Aftermarket rear lift blocks - factory shims in place or not?

lapoolboy · December 27, 2019

I want to get to the bottom of this once-and-for-all. Those of you who have aftermarket lifts installed (with rear blocks), please indicate if you or your installer left the factory shims in place between the spring perch and leaf spring (the shims will now be on TOP of the aftermarket lift block - see pic). My truck has less than 3000 miles on it and the ride was smooth as silk prior to installing the lift. There is now a very slight rumbling/vibrating at 55-65mph. ReadyLift says to remove the factory shim even though their instructions mention nothing of it and their own pictures in the instructions even show the shims left in place. I've looked at other manufacturer's instructions - SuperLift says to remove the shims. BDS, ProComp, etc. all specifically state to be sure NOT to remove the factory shims. No rhyme or reason - some use tapered blocks, some use flat blocks. My ReadyLift uses tapered blocks.

Now to complicate things further, a member here posted a pic of his 2020 and there are no factory shims installed on his truck.

I am not against removing the shims - I guess I just want confirmation that is the problem. You have to clamp the leafs together and remove the center pin bolt while holding it from spinning with ViseGrips. Never done it, but doesn't sound terrible. The angle of the driveshaft at the rear of the transmission is noticably "downward". The angle of the driveshaft at the rear differential appears to be pretty much in line with the pinion.

Edited December 28, 2019 by lapoolboy

Dunn · December 27, 2019

maybe a diff drop?..think most quality 4in lifts use em...

does seem weird to have tapered block and than use a shim to take "taper" out don't you think?....could have just made block flat in first place....no need to put an extra piece in there....just using common sense but idk

may be trial and error....shim is easy to remove it seems

Edited December 27, 2019 by Dunn

BlancoSilverado · December 27, 2019

Just like what Dunn said, i would personally take it to a diff shop/drive line shop and have them verify the drive line angles.

lapoolboy · December 27, 2019

11 minutes ago, Dunn said:

maybe a diff drop?..think most quality 4in lifts use em...

does seem weird to have tapered block and than use a shim to take "taper" out don't you think?....could have just made block flat in first place....no need to put an extra piece in there....just using common sense but idk

may be trial and error....shim is easy to remove it seems

"Diff drop" typically refers to the front diff.

Flatblack83 · December 27, 2019

Usually all tapered lift blocks when used the shims come out or it will be extra steep. The rule of thumb years ago was the pinion 2 degree off so when u step on the gas the driveline is right. If the blocks tapered I'd take em out. I put a 1.5 flat block in mine and its good... no vibes.

Think the rumbling might be the front end being too high with cvs too steep..?? With these trucks to be looks like 3 inches is it for the cv angles. I think even 3.5 is too much. The diff drop you really can't do, I'm pretty sure the member gets chopped like the 1st rough country 3 inch.

Edited December 27, 2019 by Flatblack83

lapoolboy · December 27, 2019

48 minutes ago, Flatblack83 said:

Usually all tapered lift blocks when used the shims come out or it will be extra steep. The rule of thumb years ago was the pinion 2 degree off so when u step on the gas the driveline is right. If the blocks tapered I'd take em out. I put a 1.5 flat block in mine and its good... no vibes.

Think the rumbling might be the front end being too high with cvs too steep..?? With these trucks to be looks like 3 inches is it for the cv angles. I think even 3.5 is too much. The diff drop you really can't do, I'm pretty sure the member gets chopped like the 1st rough country 3 inch.

The factory shims are installed with big end towards the front....so they are essentially cancelling out the taper of the blocks.

The vibration is definitely coming from the rear end. I can almost hear it. The front-end is smooth as silk....the steering wheel is nice and smooth too with no vibration. The cv's are longer on the T1's than on the K2's and the front diff is narrower, allowing more lift capability. There are several other members out there who have the 4" like me and report that their trucks are smooth so I don't know what's different about mine. All of them have the factory shims still installed too.

