Heres yet another 620 4 to 5 speed trans question..

[Perez620]

Looking to locate a 5 speed trans to replace my 4 speed in my L20B 620. A few questions I have which I’m sure have been covered many times on here..

 

After I locate a 5 speed would it be cheaper to rebuild it or buy refurb? What’s the average cost of a rebuild?

 

Also, depending on what trans I find, what major difference will I see in the gearing for my ‘74 rear end?

 

Abt help would be appreciated. If there’s a page already dedicated to this topic feel free to share

[flatcat19]

The 5 speed you're going to find d is going to be a 71B...which are actuay pretty strong.

 

I'd rock it until it explodes.

[Perez620]

The 5 speed you're going to find d is going to be a 71B...which are actuay pretty strong.

 

I'd rock it until it explodes.

[Perez620]

I would but im currently rebuiling the L20B abd i iust want everything mechanical fresh

[flatcat19]

Start shopping for the rebuild kit yourself.

[DanielC]

Buy "read end" I think you mean rear end.  That does not change unless you change it.

 

On a 521, the biggest change was going from about a 3.6 first gear ratio, to a 3.3 first gear ratio.  But that changed the huge gap between the second and third gear ratios for the better.  Fourth gear is almost always a 1 to 1 gear ratio.  The L-20-B will work nicely with the overdrive fifth. 

[Perez620]

Yeah I mean rear end sorry. Wasn’t sure about gearing though. I know a lot of guys say they prefer the close geared transmissions

[datzenmike]

The '74 through '78 manual transmissions had the same 4.375 differential gears so no change.

 

Your '74 will have a wide ratio transmission with a 3.592 first gear. Which is good for the smaller L18 it came with. (I know you said L20B) 

 

There were three gear sets in the 5 speeds, wide, mid and close. The first gear take off ratios were 3.592, 3.321 and 3.062. My opinion on this is, if using this to haul heavy loads find a wide ratio for easier starts even on an incline. It is a little buzzy in first around town with a first to second shift  just before the middle of the intersection you are crossing. If just transportation, get the mid range. It's a good all round gear set with a little more over drive for mileage. The close ratio is for a six cylinder car with more torque but it will work. It won't be as easy to take off with and the over drive is twice as high as the mid range transmission, meaning it won't get much use except out on the freeway. You won't be disappointed with the mid range gears.

 

L series FS5W71B transmissions that will literally 'drop in' to a '74 and up 620...

 

'77-'78 280z...................... Mid

'77-'78 620........................ Mid

'79 620.............................. Wide (optional Mid)

'78-'80 810........................ Mid

'79 280zx.......................... Mid

'80-'83 non turbo 280zx.... Close

'80 2wd 720...................... Wide

'81-'84 Maxima................. Mid

 

All these will work ok in the 620 but the Mid range is probably the better over all best choice if you can get one.

 

I think a rebuild kit is $100- $150? Labor is the killer when rebuilding and finding someone who knows what he's doing on a transmission that hasn't been around for 40 years and isn't full of shit. A new rebuilt one with a guarantee is $400- $600 I would guess. There are companies that specialize on rebuilt transmissions.

[Perez620]

Thanks Mike for the helpful info! In your opinion would you swap out my third member at all if I end up putting in a mid range trans? Didn’t know if it’d be a more comfortable drive. The trucks going to mainly be a cruiser but will haul light loads every once in awhile

 

Also, do you have any sources on a rebuild kit for a drop-in 5 speed? I honestly haven’t even looked into it yet just getting info about it this far

[datzenmike]

If changing to a mid ratio this will feel like a 4.025 differential when driving away from a stop, but 4th will be the same and you still have the over drive. If you change the differential to 4.11 or 3.889 or lower, it will affect ALL gears. Depends what you want. Keep in mind that if too low you may not be able to use 5th driving into the wind or on some slight hills. I think 3.889 has been used successfully for lowering the RPMs out on the highway. The problem is the acceleration in town will be sluggish.

 

No idea on the build kits. Google I guess.

