Automatic Transmission Upgrade

[longie]

there is always talk about 5 speed transmission swaps for obvious reasons! my question is : i have a 1979 Datsun 620 with 3 speed automatic transmission,is there a automatic transmission with over drive to swap it with ? i drive on the freeway on a daily basics and need an upgrade. any help or information would be greatly appreciated.

[Drummerboy4as]

+1! I have a '73 with a 3 speed auto trans and have been wondering the same thing recently. I have a built l20 that I'd like to put in, but I want to keep it auto.

[MicroMachinery]

The early 80's maximas had a 4spd OD in it. They're pretty close to the same length as the stock auto.

 

Read through this:

http://community.ratsun.net/topic/22842-l4n71b-transmission-into-early-10-series-car/

 

He successfully mounted a 4spd OD trans in place of the 3spd. It's in a 610, but the principles should be the same. Good luck!

[datzenmike]

There are two possibilities. The L3N71B and the L4N71B.

 

The L3N71B has a locking torque converter to drop RPMs in third gear on the highway. It's fully automatic and couples and uncouples without driver input. It wasn't available on the L series motors but was on the Z24 motors in the 720 trucks after Nov. '82. Since only the torque converter and flex plate were redesigned and some internal oil pump modifications made to the original 3N71B it looks identical to it. I didn't measure one when I had it but it is likely the same length. If so you should be able to simply remove the bolt on bell housing from your automatic and put it on the L3N71B and put in your '79.

 

L3N71B and 3N71B flex plate (engine side)

 

The L4N71B is a true 4speed automatic with a 0.686 ratio overdrive 4th gear. The overdrive can be engaged or locked out for passing, towing or hill climbing by an electric switch on the shift lever. This transmission was available on the '83 and '84 Maxima sedan and wagon. The good news is that it is used on an L series motor so it will all bolt up to your '79 truck. You will need the L4N71B flex plate and the torque converter. The bad news is that it is an inch or two longer than the 3N71B. I measured the one I had and it was definitely slightly longer. It might fit your driveshaft if it slips in a little farther, I don't know. Worst case you need to shorten the driveshaft.

 

 

L4N71B (top) and 3N71B (truck) automatics

 

 

 

 

+1! I have a '73 with a 3 speed auto trans and have been wondering the same thing recently. I have a built l20 that I'd like to put in, but I want to keep it auto.

 

You have either an L16 or L18 5 bolt flex plate and that won't fit the 6 bolt L20B. Best thing is to get a 6 bolt automatic flex plate for the L20B and you should be able to bolt it to your automatic. The early 620 automatics were short (26") just like the 510s.

 

.

[zerow]

He successfully mounted a 4spd OD trans in place of the 3spd. It's in a 610, but the principles should be the same. Good luck!

Yes he has! This upgrade is fantastic. Where I used to remember the 3 speed auto winding (more like whining) out to 4K+ RPM at highway speeds, I have a comfortable (read: quiet) 2800 RPM with my 3.70s at 65 MPH. If you have questions, "Ask and ye shall receive". There are a lot of things you will need to do that I did not have to do: Custom driveshaft, machined spacers (that may or may not be different than the 610), and the adapter from the stock rear crossmember to the 4 speed automatic transmission.

I have a great machine shop that I used for my parts (Williams Machine out of Lake Elsinore, CA - and he is a member on here "datrod")

[Drummerboy4as]

You have either an L16 or L18 5 bolt flex plate and that won't fit the 6 bolt L20B. Best thing is to get a 6 bolt automatic flex plate for the L20B and you should be able to bolt it to your automatic. The early 620 automatics were short (26") just like the 510s.

 

.

 

 

Ok so if I'm reading that right I need to get the L20B flex plate and possible shorten the drive shaft to fit one of those three trannys. If I grab the Z24 trans can I use the Z24 flex plate and torque converter? Same question for the Maxima trans?

Would any modifications need to be made to the trans crossmember?

[zerow]

Ok so if I'm reading that right I need to get the L20B flex plate and possible shorten the drive shaft to fit one of those three trannys. If I grab the Z24 trans can I use the Z24 flex plate and torque converter? Same question for the Maxima trans?

Would any modifications need to be made to the trans crossmember?

You need a 6 bolt flexplate, yes. BUT it has to be a 6 bolt flexplate from a car that had the L4N71B transmission. The first picture shows the correct flexplate. It had a "dished" look to it, the standard issue 6 bolt flexplate from a 3 speed automatic is flat, without the "dish".

