PL620 Full Balljoint front end conversion

[LS2620]

Is there anyone that makes the shock tower/upper control arm mount for the 620 that will allow for a balljoint conversion? I have heard of some people that do but have not been able to find anything. As well i have not been able to find a 720 to buy or someone who is willing to part out the front frame section needed to convert mine to balljoint and disc brake. Any help or information would be much appreciated as i have it down to the bare frame now and would be nice to get all the grinding and welding done before i paint the frame.

[datzenmike]

The '78 and '79 620 were the first ones with ball joint suspensions. The upper and / or lower control arm mounting positions are different on those frames so you can't just swap the arms to the earlier trucks. You could cut the front clip off behind the cab so that the entire torsion bar assemblies go with it and weld to the earlier frame or just swap the body to a newer frame.

 

I think BEEBANI on here makes custom control arms for the 620.

[LS2620]

False, the lower control arm fits almost perfect and the top shock towers would have to be welded on to the existing frame. I would Be swapping the Upper mounts from the 720 and setting them with the bar and plumb bob method to make sure they are true and perfectly set and i will also be using a coilover suspension instead of a torsion bar set up. I don't mean any disrespect in any of this but i would absolutely not have to cut the front clip off to swap suspension set ups. I just have to cut the top shock tower/control arm mount off of my 620 and make sure that it is flush and clean with the frame and weld on the 720 shock tower/upper control arm mount. There is a couple youtube videos of this exact swap.

[LS2620]

To clarify, I am also talking about swapping the entire front suspension, minus the torsion bars, from the 720 to the 620 as well as the shock towers/upper control arm mounts.

[jagman]

I have an entire front frame for a 720.

[datzenmike]

If you could just swap later ball joint suspension onto the earlier trucks everyone would have done it. It might be the top one but there was some reason for it not working. Show me.

[Stoffregen Motorsports]

See my 320 balljoing conversion thread. I clipped the frame just behind the motor mounts and used the original torsion bar mounts. I did this on a 320, which is narrower than the 620, but all other parts for the job should be the same. I chose that cut location because I wanted to keep the original frame numbers, which are on the passenger side, just behind the motor mount.

 

The problem with just removing the individual suspension mounting points and replacing them with the later parts is that you need to be 100% perfect in your measurements and absolutely 100% accurate on your fitting ans welding on the new parts. Doing a whole clip is simple by comparison. The most important part of doing a clip swap is setting the truck up on jackstands that won't move, then make the truck perfectly level (use shims on top of the jack stands to level it out). Use a plumb bob to measure and mark your wheel centerline on the fender and if possible, use the plumb bob to mark all important locations on the concrete floor. DO NOT MOVE  THE TRUCK at all until the job is done. You should even avoid leaning on it or getting into the cab during the job. Unless you have it rock solid on the jack stands.

 

Here's a pic of the custom stands I made for doing these types of fabrication. They are adjustable with 1" acme threaded rods and I can level out a chassis with the turn of a nut.

 

[Stoffregen Motorsports]

Here's a link to the 320 balljoint conversion thread -

 

[thisismatt]

9 minutes ago, Stoffregen Motorsports said:

Here's a link to the 320 balljoint conversion thread -

 

When you narrow the clip of the stock 720, doesn't that mess up the Ackerman angle? Not enough to matter?

[Stoffregen Motorsports]

Sure it does, but you can't have it all. The only way to fix that would be to heat and bend the steering arms.

 

COnsider this, the 320/521/620 suspension and steering are horrible. The 720 has improved geometry to begin with, so it's a massive improvement.

 

Besides, have you ever actually noticed bad ackerman? The only vehicle I have ever built that had ackerman so bad that it was noticeable was a custom suspension, custom axle BJ74 Land Cruiser. The issue was that the tie rod wouldn't clear the diff on full steering lock, so we had to run the tie rod on the front hole of the steering arms, and even then, the ackerman was only noticeable when doing a tight u-turn.

 

So yeah, theory is one thing. Reality is different.

