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521 on d21 frame


dat nasty

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I've had my 71 a few years now. Done just about every bolt on part other than a turbo. love this truck to death, even when it runs like shit I'd rather drive it then my brand-new work truck LOL. I tore down a single cam KA and went to build it and was discouraged by the price of some parts vs the final power output. Decided to run stock, so I brought a donor truck (91 d21) with a rebuilt motor less than 200 miles was my friends college project. I'm looking at just rolling the cab off of the hardbody and setting my 521 on it. As anybody seen anything about this? I have been reading for months. I'm gonna do it no matter what I have a welder and a grinder. Just looking for some helpful advice if it is out there. I will be posting pics as I go. I wasn't going to write up a post until I was done hopefully this doesn't get filled with stupid comments

Pretty much I have everything I need to do this, just need to buckle down and click on the buy it now on the airbag kit.

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Don't forget to use body mounts, don't weld cab to frame. The 620 and 720 frames are very similar. I put a 620 cab on a 720 frame... bolted right up. The 521 is likely similar however the D21 may not be. Just fabricate something to make it work.

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Wayno has a 520? On a 720 frame. He stretched his cab 10 inches to make it all fit. While not exactly what you are planning, it might help you get some ideas about the issues you will encounter.

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I'm no dummy I plan on using mounts, I'm just saying I'm doing this no matter what it takes, fab is no issue. Ive seen wayno's toys. Nice stuff. I'm just looking to see if anybody has done anything with a D 21 frame. It is wider. Curves are different. Pretty much have to body drop to line everything up but that doesn't hurt my feelings. Just looking to see if anybody has found any major hangups.

I've been following these forums forever, new to posting. Let me get the hang of it and I'll start throwing pictures up

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I'm ok with that. The 16" tsw's I have on it now are pretty inset. They barely clear everything on the 521. Literally credit card gaps inside. I'm hoping they won't stick out too far, I like them. If I have to order wheels and roll fenders I'm not too worried

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Why not just the front section of the D21 welded to frame of the 521.

 

No real advantage to using the full frame.

 

If you had a 720 frame it would be a million times more straight forward.

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Well the full frame holds all the good parts... brakes, suspension, steering, engine and transmission. And already wired for EFI. Better gas tank and may as well keep the C-200 differential.

 

Don't roll the fenders. Measure the width first and decide if worth doing.

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I think that the wheel base from front to back is going to be your biggest issue, second will be the engine fitment in the engine compartment and the front wheels centering in the fender wells, it works real well with the 521 cab on the 720 frame, if the 91 D21 frame is like the 86 720 frame it should go well.

The track width is also going to come into play, as the years have went by, the track width has gotten wider and wider, it's inches between the 521 and the 720, but I use stock rims which work well, they are offset in, I would have been fine on the 1966 520/86 720 frame combonation if I had not lowered it, but I did lower it, so I had to shorten the lower control arms when I made the drop arms I have on the truck.

The gap between the cab and the box has always been the issue for most, the difference between the 1970 Datsun 521 and 1985 720 regular cab frames is about 3 inches, so there is a 3 or 4 inch gap between the cab and box, there are a few ways to fix it without modifying the cab/box, but in the end something will need modified, a drive line mod with special drop blocks that allow the axle to be moved forward a few inches would likely be the easiest, as long as you understand what your doing, I wonder if a 520 drive line would work in this conversion?

Only the front cab mounts line up on my 521 cab on 720 frames, the rest need modifications.

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I put a 1980 720 regular cab chassis under my 66 520 and in doing so I had to relocate #2 middle and #3 rear cab mounts on the frame to line up the holes in the cab.  Also removed the bed mounts from the frame.  Trimmed them down and relocated on the frame.  My cab to bed gap is 5/8".  Closer than stock.

 

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Can not delete the PhotoBucket Trash.

