Z height position?

evm · September 4, 2021

I'm about to replace upper control arms on my 1 ton 2wd c3500 crew cab. Alldata manual says to tighten the upper control arm bolts to 140lbs with control arm at z height position? Im confused and don't really know anything on Z height position or if this even applies to my vehicle? Can I just lower truck and tighten the nuts down?

newdude · September 4, 2021

From the service manual:

Installation Procedure

Notice: Refer to Fastener Notice in Cautions and Notices.

Important:

The pivot bolts must be installed in the proper direction.
Tighten the front nut prior to tightening the rear nut.
Tighten the nuts with the control arm at Z height. Refer to Trim Height Inspection in Suspension General Diagnosis.

Install the bushings.
Install the upper control arm to the frame.
Install the bolts and new nuts.
Tighten
Tighten the nuts to 190 Y (140 lb ft).
Install the upper ball joint to the steering knuckle. Refer to Upper Control Arm Ball Joint Replacement.
Install the brake hose bracket to the upper control arm.
Install the bracket screws and the nuts.
Tighten
Tighten the nuts to 18 Y (13 lb ft).
Install the brake hose to the bracket.
Install the air cleaner extension, if necessary.
Install the tire and wheel assembly. Refer to Wheel Installation in Tires and Wheels.
Check the front wheel alignment. Refer to Wheel Alignment Measurement in Wheel Alignment.

Trim Height Inspection

Z Height

Trim height is a predetermined measurement relating to vehicle ride height. Incorrect trim heights may cause symptoms similar to alignment problems. Check the trim heights when you diagnose steering or vibration complaints.

The Z dimension measurement determines the proper ride height for the front end. The Z dimension is different for RWD vehicles and S4WD or F4WD vehicles. The S4WD and F4WD vehicles have different points of measurement to determine Z height. Use the torsion bar adjusting arm in order to adjust Z height. RWD vehicles have no adjustment and could require replacement of suspension components.

On RWD vehicles, measure Z height from the center line of the bolt of the front lower control arm down to the lower inboard corner of the ball joint. Trim all Z heights at 89.0-101 mm (3.5-4.0 in).

Edited September 4, 2021 by newdude

newdude · September 4, 2021

Here is a picture of how to measure:

Edited September 4, 2021 by newdude

evm · September 5, 2021

Awesome pic! Thank you

scott2000 · April 5, 2023

1993 GMC Yukon SLE 4wd...

The manual says to make sure the z height is correct before torquing any ball joints or control arms as it relates to replacing. My truck z height wasn't right to start with and the torsion keys were adjusted for leveling after a body lift by the guy I bought it from. Angles were always pretty bad and the sway bar links always looked tweaked.

All my suspension parts are removed (lower ca stayed but I loosened nuts) and I'm putting everything back but am confused and nervous on how to start at the correct z height...

I put the torsion key where it is supposed to be from manual (34mm from bottom of retaining plate to top of bolt head) because when I tried jacking up the lower ca to get the z correct, it would lift the truck from the stands. Even with the torsion adjustment almost all the way out after setting it per the manual, it still takes a lot of force to get the lower control arms up to get the correct z but it's there. Just seems like a lot of tension and makes me nervous torquing everything down while it's like this and I just want to make sure I'm thinking right. Does this simulate the truck's load enough?

Doesn't seem right to put the wheels on and put the truck on the ground and trying to get to the ball joints to torque them. I guess the upper and lower ca would be easy enough.

Do I leave everything snug and put the truck on the ground, get the z correct, then torque or can I jack up the lower ca and get the z correct and torque everything like that?

Thanks!

Edited April 5, 2023 by scott2000

scott2000 · April 6, 2023

So with the truck lifted on stands, I jacked up the lower control arms to get the correct z height and torqued down the upper and lower ball joints since they were easy to get to with everything off. Then I put the axles and tires back on and put the truck on the ground and my z height was off approx 2" (z=4" instead of 6"). Guessing because my tires are 2.2" taller than factory, this is why.? I should have mentioned the tires earlier.

