Tag: 1977 F100

Dear Welder Series… 1977 F100 MII?

Dear Welder Series,

I have a 1977 F100 that I’m currently trying to fit a 7.3 Godzilla into. The oil pan of the Godzilla along with the twin I-beam suspension are creating some challenges. 

The primary issue is the drag link at the rear of the crossmember. To fix this I’m considering putting in an ifs kit, however I’m weird – I don’t want to lower the truck and I’m perfectly happy to keep the 5-5.5” lug spacing for the wheels.  I know, I know that is weird but I like the stance and wheels I have now. I’ve got some sentimental attachments to this truck as I bought when I was 14 years old – 30 years ago!  The problem is all of the ifs kits that I have run across all claim 3-5” drop. 

I’m looking at the wealth of knowledge on your website and from my thinking it would be completely doable to have the upper a-arm outside the frame rail and still have a close to stock track width of ~65.75” hub to hub. This would allow the frame to ‘rise up’ relative to the suspension geometry allowing for stock ride height. 

My frame rails are 33.5” measured outside to outside. From what I have found so far, this would require a different steering rack that is longer than originally offered (or is normally ran on a Mustang II style suspension). 

Is this in fact possible or am I missing some key detail?  Any guidance to crossmembers/ a-arm kits from you guys that I could do this myself would be great!

Thanks in advance for you help!

Hi Paul,

It was great talking to you the other day; it was super helpful to get me going. 

For my specific situation, I’ve drawn up a 4” square crossmember that the lower control arm and steering rack will mount to. The positions will push the control arms out such that the upper control arm will be outside my frame rail allowing the frame to rise above the standard position of most Mustang II IFS that provide 3”-5” drop (which I don’t want).   Using dimensions of control arms and an MII spindle it will still be within the current hub-to-hub dimensions I have today. 

Based on the outside-the-frame-rail upper control arm, first and foremost, I want to first make sure my assumptions/math is correct – the horizontal separation between the upper and lower control arm pivots (passing a vertical line through each point) should be 3.625”. ((29.5 – 22.25)/2). 

Of course, I also have some other questions…

Does the center of the crossmember coincide with the center of the spindle (i.e. wheel center)?  I realize this is based on the assumption that the crossmember in a Mustang II is 4” as I’m planning to use(?).  Reason I ask is it seems all the Mustang II IFS kits position the crossmember centered on the original wheel centerline. I know the lower control arm mounts directly to the front of the crossmember with the rear needing a tubular support and gusset – pushing it rearward so I assume the lower control arm geometry in effect makes the spindle centered with the crossmember but wanted to confirm. 

What is the angle of the spring hat viewed from the front?  Also it’s relationship fore to aft with the crossmember?

The steering rack tab mounts – what would be their relative position to each other and should it be centered on my crossmember, or biased?

Paul, I know I’ve asked a lot of questions. If you prefer a phone call to talk through this versus email I’m absolutely fine with that in fact, almost prefer it.

Best Regards,


Dear Ken…
Ken, you are basically on the right track.

Please send dimensions for the attached drawings.  

‘Z’ is spindle location, front-to-rear.

In MII C1, I need inside and outside measurements at each point.

In MII C2, the frame must be blocked at the finished ride height and rake you want. At each point, dimensions should be from the ground to the bottom and to the top of the frame rail.

Dimensions to the nearest 1/16” is great; to the nearest 1/8” will be ok.

I’ll get back to you with a drawing showing, I hope, how our 60-1/2” crossmember can be used to give you what you want. Or I’ll have notes explaining how the frame has to be modified.

Thanks for working with me.


Dear Welder Series…
Hi Paul,

I got some measurements however, I still have the original suspension in the truck and the steering gear so it made getting dimensions somewhat tricky. 

For MII C1:
As accurately as I can measure at this time the outside frame width for all points TF, CF, Z, CR, TR, & TC is 33.5”. 

Inside measurements:
TF – 28.875“
CF – 28.875“
Z – 28.875”
CR – 28.875“
TR – 29.625“
TC – 29.625”

For MII C2:

Frame is nearly completely flat in the range of these measurements. With the lower crossmember and twin I-beam brackets in the way made getting very precise measurements difficult. It is at the correct ride height front and rear though. 

At all points, the dimension to the lower frame rail was 16.75” while the dimension from the floor to the upper measured 22.625”

Look forward to your suggestions about how to make this work. 

Best Regards,

Dear Ken…

Hi, Ken.

Thanks for the dimensions. I’ve put them in my CAD template for a 60-1/2” track width kit.

I have based my thoughts on a stock ’77 F100 tire size of 225/75/R15 which is about 28-1/4” diameter.

Using your ground to bottom of frame ride height dimension of 16-3/4”, the bottom of the frame would be 3-1/8” higher than the spindle. We allow for about 1/2” of tire “squat.

With a 60-1/2” kit, the nominal upper arm center-to-center would be 33-1/2”. The frame will not give the arms the clearance needed. The arms would be part way down the outside of the rails, too.

When we have this condition, a solution is to add to the bottom of the frame and cut away the stock frame above it (except for support ahead and behind). A section of 2x4x1/8” wall tubing, about 18” long would work. The tube would center on ‘Z. The stock frame would be cut away 6” ahead and behind ‘Z’. With a little planning, the front and rear of the 2×4 can be angled to blend into the stock frame and not look “scabbed” on.

That done, the upper arms would have clearance.

The bottom of the 2×4 would need a small c-notch for rack bellows clearance.

Your F100’s stance would then be as you have now mocked it up. I could give you cut lines information that you could use on our crossmember and upper towers. 

The general idea is as shown in the attached pdf. In this example the bottom of the frame is not straight so the customer has to allow for that. The shaded area is stock frame that would be removed. There should probably be more weld length between the bottom of the stock frame and the top of the added section. This is just a concept drawing.

Let me know if this is the way you would like to go or if you have questions/comments.


Dear Welder Series…

Hi Paul,

Thanks for the information. Your numbers are spot on with what I’ve measured and assumed. I was using ~3” below the bottom of my frame rail as the location of my spindle center. 

I sketched up a concept (attached) of a tubular cross member that will basically cradle my current frame rails making room for, as you said, the upper control arm pivot because it wants to go thru the frame in order to raise the chassis up. My sketch moves to a hub to hub of 64.75” (current TIB is 65.75”). I know this will require a lengthen steering rack (or at least spacers – I don’t know if all racks have threaded joints or not though. If so, it seems like a rather easy way to lengthen to get the pivot to coincide with the upper and lower control arm pivot.). 

My thought is it would be easier to make this assembly on a bench and install than to cut and modify my frame(?). Not sure if that is a correct assumption or not…

Forgive my rather crude model and hope it gets my thoughts across. 


Thank you!


Dear Ken…
Ken, it’s probably easier to do on the bench.


Dear Welder Series…
Would your 62.5” kit get the upper control arm out beyond my 33.5” frame rails?

Dear Ken…
The nominal cross shaft c-c would be 35-1/2”.  Clearance would depend on the control arm and the adjustment that might be required.