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620 driveline

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I recently raised my 77 620 pickup to stock height and put bigger tires on it.  Now when I'm driving at about 40 mph I get a loud hmmm and as the speed increases the hmm gets louder and only when I'm pushing the gas pedal.  Also at around 55 or 60 I get a shake. I'm pretty sure it's the driveline and the angle of it being incorrect. Anybody know how I can fix this without going to a driveline shop?

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 Get a drive line angle gauge.




Measure the angle between the front and rear driveshafts.... then the rear drive shaft to the differential. You tube has how to do it right. Both angles must be the same or as close as possible. I think you can buy shims to slip under the leaf spring to adjust.


Drive shafts spin very fast 4.375 times faster than the tire. It'll be more of a loud hummmm. Actual shaking is probably a tire out of balance, bent rim, tire damage, mud on rim, out of round (bulge)




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Should have asked how or did you raise the rear?  remove lowering blocks?



The bolt that holds the spring pack together has a round head and this fits up into the hole in the spring perch on the axle above it.


My 710 HUMMED after lowering it. I measured the angles and concluded the differential needed to tilt down slightly at the drive shaft. Tried a 1/8" thick spacer at the rear of the perch would fix it and it did. Lucky on the first try.


Vehicle must be sitting on it's wheels. If you can, get your g/f to sit in the driver's seat as ballast while measuring.

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Well do the measuring as see if this might be the cause. Rent or borrow the gauge. I used a plastic compass and a string with a washer on it.

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Can't get my head around those numbers. Loosen the U bolts on the leaf springs and slip a washer on the back side between top of spring pack and the spring perch on the axle and snug up the U bolts. This will tilt the front of the differential downward. Measure and see if improved or worse. If worse switch shim to the front of the axle.

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No idea. Try and see. Change as needed. Changing the rear driveshaft to differential angle will also change the front angle slightly

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Ad says it will correct your pinion angle but in reality it will probably only help to correct it. The proof is when the angles are identical or very close and why we measure them to confirm.

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On 3/27/2021 at 12:46 PM, Aron 77 said:

I bought some 2.5 degree shims is that too much to start with

try it ideally front and rear angles should be identical 

2.6 to 12.7 is a 10 degree difference so 5 degrees is half and should get you pretty close 

all depends on where the shims are placed 

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Watch a YouTube video on measuring driveshaft angles. Then hit the spicer angle calculator and punch in your numbers. Then find the shim you need and away you go.

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Angle should be close to 3 degrees. Just read that over the weekend from one of the hot rod magazines. Will try to find which one.

Shims available from Summit racing also.


Chopper Jim

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4 hours ago, Chopper Jim said:



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Chopper Jim

only in Arizona LOL

what is this long LINK

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