 

Can you use the Z24 flexplate? I can say I am not familiar with the Z24 in that manner. The torque converter may have a different stall speed between that and the L-series motor, but again I could be wrong. Datzenmike should be able to shed some light on 6 bolt flexplate fitment from a Z24 onto an L20B motor. I picked my flexplate and transmission off of an L20A motor that was (from what I understand) a Low-Mileage Japanese Engine swap done sometime during the donor car's life. From the appearance of the photo Datzenmike posted above, the L3N71B flexplate looks like the one I have for my L4N71B.

 

Would modifications need to be made to the trans crossmember? I did not modify mine in any way. The adapter (pictures 3 & 4 of post #82) fits between the OE mount and the bottom of the L4N71B. Without going into extreme detail, the L4N71B has "ears" at the back of the transmission that I utilized for the adapter and the mount. The 3N71B mounts towards the middle of the transmission and is "higher" in relation to the midline of the transmission.

http://community.ratsun.net/topic/22842-l4n71b-transmission-into-early-10-series-car/page__st__80 Post #82

 

Are you planning for this to go in your 510 or 620? I am not familiar with the 620 mount, but if the 510 mount looks like my 610 mount, then it is just the spacer that gets added. You will see that the Maxima L4N71B transmission mounts are like a halo, where the 3N71B mounts with bolts from below.

[datzenmike]

If you get a Z24 L3N71B complete with flex plate and torque converter. All you need to to change is to put your L bell housing on it. Haven't done this but see no reason why it will not work.

[istvanhlc230]

Hi there,

 

I would have a question regarding the 3N71B and L3N71B transmissions.

Right now I have issues with vibration coming out from my 3N71B trasmission and

I suspecting the flex plate and the torque converter as the source of my problems.

As I read the L3N71B converter has an "automatic" lockup, so meanwhile I deciding to replace the 3N71B torque

converter I was thinking is it possible to replace directly to the L3N71B's converter with its flex plate without

any modification? Would it be able to gain overdrived gear?

 

Thank you.

 

Istvan

[datzenmike]

Vibration? Unlikely the torque converter or flex plate unless damaged... and that's hard to do short of an accident.

 

Better to check the U joints.

 

Does the Laurel have a two piece driveshaft with a center carrier bearing? The bearings seldom go bad but the rubber ring (isolator) can rot away and the driveshaft isn't properly supported and will move around when it shouldn't.

 

Have you raised or lowered the body height? This will throw the U joint angles off and will cause vibration.

 

 

 

The L3N71B is different from the 3N71B. For one, the flex plate and torque converters are different but more importantly, the oil pump has been modified also, with the addition of a lock up piston and a speed cut valve inside it. In operation the proper speed will allow the speed cut valve to engage the lock up piston and this directly couples the turbine runner and the oil pump impeller. This in turn requires additional control lines from the valve body.

 

I guess the short of it is, you would need the locking torque converter and flex plate, the oil pump and for sure the valve body. I suspect that this would do it but no guarantees. It would be easier to get an L3N71B transmission.

[datsunaholic]

And easy answer #2, no, that wouldn't get you overdrive. You'd need the L4N71B to do that.

[istvanhlc230]

Well, I had issues with the propeller shaft as well.

I took it to a service shop for balancing, but even after that

the vibration remain. In warranty the workshop replaced the tubing

and re-balanced the shaft. Me and the workshop found the joints in

good condition. I've made a test without the propeller shaft attached, please see

these videos:

 

http://www.youtube.com/watch?v=vxQppizS7io&list=UUtpb_D7qD9tCppWZ8mSGJdg&index=2&feature=plcp

http://www.youtube.com/watch?v=UrlS5jtJ-mE&list=UUtpb_D7qD9tCppWZ8mSGJdg&index=1&feature=plcp

 

Al these cases I had the propeller shaft removed and the vibration was quite high.

After the shaft fix wasn't resolved my vibration issue, I replaced the torque converter

I had from a broken car, which made the vibration 10 times smaller, but still remain

which is still shaking the car related to the RPM. I overhauled the transmission by myself.

If you looking at the first video, the drive shaft is not running centered in the tranny,

I know it has a bushing inside to center the shaft when the propeller shaft attached.

As I attach the small part of the shaft, the vibration becomes much higher.

I have a new bearing on the propeller shaft with new rubber bed anyway.

 

Thank you for your previous answer, I really appreciate.

Istvan

[istvanhlc230]

One of my friends have a new converter, but the part number (31100-X0501) says it is for 280ZX.

You are more experienced than me, could you tell me is that would fit on the 3N71B?