holes: 30 minutes.

Time to make the holes and fish out the dropped nuts or slugs: 2 hours.

If you want to use a lock washer over the nut, glue the washer onto the nut before inserting into the hole. Reserved for the not-faint-of-heart. I strongly suggest you first insert a plastic bag into the cavity before attempting. After the bottom of the nut is flush with the bottom of the bolt, raise the bolt, remove the bag.