521 vs 720 Body Swap?

[Trev0rN0t]

Hello Ratsun Community, 

 

 

I have been rebuilding a 1971 Datsun 521(L16) with my grand father, its a frame up rebuild and we have removed the body from the frame. We have been going back and forth between an L20B and KA swap. Which was easily decided when my grandpa's friend informed him he has a 1979 & 1/2 720 at his wrecking yard and he would sell it to us for a whopping title fee + wrecking fee  of $200.00 dollars. So that decision was made easy as it runs and drives and has an L20b!! With all that said I am hoping you guys/gals can help me answer a few quick questions. 

 

- Can we simply swap the bodies? As I like the 521 body style the wheel bases online looked very similar roughly 1" difference each way. 

 

- Can you just swap the front clip only or do you have to do both? 

 

- Can you swap any other parts like doors, tailgates etc. ?  

 

- Is there any parts or drive train advantages of the later model 1979. i.e. Rearend, suspension, braking system etc.? 

[DanielC]

Wayno is the expert on this topic.   He lives in Washington.  He will reply soon.

One issue you need to find out about is how will the vehicle be titled in your state.  In Oregon, for example, if I put a 1970 521 cab, and bed on a 720, it is titled as a 720 truck to match the frame numbers.  In Oregon vehicles newer than 1975 also need an emissions inspection, in some metropolitan areas.  1970 521 vehicles are exempt from emission inspections in Oregon.

[datzenmike]

The model year starts in June of the previous year so the build dates look like half year but they aren't. If the build date is June '79 through July '80 it's a 1980 model year truck. I know, it's weird but how it's done.

 

Yes the body will swap but the 720 must be the same wheelbase as the 521, in other words a regular cab short wheelbase. The steering column is different on the 720. It's articulated with a U joint and the steering box is on the outside of the frame and separate from the column. The 521 has a one piece column and box and I don't think it will bolt to the inside of the 720 frame. If possible use the 720 column if you can make it work.

 

Differences...

 

Well the front brakes are disc so your master should be replaced with the larger 720dual  master and if possible keep the brake booster. wayno will know more about this. Larger brakes mean a larger master cylinder which means more effort on the pedal so a booster is a good idea. If you keep the single 521 master it may feel mushy because it has to travel farther to move more fluid. You will also have to put a disc brake residual valve in the front. A single master looses ALL braking is there is a leak anywhere in the system. A dual master has seperate front and rear circuits. A leak in one and you can still get stopped. 

 

The 720 will have an L20B so you can keep this or your L16 will fit right in. The 720 will also have a stronger 71B 4 speed possibly the optional 71B 5 speed.

 

The differential could be a 3.889, 4.11 or the same as your 521 with a 4.375. Look on the passenger side inner fender under the hood hinge for the engine plate. The differential is stamped in the lower right. An HF43 would be a 4.375

 

The 720 is close to 3" wider at the axles so the 720 will poke the 521 rims and tires out 1.5" per side so keep this in mind. The 521 axle could be swapped in I guess. There's no changing the fronts.

 

 

 

[Trev0rN0t]

Gents very valuable information, my 521 doesn't have a title but they are in Oregon home will be Utah so ill have to look into that. Mike killer information show this to my grandpa and might have more questions sorry. Such a great community here really appreciate everyone insight and write ups, can't imagine a world without the internet these days. 

[wayno]

Mike, the 1980 Datsun 720 has the VIN plate the passenger side inner fender, that plate your talking about that has the rear axle info started on the 1981 Datsun 720.

 

I have done several frame/chassis swaps over the last 11 years, when I first started doing them WA  State was a different place than it is now, I would likely do it differently now, especially with what I have done to my builds, but I am old and don't care anymore, when I die they can be scrapped out as I will not care anymore.

 

The 720 has a way better front suspension setup(ball joints) and disc brakes, I would likely cut the 521 frame off in front of the VIN number and weld on the 720 front clip on, I would make sure the original 521 VIN number still existed, I would not do it any other way anymore except maybe to cut the front shock tower assemblies off the 720 frame and weld them to the 521 frame, then I suspect you could use the 521 steering column/idler arm without any issues.

 

I have no idea what your laws are in Utah,  you should really look into it before doing a frame/chassis swap, and you likely need to have both titles in your name(paid your taxes) before trying to get a combined title.

 

 

 

[datzenmike]

I probably have forgotten some stuff...

 

The steering box and the idler arm are on the outside of the frame rails on the 720. On the 521are inside and this doesn't leave much room for a larger alternator. If switching to a 720 frame you can probably run the 720 50/60 amp alternator which is almost twice the output. There are a couple of wires to join together but you can look into this later.

 

Put the 720 distributor AND COIL in your L16. It's the high output EI matchbox.

 

You'll have to cut the 521 down pipe and weld to the 720 exhaust pipe.

 

 

[Charlie69]

My 66 520 is on a 1980 720 2wd frame.  My brother and I did the swap several years ago.  We had advice on this swap from Wayno.  I am running a L20B automatic in my 520.  It is not a difficult swap if you have a cutting torch, grinder, a welder and of course ability to complete the necessary work a swap of this nature requires.

 

Wayno is the King of the 720 chassis swap.

Edited June 10, 2020 by Charlie69

[Stoffregen Motorsports]

I don't think you can mix and match steering boxes without steering geometry issues. The 521 box mounted on the inside of the frame may have the drag link pivot closer to the frame centerline than the 720. I'm almost positive of this.

 

An entire frame swap is a lot easier than a front clip swap. If you are good fabricator it may be fun to cut off the old front clip and weld up the 720 clip, but it is a lot of work. Mostly in the preparation, measuring and planning. Be sure to set the truck level on jack stands and do not move it before the new clip is tacked in place. When I say level, I mean in both directions (front to back and side to side). Use thin metal shims on top of the jack stands to get it perfectly level. Check the wheel centerline with a plumb bob and mark it on the body before you cut the old one off. Hell, there's a lot to this. If you want to see details search out my old "320 balljoint conversion" thread here on Ratsun.

 

As for the steering column, I used the original column inside the cab, but made a firewall plate that was welded to the column and bolted to the firewall. I then machined the steering shaft to take a U-joint and from there went to the 720 steering box with an articulated, exposed steering shaft.