Heidt’s Subframe Connector and 4-link Mount Installation

Since my last entry, a lot of progress has been made.  After several days and nights looking over every practical approach to installing the subframe connectors and 4-link mounts, I fundamentally decided to let the parts themselves guide the work required.  I knew up front that the prospect of everything fitting “just right” was a pipe dream.  Modifications and corrections would have to be made and with that in mind, I decided to let the “hard points” of the 4-link mounts dictate the absolute position of the parts and then make the necessary modifications to the subframe connectors to fit accordingly.  After all, the 4-link mounts are the critical components in the assembly and their absolute position is dictated by the front leaf spring mounting bolt location anyway.  I reasoned that if I started from those “hard points” on the chassis, everything else would fit where it was required.  I am quite happy to say this logic proved solid and I managed to get everything in place with maximum functionality on the first hit.

As has become my habit with all similar jobs, I trial fit everything multiple times, making small adjustment as I go to make sure everything fits and functions correctly before making anything permanent.  The old “measure-twice-cut-once” philosophy in practice.  After several small tweaks and adjustments to both the 4-link mounts and subframe connectors, I was satisfied that the fit I had achieved was quite acceptable and accurate.  As I mentioned in a previous post, I was not happy with the gap that remained between the outboard side of the rear frame rail and the 4-link mount flange and determined I would bridge this gap with a substantial section of plate steel “shim” that would be welded to the frame rail and the edge of the mount for strength.  Also, the mock up process showed that under no possible circumstance would the rear mounting holes of the subframe connector ever line up with the 4-link mounts.  So, with those issues clearly defined, I set about my first fabrication and fitting tasks.

First up, I mocked everything up in the best possible configuration I could achieve in order to measure and fit the frame rail “shims” and to precisely determine what modifications would be required to the mounting holes on the subframe connectors.  Next, I fabricated two shims from ¼” x 3” plate steel and tacked them into place and verified their fit with the mounts.  Once satisfied I had the shims fit correctly, I tacked them in, removed the mount, and then fully welded the shims in place.  Once again, I installed the 4-link mounts and carefully located them such that they could be tacked in place as well.  Now, the 4-link mounts could function as a permanent and stable reference point for all of the subframe connector work to follow.

The subframe connector mounting hole misalignment was severe enough that I decided to weld the holes up completely and drill them in the proper location.  At the same time, I determined that the bolt heads would not properly clear the horizontal plane of the connector mount as the hole was too close to the corner of the bracket.  So after I located and drilled the new mounting hole, I crafted a tidy solution to the bolt head clearance problem that would allow ample room for the bolt as well as extra strength and weld area for the subframe connector mount and 4-link bracket.  Quite simply, I fashioned a simple “channel” relief in the subframe connector mount that allowed me to clearance the bolt head with ease and to weld the bracket solidly across the frame rail and into the 4-link mount.  This thing ain’t going anywhere!  As an aside, I also came up with a nifty way to weld the crush sleeve to the back side of the connector mount so that if ever I were to remove the main mounting bolt, the sleeve would not simply fall down into the frame, never to be see again.  This solution was simply to drill two 3/8” holes opposite each other such that the edge of the crush sleeve was visible in the holes from the outside.  Then I just welded the edge of the sleeve to the plate while filling the holes.  After a few passes with the angle grinder, the welds were invisible and so was the worry of losing the crush sleeve.

Now that the subframe connectors would fit the chassis properly, I removed them one more time to prime the tubes since I would not be able to do this with them permanently mounted to the car.  After an overnight cure, I re-installed the connectors to the chassis and proceeded to tack them into place.  After one more check that everything was fitting as planned, I began welding everything in permanently.  A few evenings later, and the welding job was complete and I am now ready to clean up the weld edges with the grinding disk and prep the weld areas for priming, including the few spots on the interior floor that were scorched by the welding heat during the front subframe installation.  Once in primer, the whole works should look very integrated and unassuming.  In fact, once the final textured SEM bed liner finish is applied, they should be so non-descript as to go mostly unnoticed by the untrained eye………..exactly as I have intended.

