Short Body 4 Speed w/ Overdrive?

[Eomund]

I'm considering a swap to a 4 speed with an overdrive as I'm commuting in my truck. I think it'll help keep my revs down a bit while on the highway.

Do they make a shortbody for trans for this? And will it fit in my well or is it any bigger physically?

And if all is yes, what is a fair price for one. I am willing to get one not perfect and take it to my local trans shop to have it rebuilt. But is it common to find these for under $100?

 

Any info is much appreciated.

[wayno]

5spd is your best choice, you will have to either make one out of an 85/86 Nissan 720 regular cab/short box short shaft 5spd or start modifying stuff to make a long shaft fit, you will still need to modify the transmission mount.

[Eomund]

8 minutes ago, wayno said:

5spd is your best choice, you will have to either make one out of an 85/86 Nissan 720 regular cab/short box short shaft 5spd or start modifying stuff to make a long shaft fit, you will still need to modify the transmission mount.

Thing is I very specifically want the overdrive. A proper 5 speed is still 1:1 in top gear, where overdrive is something like 1.25:1 or 1.5:1 gearing it up. I know they made a 4spd w/ overdrive for the 620's (my dad had one). The over drive being geared up is what I'm after.

 

So I will need to alter my trans mount no matter what? I was hoping for a straight bolt over. If it requires too much I'll just keep what I have. I can alter the well really easily. But making mounts are a pain and I'd like to avoid it if at all possible.

Edited February 7, 2020 by Eomund
adding info

[wayno]

As far as I know all Datsun transmissions are 1:1 in 4th gear, the 71b 5 speeds have better 5th gearing after the 620, meaning that 720 5spds will lower your rpms in 5th gear more than a 620 5spd, some 620 5spds are like the 720 5spd but I don't know how to tell the difference just looking at them so I stick with transmissions made after 1980.

 

I don't recall now how much the transmission mount needs to be shortened/lowered on the 521, but it needs to be modified, here is what the transmission mount height looks like.

 

This is the height it needs to be for a 5spd on a 521.

 

Here is what the transmission mounts I used to make look like, they used a 720 transmission mount.

 

Here is the stock 521 transmission mount used on my mount I made.

 

This is what someone had done on my truck before I bought it, they cut the transmission mount shorter and welded it onto a flat plate and used the 2 lower bolts ti hold it on the cross member, eventually them mounts stripped out but I made a new mount before it fell apart.

Edited February 7, 2020 by wayno

[Crashtd420]

29 minutes ago, Eomund said:

Thing is I very specifically want the overdrive. A proper 5 speed is still 1:1 in top gear, where overdrive is something like 1.25:1 or 1.5:1 gearing it up. I know they made a 4spd w/ overdrive for the 620's (my dad had one). The over drive being geared up is what I'm after.

 

So I will need to alter my trans mount no matter what? I was hoping for a straight bolt over. If it requires too much I'll just keep what I have. I can alter the well really easily. But making mounts are a pain and I'd like to avoid it if at all possible.

 

For a 521 what wayno said is pretty much the easiest way to go.... but you have to make the 1 rear mount and swap the bellhousing to the proper L series front....

Watch out for the stumpy 4x4 transmission... the are the same as the 2x4 version but dont have a speedometer drive.... 

 

Any L series 5 spd will bolt up but they are longer... you still need to do the rear mount and also you have to shorten the drive shaft.... and the shifter gets pushed back a few inches......

 

I doubt you'll find what you want for $100 unless the transmission is junk....

Edited February 7, 2020 by Crashtd420

[Eomund]

31 minutes ago, wayno said:

As far as I know all Datsun transmissions are 1:1 in 4th gear, the 71b 5 speeds have better 5th gearing after the 620, meaning that 720 5spds will lower your rpms in 5th gear more than a 620 5spd, some 620 5spds are like the 720 5spd but I don't know how to tell the difference just looking at them so I stick with transmissions made after 1980.

 

I don't recall now how much the transmission mount needs to be shortened/lowered on the 521, but it needs to be modified, here is what the transmission mount height looks like.

 

This is the height it needs to be for a 5spd on a 521.

 

Here is what the transmission mounts I used to make look like, they used a 720 transmission mount.

 

Here is the stock 521 transmission mount used on my mount I made.

 

This is what someone had done on my truck before I bought it, they cut the transmission mount shorter and welded it onto a flat plate and used the 2 lower bolts ti hold it on the cross member, eventually them mounts stripped out but I made a new mount before it fell apart.

The 5th gear running at a lower RPM is the overdrive. 1:1 means it will be the same no matter what. 3 speed 4 speed, even a 6 speed. One engine revolution is one trans revolution, (basically). How many gears won't change that. Altering the gearing making it higher than 1:1 will make it have lower revs. I've done a lot of reading on the matter.

