Chassis Swap for 1975 Datsun 620 – Seeking Frame Recommendations

[CursedOutlaw]

Hey everyone,

 

I’m planning a chassis swap for my 1975 Datsun 620 and would love some input from the community. My goal is to enhance the truck’s performance and handling while maintaining its classic look. I’m particularly interested in frames that would fit without extensive modifications.

 

Here are some details about my build:

• Current Engine: Planning a motor swap for more power.

• Project Goals: Improved performance, handling, and reliability.

 

I’ve heard that certain frames from other vehicles might be a good fit, but I’d like to know what has worked for others. Specifically, I’m looking for:

• Compatibility: Frames that align well with the Datsun 620 dimensions.

• Ease of Installation: Preferably options that don’t require significant cutting or welding.

• Performance: Frames that offer a solid foundation for performance upgrades.

 

Some potential candidates I’ve come across include frames from the Nissan Hardbody (D21) and other small pickups. Has anyone here done a similar swap? If so, which frames did you use, and what challenges did you encounter?

 

Any advice, recommendations, or insights would be greatly appreciated. Thanks in advance for your help!

 

Cheers,

Jairus

[datzenmike]

A frame isn't going to do much for performance, it's just trading one frame for another. '75? that would be a regular cab short wheelbase? A 720 short wheelbase regular cab frame is close enough a match but no gain unless your frame is rusted out.

 

Enhance 'performance'? What does this mean?? Handling? You can vastly improve handling by lowering the truck's center of gravity. The front has adjustable torsion bars. But within reason, or it adds problems. The rear can be dropped with 'drop blocks' under the leaf springs.

[EDM620]

Since you're looking for easy frame swap, the 720 is nearly identical to the 620 in all respects but for a few differences.

 

The 720 rear frame rails are slightly longer but can be cut down. The extra length is due to the rear bumper on the 720. What is notable is there is a difference with where the steering box mounts onto the frame, the 720 mounts to the outside of the frame where the 620 is inside the frame. All other mounts cab & box are the same - regular cab to regular cab/KC to KC. You can change from reg to KC by relocating a couple of mounts.

 

While no "performance" gains result, you do gain front disc brakes, power steering, a larger engine and a 5 speed manual.

 

To the best of my knowledge, the D21 frame remains the same dimensionally. Here you gain the potential for a V6, at least the KA. 

 

Fitting the smaller 620 body to the 720 or D21 running gear presents challenges. The engine bay on the 620 is pretty small and the 720/D21 simply have more stuff under the hood. Look at my thread 

for some of the details of making things fit. Honestly if I'd had a D21 to start with, that would have been the direction I went. Not easier though...

 

[datzenmike]

4 hours ago, EDM620 said:

Since you're looking for easy frame swap, the 720 is nearly identical to the 620 in all respects but for a few differences.

 

The 720 rear frame rails are slightly longer but can be cut down. The extra length is due to the rear bumper on the 720. What is notable is there is a difference with where the steering box mounts onto the frame, the 720 mounts to the outside of the frame where the 620 is inside the frame. All other mounts cab & box are the same - regular cab to regular cab/KC to KC. You can change from reg to KC by relocating a couple of mounts.

 

 

 

You can only convert a regular cab long whee base to a  KC frame by moving the two front box mounts. A '75 makes it a short wheel base truck. I put my '78 KC on an '82 720 regular cab long wheelbase by doing this. The steering column is more or less in the center of the left frame rail so for a time I did use the 620 steering box by ovaling the mount holes in the frame and running it on the inside.

[EDM620]

56 minutes ago, datzenmike said:

moving the two front box mounts

Which I did on my build to convert reg cab long box frame to KC short box. Going the opposite direction is effectively the same thing but in reverse. KC uses a stepped fuel tank on the later frame mounted tank. The shorty would have to remain a shorty.

 

The steering box wasn't an issue operationally - I used the 720 column and adapted the firewall mount as needed. What is a minor issue is body clearance over the steering gear, some recontouring (hammer work) was required. That help was also needed to make other components fit properly.

 

Just saying, the frame fit is the easy part, it's all the other stuff that changed between generations of truck that creates the challenges. Wierd things like the wiper motor changing from inside the cab to outside. Different size brake booster. And the electrical... 

 

CursedOutlaw are you intending on keeping the 620 running gear/electrical etc or changing it all? There is a can of worms here, driven mostly by the fact the 620 engine bay is small. The 620 radiator is small. The electrics are simple with only a few circuits. I could go on... oh yeah my build goes through it.

[Duncan]

Just spit balling here, but a Chevy S10 frame should be fairly close, and there are TONS of aftermarket upgrades such as rack pinion, big disc brakes, much better choice of wheels, 4 link kits, etc, etc.  You can even bolt it an LS motor using aftermarket mounts if you wish..

 

 

[datzenmike]

Unless the 620 frame is ruined this is pretty much a lot of work for not much gain. The 720 frame is virtually identical, anything else has to be sourced and a lot of work for little to no gain. If your frame is good leave it alone, if not keep it simple... weld it up or get another 620/720 frame. 

 

You say it's a  '75 but not much else. So is it a regular cab short wheelbase? or a regular cab long wheelbase. There were no KC long wheelbases in '75 but there were long beds on long wheelbase. If you have a long wheelbase only another long bed or KC frame will work. If you have a short wheelbase then only another short wheelbase will work. 

[CursedOutlaw]

On 12/7/2024 at 6:47 AM, datzenmike said:

A frame isn't going to do much for performance, it's just trading one frame for another. '75? that would be a regular cab short wheelbase? A 720 short wheelbase regular cab frame is close enough a match but no gain unless your frame is rusted out.

 

Enhance 'performance'? What does this mean?? Handling? You can vastly improve handling by lowering the truck's center of gravity. The front has adjustable torsion bars. But within reason, or it adds problems. The rear can be dropped with 'drop blocks' under the leaf springs.

It’s the 620 single cab short bed L18 4 speed. I’m just trying to get away from the king pin front end and have a chassis and suspension I can rebuild and buy parts for. The whole performance aspect is asking what frames can handle a good amount of power and won’t have a ton of body roll. My Datsun is on 4 inch drop blocks and I have the front turned down

[datzenmike]

L18? Engine was swapped if this is a '75.

 

Then look for a '78 or '79 620, they had disc brakes and ball joint steering. If you get an entire truck  and you also get an L20B, electronic ignition, and alternators with internal regulators.

 

An '80through '82 720 had the same suspension/steering and the '83-'86.5 had upgraded caliper and vented rotors similar to the early D21.

 

Both 620 and 720 short wheelbase trucks have frames that your '75 body/box will fit right on.

[CursedOutlaw]

40 minutes ago, datzenmike said:

L18? Engine was swapped if this is a '75.

 

Then look for a '78 or '79 620, they had disc brakes and ball joint steering. If you get an entire truck  and you also get an L20B, electronic ignition, and alternators with internal regulators.

 

An '80through '82 720 had the same suspension/steering and the '83-'86.5 had upgraded caliper and vented rotors similar to the early D21.

 

Both 620 and 720 short wheelbase trucks have frames that your '75 body/box will fit right on.

I don’t know the exact engine, I thought the l20 is an i6 instead of i4. And from what you just said I’m thinking about selling my Datsun and looking for a 78 620 king cab.

[datzenmike]

The L20 IS a six cylinder from the mid '60s used in Japan. It's a 2 liter engine but when the 2 liter L series four cylinder was built they couldn't use the L20 name as it already existed so... they called it an L20B.

 

Look behind the dip stick for a flat boss on the block just below the aluminum head. The engine displacement is stamped there...