So I adjusted the torsion keys to get the height back to spec. Just wondering what torquing the ball joints at the incorrect height will do. May try to get at them and see if I can re-torque them. Guessing they actually need to be loosened but I can't picture it in my mind yet...

newdude · April 6, 2023

23 hours ago, scott2000 said:

So with the truck lifted on stands, I jacked up the lower control arms to get the correct z height and torqued down the upper and lower ball joints since they were easy to get to with everything off. Then I put the axles and tires back on and put the truck on the ground and my z height was off approx 2" (z=4" instead of 6"). Guessing because my tires are 2.2" taller than factory, this is why.? I should have mentioned the tires earlier.

So I adjusted the torsion keys to get the height back to spec. Just wondering what torquing the ball joints at the incorrect height will do. May try to get at them and see if I can re-torque them. Guessing they actually need to be loosened but I can't picture it in my mind yet...

Tires would make no difference. The control arms are still in the same position and angle. Those will change based on Z height change.

Are you sure you are measuring correctly? The pictures I posted almost 2 years ago above are a good reference. Grab a long level, bottom of the lower ball joint and then hold it level out to where you can then measure down from the LCA mounting bolt to the top of that level.

Also, if you have leveling keys in there that you don't know about (PO changed the stock for leveling keys for example), that will throw off all of this entirely as they won't likely be able to allow you to achieve the correct Z height at all.

As for ball joint torquing, the OE manual states to support the lower control arm so the arm is supported in the proper position for removing and installing the nut. It does not state to adjust your Z height and then proceed with the work.

Edited April 6, 2023 by newdude

scott2000 · April 7, 2023

18 hours ago, newdude said:

Tires would make no difference. The control arms are still in the same position and angle. Those will change based on Z height change

Thank you! So you're saying,if my Z height angle was correct when I measured while the control arm was jacked up, then even though when I put the wheels back on and the z height changed 2" (6" to 4") , that's just reflecting the different load? Sorry, I know I'm overthinking this.

18 hours ago, newdude said:

Are you sure you are measuring correctly? The pictures I posted almost 2 years ago above are a good reference. Grab a long level, bottom of the lower ball joint and then hold it level out to where you can then measure down from the LCA mounting bolt to the top of that level

Yes. I understand the measurment

18 hours ago, newdude said:

As for ball joint torquing, the OE manual states to support the lower control arm so the arm is supported in the proper position for removing and installing the nut. It does not state to adjust your Z height and then proceed with the work.

It's just in my Service manual (1993 Sierra,Yukon,Suburban Crew Cab and 3500 HD) it says nut must be tightened with the control arm at z Height. Same with the control arms themselves and some other items.

I should have mentioned earlier that I never checked the Z height before removing any suspension parts. I didn't learn about it until going to put things back. Obviously I didn't have any way to check when there were no parts so, I lowered the torsion keys to what was in the manual, left the upper and lower ca nuts loose, and jacked up the lower ca to get the z height as was in the manual and installed the upper and lower ball joints to both arms and the knuckle. I think this may have been a wrong way to go about it but not sure.

I then put the axles in and the tires back on, and put the truck on the ground. When I measured the z height, it was off by 2". So I adjusted the torsion keys to get the z height where it is supposed to be and torqued down the control arms while it was on the ground. I've since put everything else back in, shocks , tie rods, etc.

I noticed I now can't achieve z height with the truck on stands and jacking up the lower ca but, I can get enough of an angle to do anything necessary. Can get it to about 7" before it'll lift of the stands .

I don't know, lots of replacement instructions in the manual indicates releasing tension on the torsion bars as a first step so I'm a little nervous something isn't right but it seems to be going together nicely. I should be able to have everything finished up this weekend and go get it aligned.

I appreciate the insight and hopefully I haven't diluted the scenario with too many details.