Rear diff

[Crytek89]

Hey Guys!

I'm still kicking, as is the ol' 720. Thus far the KA has proved to be fun but at this point I'm looking for some better gearing in the rear end to help it out a bit.

 

At this point I'm not honestly sure what rear end i'm running (I'm sure the VIN plates show though). Is there any stamps on the rear diff itself to identify what gearing it is or does the cover have to be removed and teeth counted or just check VIN plates?

 

Also, I know not classified section, but anyone know the typical going rate for a diff/best option for the KA?

[datzenmike]

Engine plate tag on inner passenger side fender under the hood hings. Bottom line will read

 

ENG/TRANS....... FS5W71B...................HL38  (assuming you have an H-190)

(may not be 38 which is 3.889 gear, may be 37 or 3.70 or 41 for a 4.11.... you get the idea.)

 

This is what was installed at the factory. It's too much work but if you remove the 3rd member and look on the outer edge of the crown gear the ratio is stamped there. For example 37/9 or 37 divide by 9 is 4.11

 

 

You don't say if you want more top end speed better mileage... or more acceleration.

[Crytek89]

Gotcha, makes sense.

Thanks!

Probably more top-end speed. Acceleration is really decent but doesn't seem to want to get very fast haha

[datzenmike]

Well the Z24 has stump pulling bottom end but drops off at about highway speed. A KA should wake it up some. Better look and see what you have before you jump into this.

 

Some Z24s had a 3.545 which would be almost OK for cruise mileage but boring. A KA should make better use of this at higher speed but a 3.889 or 4.11 would wake it up.

 

My 710 came with a 3.70 diff and acceleration was hardly there above highway speed. I put a 4.11 in it and although this was supposedly counter productive to top speed and mileage I could still get to over 100 MPH and it pulls hard all the way. I don't even down shift to pass, just floor it.

[Crytek89]

Awesome, thanks for the tips Mike!

Always appreciate it.

[datzenmike]

Oh, forgot. Canby trip of 2,600 miles I got 30 US MPG average. One 6 gallon fill up was 35!

[Charlie69]

I found a 3.545  in a 84 720 regular cab Z24 5 speed, a 3.70 in a 85 720 regular cab Z24 5 speed, a 3.889 in a 1980 720 regular cab L20B 5 speed, and a 4.11 in a 86 720 4x4 King Cab Z24 5 speed, all of these trucks are short beds and are in the same wrecking yard.

 

Shawn got the 3.545 last week.  I am going back next Saturday and pull the 4.11and the 3.889.  I will get the 3.70 the Saturday after as that is the newest truck placed.

 

3 out of 4 of these trucks are front end collisons.

Edited July 9, 2018 by Charlie69

[datzenmike]

Beware the 4x4 diff. The U joint size changed around '83? and no the flange won't swap the spline is different. Just check first is all.

 

The '83-'86 Mileage Option 720 with the Z290 engine had a 3.364 ratio. Insane!-

[tr8er]

12 minutes ago, datzenmike said:

Beware the 4x4 diff. The U joint size changed around '83? and no the flange won't swap the spline is different. Just check first is all.

 

The '83-'86 Mileage Option 720 with the Z290 engine had a 3.364 ratio. Insane!-

3.36!  Good to know.

[Stoffregen Motorsports]

12 hours ago, datzenmike said:

Beware the 4x4 diff. The U joint size changed around '83? and no the flange won't swap the spline is different. Just check first is all.

 

The '83-'86 Mileage Option 720 with the Z290 engine had a 3.364 ratio. Insane!-

If the register is the same diameter, you can drill new bolt holes. I do this all the time here in my shop. It's best to use a transfer punch and drill press, or have someone custom machine the bolt pattern on a mill.

 

Being flexible opens up all sorts of doors.