FC subframe into FB thread

[Whizbang]

(i might change / update pictures with better ones as I go)

seems to me that we dont have any official thread on this. I have converted five cars to FC subframes now and worked out a few kinks along the way. Also, I have some "options" to consider when doing your own that can lead down some different paths.

But first, let's discuss a little history.

The 2nd generation RX-7 had much effort placed in the engineering process. To protect the image of the new chassis, Mazda utilized the FB chassis for testing of the various concepts of the FC suspension. I read about this in a hardback book from the era plainly called "Rx-7" written right as the car had come out. Pictures showed the test mules, and suspension configurations. So this got me thinking that it could be similiar but I never bothered to continue much with it.

Then Pete, rotary fellow in Cleveland, started messing with actually doing the swap. He confirmed that the front studs of the FB subframe can be used to located the FC subframe! Now, this does cause an increase in the wheelbase and therefore, gains in caster and issues with clearance to the fender. This can be accomodated however as having something to locate the subframe minimizes your chances of being "off" during install. And also, more caster and wheel base is a good thing!

So what do you get?
FC brakes, suspension (including sway bars and struts), steering, subframe while removing the clunky steering box and linksages and rather hefty old FB suspension. Moreover, you can get rid of the crap lug pattern, the out of date strut design and modern brakes. Win-win.

So what goes into doing the swap?

There are a few ways, but here is how I do it. We can discuss alternatives, results, etc afterward.

[Whizbang]

Considerations

Materials Needed

Tools I Used

[Whizbang]

STEP 1: Remove the factory suspension and steering components. *
*do I REALLY need to state the engine should be out?

Remove the struts, hubs, subframe, steering, steering box, and all components of the suspension. The tension rodss can be left attached to the control arm. Just loosen the bolts holding the bracket to the frame.

[Whizbang]

STEP 2: Remove the REAR studs for FB subframe.

You need the front studs to locate. The rears will be in the way. They are welded to the frame and thus require some grinding in order to be removed. Usually, you can grind a fair amount of weld off, then tap with a hammer from below to dislodge them. The resulting hole will go unused.

[Whizbang]

STEP 3: Located the rear studs.

This is the most difficult part in my opinion. Here is where you need your 7" long bolts of whatever pitch you choose, and the plate steel. And welder. And other things mentioned.

A. Before going forward, you need to add a 1/2" spacer to the rear of the subframe around the bolt holes. Could be just a hair more than that, but 1/2" steel is easier to find. A round piece, two cut square tubes, whatever. I welded mine. You need to space the rear of the subframe off the car by that 1/2" (or slightly slightly more) to align the frame correctly. This compensates for differences in the chassis.

B. Next, start mocking up the subframe to the car by using the front studs. Best way is to remove the steering rack, hubs, etc to make it as light as possible. I usually leave the sway bar since you need to clearance something here shortly. Also, it serves as a nice handle.

C. You'll notice the pinch seam prevents the subframe from sitting flush. You will have to clearance for the sway bar, the front stud area and the rear stud area like the example below. I leave how much to clearnace to you, but do not overcut, and try to maintain the cut on the x-axis so you have a flush surface and no points to deflect the subframe. I usually weld any stray penetrations and grind / file smooth.

D. Using something to mark the proposed location of the new rear stub through the subframe. Sharpie marker works rather well. Then using a pin punch, mark the center for drilling. You need to mirror this to the top of the frame as well. Carefully measure and translate this marking to the top of the frame. I use a combination of a metal engineering rule, a square, and check several times. Then mark the top.

E. Now using the appropriate set of drill bits, drill your holes from the bottom and the top (probably independantly) so you can have a passage all the way through. It comes very close, borderline engaging the frame insert to the unused steering attachment points.

F. On your steel reinforcement plates, measure and mark the distance off the the side of plate so that with a hole of the same size drilled through it, it can fit snug to the frame, but align through the holes you have drilled. This spreads the load and prevents the bolt from pulled through or deforming the sheet metal.

G. Drop the bolts through and tighen the frame up proper. Then weld up the plates and bolts. Your mounts are now done.

H. Paint for rust protection as needed.

[Whizbang]

STEP 4: Steering

There are a few options for the steering.

First is which rack you want to use, second is which column.

My arrangement is the 3 hole power-steering rack, de-powered per the thread in the archive. Being that I have a GSL-SE, I can use the stock FB steering column BUT I need to span the distance from the column to the intermediate shaft. The FC column will do the same thing. The FB column adapts the factory electronics easier though, but odds are you don't have a GSL-SE.

I ended up using the FC column just for the hell of it.

The FC column needs the original brackets removed, then the FB brackets used (requiring careful removal so you CAN reuse). This gives you the second point of support so the column doesn't wobble. Make sure you have the clearance so the U-joint can rotate and not hit the frame. Also, note where the locking mechanism is for the ignition so its not rotated strangely and you cant mount your ignition like i did... oops.

[Whizbang]

STEP 5: Suspension and Brakes

Technically, you can use the FC struts, with FB springs and tops. But even with something like an Eibach lowering spring. The car seemed to ride... a little high.

The other route is choosing one of the many FC coilover options! I especially recommend the pillowball tops considering the deflection we have due to the caster gains. Myself, I found a nice set of HKS Hipermax, used of course. You can even make the rear coilovers "work" but I'll cover that another time. The only work required to use the top hats is the holes must be lengthened outwards in order to accomodate the slightly larger pattern. Simple work with a round file, or ferrous bit.

Then you must choose what do you about your brakes. There are three options. Well four, but no one is going to bother with using the 5 lug hubs with the single piston caliper...

• 5 Lug / 4 Piston Caliper
• 4 Lug / 4 Piston Caliper
• 4 Lug / Single Piston Caliper

If using 5 lug / 4 piston...
This is pretty easy. It all bolts together as you found it.

If using 4 lug / 4 piston...
Two things must be done. The first a small spacer must be made that goes between the hub and the brake rotor of 4mm. Aaron cake has a thread on this matter. That thread is HERE. The second is redrilling your rotors to fit the hubs.

If using 4 lug / single piston...
Like the 5/1, this is all a bolt together process.

[Whizbang]

STEP 6: Misc. Wrap Up

[estevan62274]

Awesome work man!
Thanks for doing this.
I hope to do this swap on my SE in the near future, still gathering up some parts.

[fc nut]

Great stuff,but don't leave me hangin. Where's the steering section? That is where most of my questions are. I've been considering this for a while now.

[Whizbang]

Originally Posted by fc nut

Great stuff,but don't leave me hangin. Where's the steering section? That is where most of my questions are. I've been considering this for a while now.

Im working on it

I need to get pictures to help the text, so Ill likely write it out, then expanded and get pictures as i finish current project.

[RedR1]

subscribed!!

[fc nut]

Good deal. Can't wait

[TOOL]

A couple of my mates in the UK have done this swap.

You can have a look on here: www.rx7fb.com

[Whizbang]

Originally Posted by TOOL

A couple of my mates in the UK have done this swap.

You can have a look on here: www.rx7fb.com

just registered on there, once i'm approved i'll do some more poking around.