Fit of every component is critical.  Here, the front subframe connector saddle fits very well against the passenger side front subframe rail.

This shot show a pretty nice fit between the 4-link bracket and the subframe connector flanges.

Here you can see where I traced around the brackets once they were in their proper position.  This helps as a quick visual reference when refitting and allowed me to mark where the frame shims would need to be located as well.

The gap between the 4-link bracket and the frame rail was (in my opinion anyway) was absolutely unacceptable.  I decided a shim was necessary to ensure good strength and integration to the chassis.

Another look at the 1/4" gap.

Here's where the wheels started to fall off the wagon.  With the 4-link bracket fit to the car and the subframe connector position established by the front spring mounting bolt, the lower control arm mounting bolt hole was nowhere near in alignment.

The mounting hole misalignment on the driver side is obvious here.

And on the passenger side, the fit is better, but not at all good enough.

First things first!  Here, I tacked in the 1/4" steel shims to fill the gap between the 4-link bracket and frame.

With the shim fit exactly as I wanted, I welded them in completely.

With the gaps properly filled, I could then tack in the 4-link brackets so I could concentrate on getting the subframe connector mounts corrected without loosing my master positional references.

I determined that the only way to get the subframe holes where they belonged was to basically start from scratch.  So.....I welded them up!

With the welds ground flush and smooth, the flange was ready to be drilled for a proper mounting hole.

With the mounting hole properly located, I found the bolt head clearance to be too tight to the frame flange.  As such, I devised a simple clearance modification (shown by the scribed black lines) that would allow plenty of bolt clearance and allow much more weld area to strengthen this installation.

With the section cut out and smoothed, you can see that bolt clearance is no longer an issue.

While I was working on subframe connector flange fit, I decided to drill two opposing holes just at the edge of where the inner frame crush sleeve was located so I could tack weld the sleeve in place on the flange from the outside so it would not get lost in the frame rail if I ever decided to remove the locating bolt.

A closer view shows how the holes I drilled expose the edges of the crush sleeve.

A few small plug welds is all it took to permanently weld the sleeves in place.

With the welds ground smooth, the subframe connector was now ready capable of fitting the car as it should.

Before the subframe connector could be permanently welded, I primed the connector tubes because I would never be able to get primer on the surfaces between the floor and the top of the connector tubes any other way.

After final fitting of the connectors, in my usual fashion, I tack weld all over the place to keep everything from moving out of position.

I tacked the connector flanges to the 4-link mounts as well.

The same goes for the front connector saddles as well.

I started by welding the front saddles.  Here is the driver side saddle fully welded.  I ran the heat up a bit on the welder to get the beads to lay a little flatter and they looked pretty nice when I was done.

Rear bracket to connector interfaces were welded next.

Next, the 4-link brackets were welded in.

And finally, the subframe connector rear mounts were welded in to complete the installation.  Here, you can see just how much extra welded area the modifications I made created.

Other than a few scorched spots in the interior floor that I will fix in my next phase, there was absolutely no evidence from inside the car that any work had been done.  Sneaky.....

Priming the Exterior Floor & Unisteer Rack & Pinion Teaser

It’s hot!  When the heat goes up, the enthusiasm ‘round here goes down in a hurry.  But somewhere in there, I managed to piddle just enough to get a few things accomplished in the last few weeks.  Most importantly, I was finally able to concentrate on getting the lower floor surfaces in primer in preparation for the installation of the subframe connectors and Heidt’s 4-link suspension mounts. 

As with much of the under body preparation, I decided to sand-blast the rear subframe rails and axle tunnel to remove the surface rust.  I am very lucky that the rear subframes are in excellent condition in spite of the fact that the trunk floor is a mess.  Once I had this work complete, I was able to better assess the next steps in the preparation to install the 4-link suspension mounts and plan ahead for the trunk floor repairs this fall.