 

It sounds like a lot more work than I want to do at this given moment though. especially if it's longer, that means I'll have to alter my drive shaft too. Not something I'm ready for just yet.

[Eomund]

27 minutes ago, Crashtd420 said:

 

For a 521 what wayno said is pretty much the easiest way to go.... but you have to make the 1 rear mount and swap the bellhousing to the proper L series front....

Watch out for the stumpy 4x4 transmission... the are the same as the 2x4 version but dont have a speedometer drive.... 

 

Any L series 5 spd will bolt up but they are longer... you still need to do the rear mount and also you have to shorten the drive shaft.... and the shifter gets pushed back a few inches......

 

I doubt you'll find what you want for $100 unless the transmission is junk....

I have a spare 4 speed I got for less. I was hoping I could get a 5 speed for around the same cost. But it's more work than I want to do at the moment unfortunately. I can't have my truck down for any time right now.

[Crashtd420]

3 minutes ago, Eomund said:

I have a spare 4 speed I got for less. I was hoping I could get a 5 speed for around the same cost. But it's more work than I want to do at the moment unfortunately. I can't have my truck down for any time right now.

Ya people just about give away the early 4 spds... I pickup a spare for parts for under 100 shipped....

I got lucky I almost sold my stumpy 5spd.... I found out about the ability to swap the bellhousing to the correct L series and have been running it since last winter.... its definitely nice to lower the rpms while crusing on the highway...

[Eomund]

1 hour ago, Crashtd420 said:

Ya people just about give away the early 4 spds... I pickup a spare for parts for under 100 shipped....

I got lucky I almost sold my stumpy 5spd.... I found out about the ability to swap the bellhousing to the correct L series and have been running it since last winter.... its definitely nice to lower the rpms while crusing on the highway...

And this makes me wonder if this is an option too. I can get a Toyota 5 speed for nothing. The 80's 5 speeds are a dime a dozen. I wonder if I could swap the bell housing. Not sure if that's an option. I do have a spare trans. but it might be a lot of work you know?

[wayno]

You can change your rear gearing to something more freeway oriented but it is harder to get it going from a stop(you loose the hole shot), you likely have 4.37 gears right now unless someone has changed them, you could put 4.1 or 3.89 gears in your axle in a few hours or less for me if everything goes smoothly, it requires jacking it up, draining the gear oil, releasing the park brake and likely removing the park brake cable pin in the middle, removing the wheels, removing the drive line from the pinion flange, disconnecting the brake lines from the backing plates, removing the lock nuts holding the backing plates on the axle housing then slide the axles out at least 4", then remove the 10 nuts/lock washers holding the drop out in, then remove the drop out, if it comes out clean without ruining the gasket on the axle housing then clean up the new drop out mating surface and reverse putting it back together.

[Eomund]

1 hour ago, wayno said:

You can change your rear gearing to something more freeway oriented but it is harder to get it going from a stop(you loose the hole shot), you likely have 4.37 gears right now unless someone has changed them, you could put 4.1 or 3.89 gears in your axle in a few hours or less for me if everything goes smoothly, it requires jacking it up, draining the gear oil, releasing the park brake and likely removing the park brake cable pin in the middle, removing the wheels, removing the drive line from the pinion flange, disconnecting the brake lines from the backing plates, removing the lock nuts holding the backing plates on the axle housing then slide the axles out at least 4", then remove the 10 nuts/lock washers holding the drop out in, then remove the drop out, if it comes out clean without ruining the gasket on the axle housing then clean up the new drop out mating surface and reverse putting it back together.

I've looked into this. If I do this, I'd snag a rear end from a D21 as it is much newer. What I have is in fantastic shape tho.

[NC85ST]

I put a 5 speed out of a 280z and used a driveshaft out of a regular cab, short bed 620. No cutting required. Can’t remember what I did for the mount. I would have to look. Big improvement over the 4 speed. Of course I put an L20b in it when I blew the head gasket on the L16. I did the 5 speed with the L20b. When I did it, it was easy to find 5 speeds and driveshafts.

[wayno]

The drop out is much easier to deal with than a transmission conversion, that piece of shit dogleg 5spd is the closest to a bolt in as one will ever get, the transmission mount has to be modified/fabricated and a soft plug needs to be installed in the drive line to keep oil from leaking.

The Datsun transmission has never had an overdrive option, the Datsun Z car did have a transmission that had a really good overdrive 5th gear for a couple years if you want to call that an overdrive as it really is just 5th gear with a better gear ratio than all the other 5spds out there, and it is a long shaft, so the shift lever would be 5 inches farther back and the drive line would have to be shortened.

