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My 1971 521


d.p

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I don't think this will impact the gasket because it's well inside but definitely should have been cut off level with razor. If those metal sleeves are up tap them down or nick off with grinder. Cork gasket is fine although I have used a neoprene rubber matrix with cork granules in it.

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  • 2 weeks later...

Well new oil pan gasket and flattening those needle seals did the trick.  No a speck of oil from the pan.  And removing the cross member and the center link made it super easy. Why anyone would do it any differently with the engine in the truck is beyond me.  
 

I am wondering now what I can do about my rear end?  I would really like to get my RPMs down on the highway.  Are people swapping out diffs/rear ends on these fuckers? 

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Stock is 4.375 H-190

 

Other trucks like the 720 have H-190 in these ratios...

 

3.363

3.545

3.70*

3.889*

4.11

 

* these are probably what you want.

 

If you have 26.5" diameter tires and at 70 MPH  in 4th gear you are at about 3,880 RPM

 

3.889 diff......... 3,450 RPM

3.70.................. 3,283 RPM

 

Fifth gear would be even lower. At some point the lower RPMs lower the torque output of the L16 so that it seriously affects it's ability to maintain highway speeds specially into a wind or even gentle hills. It would affect an L20B less but, will affect it. In addition ALL gears will be more lethargic, you'll have to step down on the gas more and have less performance. If you have a 4 speed, a 5 speed over drive keeps the in town performance and lowers the highway RPMs.

 

At 30 MPH in town third gear is 1,900 and almost unusable for driving, only steady speed. You'll seldom get out of second.

 

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@datzenmike I have a cam'd and bored L20B, dual mikunis, a 5 speed out of a 79 280ZX and 5th gear on the freeway at what I think is 70 is right around 4-4.5k RPMs.  Hard to tell because my speedo is off but using GPS on my phone I am pretty sure that is in the ballpark.   When GPS says 70 my speedometer says a 100 lol.  

 

 

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Tire size also affects the RPM. Lets assume a slightly smaller tire and call it 4k @70.  If you have the tire size I can re compute for 3.889 and 3.70.

 

An L20B should have little problem with this.

 

The 5 speed likely has the 280zx speedometer pinion in it still. The '79 had a 3.364 and you have 4.375 or 30% different and 70 MPH + 30% is just over 90 MPH so it's in the ball park

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Yeah it does have the 280zx pinion in it cause I never changed it.  Should I have swapped pinions from my 4speed (long gone)?  Tires are 195/50/15s but again I want lower RPMS on the freeway so what is the easiest way to get that without having to drive slower?

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It should as it has the same 29 spline side gears.

 

Almost every year of 2 wheel drive 720 might have had the 3.889 gears. Look for the engine tag on the passenger side inner fender below the hood hinge for differential gearing

 

buF8TkE.jpg

 

Bottom line far right HF 38 is 3.889. 37 is 3.70 and 41 is 4.11 and so on.

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2 hours ago, d.p said:

@datzenmike I have a cam'd and bored L20B, dual mikunis, a 5 speed out of a 79 280ZX and 5th gear on the freeway at what I think is 70 is right around 4-4.5k RPMs.  Hard to tell because my speedo is off but using GPS on my phone I am pretty sure that is in the ballpark.   When GPS says 70 my speedometer says a 100 lol.  

 

 

4-4.5k really....

I'd get that with my 4spd....

In 5th doing 70 i am just over 3,000 rpm maybe 3500.... and I still have the 4.375 gears.... 

 

Does your timing light have a tachometer reading?

Maybe you could check the rpms at 3000 and see if the gauge and timing light read the same....

 

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1 minute ago, Crashtd420 said:

4-4.5k really....

I'd get that with my 4spd....

In 5th doing 70 i am just over 3,000 rpm maybe 3500.... and I still have the 4.375 gears.... 

 

Does your timing light have a tachometer reading?

Maybe you could check the rpms at 3000 and see if the gauge and timing light read the same....

 

 

Yeah, unless I am going faster than that 70 but it would be odd if I was.  Because I am not blowing by cars (55MPH speed limit) at 4-4.5K RPMs either so its not like I am going 80 or 90.  I will test it out in a little bit and report back as I have to run some errands.  

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9 minutes ago, d.p said:

 

Yeah, unless I am going faster than that 70 but it would be odd if I was.  Because I am not blowing by cars (55MPH speed limit) at 4-4.5K RPMs either so its not like I am going 80 or 90.  I will test it out in a little bit and report back as I have to run some errands.  

Only other thing I can think of is the difference between our transmissions.... 

I dont know if the 5th gear ratio is different or if it's only the lower gears when they talk about wide or close ratio...

I'm using an 85 720 transmission....

Forget to add we are running similar size tires too...

195/60-15.... 

Edited by Crashtd420
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Without a doubt 70mph @4k give or take.  I think my trans has the wide 5th but like you no idea what that correlates to.  
 

Also my throttle is so jerky in low gears. Like the gas peddle or the throttle cable gets stuck/hung up and when it lets go the truck will lurch forward.  I end up feathering the clutch to get it to smooth out but it’s annoying as hell.  It’s been doing this since I swapped all this over and for the life of me can’t figure out why.  

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3 hours ago, d.p said:

Would a 2WD from an 81 or 85 work? 

 

It might. You will have to check the engine tag.

 

There were three different displacement gas and two different diesel engine displacements, different tire size options plus auto or standard and regular and long box carrying capacities. There was also a mileage option with a 3.364 differential. The tag lists what was put in at the factory.

 

 

 

1 hour ago, d.p said:

Without a doubt 70mph @4k give or take.  I think my trans has the wide 5th but like you no idea what that correlates to.  
 

Also my throttle is so jerky in low gears. Like the gas peddle or the throttle cable gets stuck/hung up and when it lets go the truck will lurch forward.  I end up feathering the clutch to get it to smooth out but it’s annoying as hell.  It’s been doing this since I swapped all this over and for the life of me can’t figure out why.  

 

If you have the '79 zx then it has a 0.864 over drive 5th. 

 

4k @ 70 in 5th with 4.375 gears and a 26" diameter tire is hard to explain.

I get 3,400 RPMs

 

If you had 4.875 J13 520 gears then it's closer at 3,800RPMs

 

 

 

I believe your GPS but your tach may be wrong.

 

What tire size are you running. It won't change anything much  but will tweak it slightly.

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27 minutes ago, Crashtd420 said:

So at 70mph mike was dead on....

3400 rpm....

I was at 4000 rpm in 4th doing 70mph....

 


so what the fuck does that mean for me? Either my tach is wrong or I have some weird rear gearing?  

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On 11/6/2021 at 12:22 PM, d.p said:

wondering know if I should use a cork gasket, gasket maker or both?

Use a gasket sealant but only against the block (always to the machined surface, then clean & dry against the pan. Make sure the pan is nice & flat then torque to spec.

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12 hours ago, d.p said:

Either my tach is wrong or I have some weird rear gearing?  

Both can be checked, definitely start with these 2....

Tach is the easiest to verify with your timing light... make sure you rev the motor up to 4000 to compare....

 

Then check the rear end ratio hopefully Mike can tell you how to accomplish that.....

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On 11/19/2021 at 7:15 AM, datzenmike said:

Lift one rear wheel off ground, transmission in neutral. Turn tire 2 times and count the drive shaft turns = differential ratio

or other way around. mark the drive shaft as well as the tire.  turn the drive shaft one full turn and see how many times the wheel turns

 

if the tire turns around about 3.88 times........  

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