[thisismatt]

I always suspected it was bad on my I-beam front end F250. It would chatter and slip making tight turns, especially if one wheel hit some gravel.

[Stoffregen Motorsports]

If your F250 is full time four wheel drive, then that probably has more to do with it than anything else.

 

One other thing, wheelbase. Vehicles offered with different wheelbase lengths don't have specific steering arms between them. If you buy a long bed, extra cab truck, is has the same steering arms as a short cab, short bed truck. Since wheelbase also plays a part in ackerman, it's such an inconsequential detail that even the engineers don't make adjustments.

 

For ackerman to be noticeably bad, something has to be very wrong.

[thisismatt]

It was two wheel drive but it was the XLT king/crew cab (still only two door). Thing has the turning radius of an aircraft carrier.

[Valvebounce]

I would 100% just find a late model 620 chassis, and swap all your parts onto it. 

[Valvebounce]

Then d21 hubs and stubs just bolt in

 

[LS2620]

On the 620s vs the 720s front suspension geometry it is nearly the same as far as how and where the control arms mount. The only difference is the type of shock tower/upper control arm mount. If you get a 720 frame off of someone and cut very carefully and get that shock tower mount off the frame without making the mounting plate off center and crooked you can use that piece and weld it on to where the existing shock tower was on you 620. it is a very invasive job and not easily done but is very doable if you have the right knowledge and trust you welding skills to do it. you use a level rod of some sort and tie a plumb bob onto it and set your new control arm mounts where they go and use that to make sure that they are true and center to the lower control arm mounts. you do this by centering the plumb bob on the lower brace that is connected to the lower control arm mounts. you have to be very picky on this as this will be whether everything lines up the way it should. 

 

look up this url and it should take you straight to one of the only youtube videos on this.

 

[datzenmike]

Just like I said earlier the 720 stuff does not swap on for the reason given: one of the control arms is the wrong length. If it did, it would be done. Cutting the upper control mount off and swapping it is beyond most people's ability and equipment. Anything is possible but if it was easy everyone would have done it.

[Stoffregen Motorsports]

13 hours ago, LS2620 said:

On the 620s vs the 720s front suspension geometry it is nearly the same as far as how and where the control arms mount. The only difference is the type of shock tower/upper control arm mount. If you get a 720 frame off of someone and cut very carefully and get that shock tower mount off the frame without making the mounting plate off center and crooked you can use that piece and weld it on to where the existing shock tower was on you 620. it is a very invasive job and not easily done but is very doable if you have the right knowledge and trust you welding skills to do it. you use a level rod of some sort and tie a plumb bob onto it and set your new control arm mounts where they go and use that to make sure that they are true and center to the lower control arm mounts. you do this by centering the plumb bob on the lower brace that is connected to the lower control arm mounts. you have to be very picky on this as this will be whether everything lines up the way it should.

 

I am 80% sure that the geometry is different. I was only able to compare the 720 clip to a 320 clip, but as far as I know, the 320 setup is near identical to the 620. If that's true, then the 720 has different geometry than the 620.

 

Welding skills are good to have, but measuring skills are far more important when tackling a job like you describe. You need to measure on a perfectly level frame. There are two centerlines, one from front to back (wheel well placement) and side to side. Then you need to make sure everything is square and plumb. Sometimes the hardest part is finding a static place to take measurements from, ie - how do you measure the kingpin CL when it is not vertical in any direction? Hint, you measure the spindle location instead, but then you need to make sure the toe is set to zero and the suspension is either at full bump or full droop. Ideally you would build the suspension at the proposed ride height, without springs or torsion bars in place, so that needs to be measured and mocked up too. And then if you're removing motor mounts during the process, you need engine height, side to side placement and engine fore/aft and side/side angle.

 

If you're trying to replicate the original measurements on a suspension you have just removed, you need all those measurements before you cut the donor pieces off the clip, so all that leveling and measuring needs to be done on the clip (or donor chassis) too.

 

So yeah, there's more to it than watching a youtube video.

Edited August 3, 2021 by Stoffregen Motorsports