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Edited by Charlie69
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I come from the 4 x 4 world and my friend helping me with this has a lifetime of building Toyota crawlers. The wheelbase front to back is a couple inches longer, moving spring perches is once again no issue, Considering I'm bagging it anyway. I want this frame for The long run. I plan on leaving the full wiring harness with the frame. Since I have both of them it will be easy to splice connectors in where needed and remove what isn't. Parts are cheap and abundant and in stock. I'd rather do all this once rather than every time I upgrade something have to fab something. Just to put those rims on I had to drill out the spindles, they are held on with goldplated 3 inch grade 8 bolts LOL . I left a little room for a disc brakes while but now I cannot get them off. People put the ka in the 521, so I know it fits. No problem tubbing a wheel well to fit. I guess my main concern was steering column. I have yet to find anybody that has done the swap so I guess I will have to post as I go. I'm off to remove cabs.

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No one was saying that the motor wouldn't fit. Only that you might find that the front tires are totally off center to the wheel wells when you set the cab where it needs to be to fit the engine.

 

We are looking forward to updates.

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The 521 steering box and column are one piece. The D-21 will have a U joint, a rag joint or probably both and splined so it is collapsible, like the 720. I put a 720 column in my 620. (which also had the one piece box and column) With luck it will also have power steering.

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One can get just about any engine in something if they make the effort, doesn't mean it will do anything other than move, but it is in there.

It sounds like you have the understanding on how to move the axle around without cutting the frame or relocating the leaf mounts, so on that you are set.

I use the 720 wiring harnesses in all my builds as long as they are not cut up, they seem to work fine for me, but I know what I have to do, as I have done it before, so I will out line what I have done.

Round headlights are wired differently than square headlights, so I take other headlight plugs from scrap harnesses and put connectors on them and plug them into the proper plugs to make the headlights to work correctly.

The heater only needs a hot wire to work, so I use the fused heater wire from the 720 and use the 521 heater fan switch.

I use the 720 tilt column in all my builds, but the stock 720 non-tilt steering column bolts right into the 521 without mods.

As I said, I use the 720 wiring harnesses in my builds, so I also use all the column switches, 4-ways, wipers, headlights, turn signals, and ignition key, so all the 521 switches except the heater fan and manual choke get deleted.

The instrument cluster wiring is a different story, the turn signals, oil and ign. lights are wired normally, so is the power for the gauges(I find a keyed hot wire), but the dash lights and bright indicator light are a different story, it has been suggested to me to use the power for the side marker lights so when you turn the headlights on the dash lights come on, I just found a keyed hot wire to turn my dash lights on, they are on when the key is in the on position, my next conversion I will try the side marker light power for the dash lights, as for the bright indicator light I buy a 2 wire plastic light plug that will fit in the hole from radio shack and connect it to the 2 wires that operate that bright indicator light, that light cannot be grounded for any reason, if you ground it like it is grounded in the 521 instrument cluster, your battery will be dead in the morning as the headlights barely come on, it has to be dark to see that they are on, and they will not work properly either.

The 521 wiper motor can be used with the 720 wiring harness, I figured it out, I smoked 3 intermittent boxes figuring it out, on one or more of my build threads I described how to do it, I also think I described how to do it(which wires go where) in Charlie69s 520 thread, I don't know if the hardbody wiring harnesses are the same. 

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This type of work is right up my alley and anything can be done. It would actually be a nice conversion if it were done properly.

 

Advice I can give is to build some tall adjustable jack stands, set the chassis on the stands and level it off (using laser level or digital protractors) and DO NOT move it until all the mounts are built. It sounds like you're pretty confident in your fabrication skills, so I won't elaborate on what's required, but take good measurements before tear-down. Find a good reference point that is the same on each side. Take wheel centerline measurements (wheel position in wheel well). Don't assume anything. Mock up, test fit, adjust, mock up again.

 

I'll snap a pic of the stands I built. They weren't cheap to build, but they are the perfect tool for this job.

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The steering column is just another hurdle to overcome.  I am running a shortened 720 column in my 66 520.  the rag joint lets you swap columns easily.  The turn signal clam shell on the D21 column is huge in size and might cause some issues.

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I am not trying to discourage you.  In fact I would like to see this done and well documented here, listing and documenting all the hurdles you jump in the process.  Documenting adds a lot of time to a build but it is necessary for future use.  I know some of the hurdles I crossed with my 520 build I would not remember clearly if I had not documented.

 

I have documented every part I have used on my 520 for the reason of replacement down the road.  I have mixed a lot of years and models of parts in my build over the years.  I have tried hard to stay all Nissan parts.

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