I wrote an article on how to build a short shaft 5spd for your 521, but it requires a short shaft 5spd out of an 85/86 Nissan 720 regular cab/short box, a front case from 71B 4 or 5spd in a 1980 Datsun 720 with an L block(maybe a 71B case from a 620 with a 4 or 5spd will work), I wrote it because I had just went thru 4 dogleg 5spds in 5 years, changing transmissions got old, so when I figured out how to make that heavy duty 5spd I was so thrilled I had to tell everyone, that transmission lasted me 5 years before it started grinding into 2nd gear and it would pop out of first gear under compression, so I made another one, but back then there were as many as 10 Datsun/Nissan 720s in the wrecking yards, they are getting harder to find now and I can't build what I need in their yard and walk out with what I need.

[datzenmike]

5 hours ago, Eomund said:

I'm considering a swap to a 4 speed with an overdrive as I'm commuting in my truck. I think it'll help keep my revs down a bit while on the highway.

Do they make a shortbody for trans for this? And will it fit in my well or is it any bigger physically?

And if all is yes, what is a fair price for one. I am willing to get one not perfect and take it to my local trans shop to have it rebuilt. But is it common to find these for under $100?

 

Any info is much appreciated.

 

There are limits to what an engine is strong enough to pull on the highway when the object is to drop the RPMs that produce the power to do so in the first place. Simply put, you can't get something for nothing. Generally a truck is over geared to rev it up to make as much power as needed to pull it's heavy self plus cargo at a reasonable highway speed. To a certain extent you can gain a very small amount by dropping the cruise RPMs if not hauling a load. Wind resistance increases four times for a doubling of your speed. It takes 4 times the power at 80 MPH as it does at 40! Too much OD and you can't use it on windy days or on some hills. For this you really need to down shift to 4th.

 

I've used a mid ratio zx transmission in my car and 5th is only a 13% over drive and just noticeable as a 500 RPM drop on a 4,000 RPM shift from forth. Right now I have a wide ratio of 18% or a 720 RPM drop. A 25% zx 5 speed would drop a huge 1,000 RPM. At 80 MPH dropping down to 3,000 is a serious drop in power.

[yenpit]

One point I did not see mentioned, is that all of the 280Z 280ZX 5speeds are considered an overdrive by the 5th gear ratio.  There are three different overall gear ratio's, but 5th is what you are after.  The easiest to find & cheapest 5speed is the earliest 280Z & early 280ZX with a .864 overdrive 5th.  The later 280ZX's had a .745 overdrive 5th & a .733 overdrive 5th.  There are visual ques to identify these three different 5speed ratio's, otherwise you need to rotate the trans on the bench & count rotations to identify gear ratio's.  The number stamped on the top edge of the bellhousing near the block does NOT help you identify the gear ratio's!  As mentioned above, all of these are the long tail, which require a shortened driveshaft..........somebody above mentioned a 620 driveshaft that would fit.........??

 

I typically get $200-250 for a good rebuildable 5speed CORE, unknown condition, I get more if KNOWN to be good.  The days of $100 good 5speeds are going away quickly with the values of the Z's & 510's going thru the roof & that they are 45++ years old!  I have given away the 4cyl 4speeds before, cuz most are upgrading to 5speeds.

 

I've had two of the short tail TRUCK 5speeds, but they are gone now.  I do not know what gear ratio's those were..............

[Eomund]

On 2/7/2020 at 5:23 PM, datzenmike said:

 

There are limits to what an engine is strong enough to pull on the highway when the object is to drop the RPMs that produce the power to do so in the first place. Simply put, you can't get something for nothing. Generally a truck is over geared to rev it up to make as much power as needed to pull it's heavy self plus cargo at a reasonable highway speed. To a certain extent you can gain a very small amount by dropping the cruise RPMs if not hauling a load. Wind resistance increases four times for a doubling of your speed. It takes 4 times the power at 80 MPH as it does at 40! Too much OD and you can't use it on windy days or on some hills. For this you really need to down shift to 4th.

 

I've used a mid ratio zx transmission in my car and 5th is only a 13% over drive and just noticeable as a 500 RPM drop on a 4,000 RPM shift from forth. Right now I have a wide ratio of 18% or a 720 RPM drop. A 25% zx 5 speed would drop a huge 1,000 RPM. At 80 MPH dropping down to 3,000 is a serious drop in power.

Only a 500rpm drop??? Fuck me I though it'd be more. For that little it's not worth it. Thanks for that info. With that I'll stick with the 4 speed then. Mine is strong and I have a rebuilt spare. 

[datzenmike]

Well look at it this way you can't over drive it to drive at 80 at 1,000 RPMs. It has to have a reasonable amount of over drive or you'll start slowing down and have to step down on the gas pedal. Fifth gear is for cruising speed with no or very little load on the engine.

 

 

If you drive primarily in town then a 4 speed is fine. If the majority of your traveling is on the highway then a 5 speed is handy. Slight increase in mileage and the drive is quieter.

[Eomund]

And that's what I'm looking for. But from the sounds of it. It's a bit more than what I can do right now money wise and a lot more to it than a straight swap. So I'll hold off for the time being. Especially if it will only make a small difference like that.