Snowcamo · December 27, 2019

All you need to do is get the pinion angle right. I would not continue to drive it like that. If it takes you removing the Shims so be it.

Sent from my LG-G710 using Tapatalk

Dunn · December 27, 2019

It's not like its longer axle on same truck (2018)..maybe becuz truck is wider or since diff is narrower like you say it needs longer axle, idk..."point a to point b" ...they put new CV axles in GM factory lift and it's only 2 in....I just hear peeps with vibration issues relate it to this alot....

If it were my truck I would take out wedges....makes no sense to have taper block and use wedge to take taper out and it's another piece that has "joint" so to speak

Mechanics are funny thing, everything effects everything it seems, leaving alot to "scratch their head wondering"

Start with easiest and work your way from there

Good luck hope it works out

Edited December 27, 2019 by Dunn

Flatblack83 · December 27, 2019

Take the blocks out, put it to stock and see what happens.

lapoolboy · December 27, 2019

5 minutes ago, Flatblack83 said:

Take the blocks out, put it to stock and see what happens.

Then the ass-end would be 3" lower than the front. I guess it "could" rule out the blocks, but the mis-match in height front-to-rear could induce other vibes. The shims are attached to the spring pack via the centering pin so I have to disassemble that to get the shims out.

Flatblack83 · December 27, 2019

Long post, trying to help...I only said to remove the blocks to be 100% sure that's your problem. I understand it would be higher upfront. I didnt say leave it like that...a little cali lean..lol

Is the driveline high or low when u look at it..? Measure the taper on the block, measure the taper on the shim. U either have too much angle or not enough...so..

If the taper is more on the block than the shim I would pull the shims out and put the tapered blocks in. If not I would get a flat set of blocks and try those with the stock shim. Or order a set of blocks from a company that said to remove em, or vise versa.

When you call these companies 8 out of 10 times they have no idea and just answer you with a guess so u go away. For instance I called icon to see what the coilovers were set at, she said 1 inch over stock. Few hours later I called and a guy answered, he put me on hold and came back with set at 3 inches over stock with a silverado lt. So...lol. he was 100% right when I installed them.

Edited December 27, 2019 by Flatblack83

lapoolboy · December 28, 2019

Ok, I just crawled under and measured. The ReadyLift blocks are 3" tall at the back and 2-3/4" tall at the front. The factory shim is 5/16" So, it is confirmed the angle in the ReadyLift block is cancelled by the thickness of the shim. (Even makes the block TALLER in the front by 1/16" with it in there!). The rear diff has an upward tilt and the angle with the driveshaft appears to be very close to 0 degrees. Removing the shims will give the diff even more upward tilt.

Edited December 28, 2019 by lapoolboy

lapoolboy · December 28, 2019

1 hour ago, Flatblack83 said:

If the taper is more on the block than the shim I would pull the shims out and put the tapered blocks in. If not I would get a flat set of blocks and try those with the stock shim. Or order a set of blocks from a company that said to remove em, or vise versa.

Thanks. After getting under there and measuring, this makes sense. If I leave the ReadyLift tapered blocks and remove the shims, the diff pinion will be angled upward. This definitely won't be "equal and opposite" the angle at transmission. I'm thinking I need to leave the shims and just put in some flat blocks. This will push the front of the diff down.

Edited December 28, 2019 by lapoolboy

lapoolboy · December 28, 2019

Removing the shims and leaving tapered blocks will shift front of diff upwards and create funky angle that would be opposite direction of last one I show in proposed setup.

Flatblack83 · December 28, 2019

Think about that way the diff moves when you step on the gas, you want the pinion to be 1-2 degrees lower so that when u step on the gas the pinion goes up. So that's how it should be close to that. They make lots of shims to fix this problem. Or are the blocks in the right direction..?? If it's too high now bring it down...

Edited December 28, 2019 by Flatblack83