On the cooler evenings, my dad and I scuffed the entire underside of the new floor and surrounding components to prepare the surface for a new coat of PPG primer/sealer.  Since I knew I would be welding the subframe connectors in, I taped off the weld areas at the same time I masked the floor flange edges.  There is no point contaminating the weld areas with primer if you don’t have to and hopefully this will minimize heat-blistering from the welding process and thereby reduce the work required to get everything primed after installation.

Priming was an absolute dream on the rotisserie and I was finally able to use the “faster” DP402 catalyst which really worked well.  In fact, I will be trying to stay with this catalyst in all future priming jobs if at all possible.  Good stuff.  The finished job looked very good and I was happy with how uniform everything looked.  Now, the stage is set for the subframe connector and 4-link mount installation……….and I LOVE any opportunity to fire up my welder for just this kind of thing!

The teaser for this entry is the preview of the Unisteer power rack and pinion steering kit that will be going in to the car along the way.  The kit is very nicely made, but as with most aftermarket “kits”, it follows exactly Pruett’s Restoration Rule #1:  If it says “bolt-in”, it isn’t.  While there is nothing major at play, the modifications that will be required to make the kit fit a 1970 Mustang chassis properly will be fairly numerous, but I will do what is necessary and correct (in my opinion anyway) to make this system work correctly.  On the “up” side, the Unisteer rack unit looks very clean in the car and should allow tremendous extra room to fit headers and vastly improve the feel and performance of the steering system.  Yes, it’s a lot more work than I had hoped, but I have become tempered to these necessities to get what I want.  Like they say, “If it was easy, everybody would be doing it!”

After draggin out the dreaded sand blaster, I cleaned up the axle tunnel and rear frame rails in preparation for the future suspension and trunk floor.

Passenger rear frame rails is in excellent condition in spite of the rough surrounding trunk floor panels.

Driver rear frame rail is equally good!

Rear bump stop brackets are in great shape on both sides of the car.

After a few hours with Scotch Brite pads, the lower surfaces of the floor pan were ready for primer.

In addition to masking the edges that didn't need primer, I also masked the general areas that were going to get welded when the subframe connectors and rear 4-link mounts are installed.



Floor ready for primer/sealer ("Before" shot).



Amazing how a simple coat of primer makes everything look complete even when the primer is still tacky! ("After" shot).

Another look at the primed floor pan from the front.

Here is a look at the front subframe connector "weld" areas after removal of the masking tape.

Rear subframe connector weld locations after masking tape removal.

I didn't mask all of the places that will require welding on the rear 4-link mounts due to uncertainty in location of the welds in a few spots, but the few places that were nailed down were masked as shown here.

Unisteer Rack & Pinion Mock-Up

I settled on the Unisteer power rack & pinion conversion for my Boss.  There were many reasons for this, not the least of which was the fact that I could get them on the phone for answers and they were willing to work with me on some small requests I had regarding some details of the kit components.  Here you can see the rack assembly mocked up in position.

Because the 69-70 front lower control arm mount wings are flared out compared to earlier Mustangs, the Unisteer kit is not a straight-up "bolt-in" as advertised.  Thus, it meets Pruett's restoration rule #1:  "If it says "bolt-in", it isn't going to".

Here you can see the passenger side interference in more detail.  I'm noodling on how I want to address this in a manner that I feel appropriate.  Grinding and hacking on the mounting bracket is not currently a consideration.

Here is the same area on the driver side.  Looks like the same "fix" I come up with should work on both sides.

The small tabs on the rear camber bolt reinforcement will probably require some modification to get the alignment that will be required for the rack mount.

Here is an idea of how tidy the system is relative to where the original steering box was located.  The system opens up TONS of room and reduces steering system complexity by a huge margin.

Another look at the relative simplicity of the system on the "busy" side of the steering equation.

One complaint I had with the kit is the interference between the inner tie rod boot and the pressure line loop on the right side of the rack.  No excuse for this oversight, but fortunately a delicate tweak of the bend is all that was required to correct this.

Thought it appears the fluid line fittings will work fine, there may some adjustment required to get the lines to enter and exit the rack smoothly.  We'll see.