[Perez620]

Ok makes sense. I’m currently rebuilding an L20B in place of my L18. The L18 actually had 3spd auto trans on it. Does it have the same 4.375 gears as the manual trans setup or are those different?

[datzenmike]

Ah, that's much different. The title says 4 into 5 speed. The '74 L18 with automatic had a 4.625 differential. You would definitely want something closer to 4 with the higher torque L20B. .

 

'73-'78 manuals......... 4.375

'79 manuals............... 4.11

'73-'74 automatics..... 4.625

'75-'79 automatics..... 4.375

[Perez620]

Yeah my apologies. I was just trying to get some quick info based off of the L20B with the swap. I’ll definitely have to swap a third member soon then. I’ll try and pick up a 4.375 rear

[Perez620]

Also Mike, what’s a good way to determine what gear ratio a rear end is? Any specific stampings on the third member to look for speaking of a 4.375 rear?

[flatcat19]

Oh, mike...explain this optional mid gear trans that came in the 79 620.

 

Does this option also come in trucks that had the optional 4.375 rear gear?

[datzenmike]

Well once out look on the outer edge of the ring gear for numbers and divide them.

 

37/8 would be 4.625

35/8 would be 4.375

37/9 would be 4.11

35/9 would be 3.889

 

I think you can raise one wheel an turn it with the transmission in neutral. Mark the tire and the driveshaft with tape or chalk. Turn the tire exactly 2 turns while counting the driveshaft turns. This is very rough and has to be done carefully. If you have the time turn it 20 turns and count the driveshaft turns. Now just divide by 10 for a more accurate ratio.

[datzenmike]

 

 

Oh, mike...explain this optional mid gear trans that came in the 79 620.

Does this option also come in trucks that had the optional 4.375 rear gear?

 

 

 

I think the '79 transmissions were all the same (wide ratio with 3.592 first) but there was an optional 4.375 differential over the regular 4.11. This would combine the two lower ratios (3.592 and 4.375)  for much more torque. Would be great for a camper or farm truck. Does that sticker you have say different?

[flatcat19]

It only says 4.375. The other VIN tag under hood is beyond any recognition.

 

I'll plan on pulling trans this weekend.

[kelowg]

3.89 gears .075 od on 16's with stock l20b works great for my truck. 18-22 mpg at 70-80 mph at 3300-3400 rpms

[datzenmike]

It only says 4.375. The other VIN tag under hood is beyond any recognition.

 

I'll plan on pulling trans this weekend.

 

You can easily check by placing in first and turning 20 times while counting the output spline turns. 20 in and 5.5 out is a wide ratio. At least this is what I expect.

 

 

 

The 620 engine tags only showed the engine displacement. It wasn't until the 720 that the trans and axle ratios were stamped.

[Perez620]

3.89 gears .075 od on 16's with stock l20b works great for my truck. 18-22 mpg at 70-80 mph at 3300-3400 rpms

How is it shifting through the gears while driving? Any gear in particular that is bothersome? That mph at those rpms would be perfect

[Perez620]

Well once out look on the outer edge of the ring gear for numbers and divide them.

 

37/8 would be 4.625

35/8 would be 4.375

37/9 would be 4.11

35/9 would be 3.889

 

I think you can raise one wheel an turn it with the transmission in neutral. Mark the tire and the driveshaft with tape or chalk. Turn the tire exactly 2 turns while counting the driveshaft turns. This is very rough and has to be done carefully. If you have the time turn it 20 turns and count the driveshaft turns. Now just divide by 10 for a more accurate ratio.

That’s what I figured unfortunately. Was just wondering in the case of purchasing a rear from someone. Guess I’ll just have to go by year if possible

[datzenmike]

Well if buying a differential you won't want the housing and axles anyway. The differential is self contained and just unbolts so it would be easy for a seller to read the numbers.

 

[Perez620]

Makes sense. I’ll see what I can come up with. By the way when I got to the shop this morning I performed the driveshaft to tire rotation count and I did get 4.625 on the dot. It was 46.25 rotations of the driveshaft to 20 rotations of the rear tire. Great method for finding the rear ratio

[datzenmike]

:thumbup:

 

Just so you know an LSD would require both wheels off the ground.