Understanding the ZF box?

[Chip Flor]

Hello all,

Last year I finally put together my '56 356 A coupe. After I was done with the car, when driving it moved around on the freeway on its own volition.
With this I sent the car to a reputable shop that specialized in 356's and other Porsches to sort this out.

Anyway $1800.00 later the car still swerves on the freeway and is kind of scary to drive.
This week I took a look at the front suspension.
1) the steering damper bolt to the pitman arm was loose.
2) there was about a 1/64" play at the pitman arm (dead center straight)

Ok....ZF Box is bad. I thought I would take out the transplanted ZF box and get it rebuilt. But before I removed the box, I did notice that in straight ahead position the marks on the ZF box were not even close to lined up. They were about 40-60 degrees off from dead center(i.e. marks lined up) (SEE PICS).
In other words, when the marks were line up (dead straight) there was zero play on the pitman arm (before rebuild). With about 40-60 degrees set on the box as straight ahead there was a little play.
Also, the further you rotated from center the play increased. So not having the marks lined up this explains why I was driving with play at the pitman arm in the straight ahead postion on the steering wheel.
Had this been aligned with the ZF marks aligned there would have been zero play driving dead straight.

Got on the phone with the ZF rebuild guy. He told me after a rebuild there was ZERO play in his boxes when he sends them back (anywhere in the rotation).
Great $800.00 to get my box perfect. (zero play expected)
Also to get the car running straight with the ZF mark lined up correctly should cure my problem.

I get my box back... I can feel a discernible lump when turning past the marks on the center line. This was not how it was adjusted before. Before you could barely feel this lump but not nearly like this. I called the builder he told me that was normal.
OK... I get the rebuilt box back in the car lump and all, and at 40- 60 degrees past center there is a slight play (better than before, but clearly this was a result of adjusting the peg deeper) and when you go all the way to one side there is still a significant amount of play.
I was assured there would be "zero Play" . wrong.

So Please will someone tell me how a ZF box is supposed to behave?
zero play (with lumpy feel) at dead center.
minimal play at 40- 60 degrees turned?
and a 1/64" to 1/32" play when all the way turned?

Is this how these ZF boxes work?
(with zero play at dead center with built in play the farther you get from center.)

Any help UNDERSTANDING how a typical ZF box is supposed to work, would be greatly appreciated.
I would hate to think that all I needed was the box readjusted to create this lumpy feel with the same play from center as before.

FYI The builder uses 140 wt EP (Extreme Pressure) Sulfur laden---soft metal corroding (i.e. bronze bushings) gear lube.

[Paul Lima]

I believe Porsche calls out 90 weight oil for the ZF box, and that has worked well for me. I can add that a good, well set up ZF box (including specifically lining up the mark and setting tie rods for straight ahead) is very smooth, no noticeable play in normal driving. BTW, this is how I aligned my Roadster: https://www.smartracingproducts.com/smartstrings.html

[Chip Flor]

Thanks for the reply....
The alignment tools are way better than me kicking my jack stands holding a string.

Should be noted..... At the steering wheel you can NOT feel the lump (center) You can only feel it when turning the box by hand.
How do I know this?.... Went to a lot of back and forth getting the steering wheel centered with the ZF box marks.

[Geoff Fleming]

The ZF steering box provides a beautiful, smooth input to the driver in all phases of turning, straight ahead, etc. That is, a properly set box along with correct toe-in of the front wheels and having the steering 'centered'. Absolutely NO binding or lumpy feel in any part of the turn, either left or right.
If the box is in good condition, filled with 90wt hypoid oil, ( same as the transmission), and the steering wheel is set in the straight ahead position, you should be able to make 1 1/2 turns left and the same 1 1/2 right. In the straight ahead, the casting seam on the box should be lined up with a distinct cut mark on the steering shaft. The steering wheel should be placed on the shaft ONLY when these marks are aligned and the front wheels in the straight-ahead position.
Now the toe-in should be set...1/8" toe-in, ( never out), is fine. All four bolts on the clamp that holds the box in place must be tight as should be the steering damper and the two tie-rod end attachment points.
If the link and king pins are in reasonable shape, there should be no problems and the car should track perfectly on a level surface.
If the steering wheel maintains a slight pitch to, say, the right when you are steering straight, then the adjustment is made at the tie-rods. NEVER remove the steering wheel to make a 'cosmetic' fix, as this will result in undoing the careful alignment of the box and destroy the centering setting. To correct the RIGHT pitch you have to carefully turn each tie rod so the front wheel turns MORE to the RIGHT. Do this on both sides, not just one. You have to maintain the toe-in. It may take many attempts before you achieve a nice, straight steering wheel position but it is well worth the effort. Mark each tie rod so you know where you started the work and can measure equal turns for both sides. I find adjusting the rods about 1/4" at a time usually works well. Remember...steering wheel favoring the RIGHT, turn both wheels to the RIGHT...if the steering wheel wants to stay a bit LEFT, both wheels must be turned to the LEFT, via the tie rods in either scenario.
When set up, the steering wheel will not feel stiff or radically loose. The 19mm adjusting nut on the steering box should be set so it does not cause binding in any position. It can be slackened a bit and the steering wheel should show about 10mm play side to side.
One other item to check...the Pitman arm, which attaches to the steering box, has an alignment mark struck onto the rim of the open section, where it aligns with a mark on the lower shaft of the box. There is no other position for the arm to be in. If it was improperly placed, you will never achieve even decent steering, so take a flashlight and examine this critical setting. Of course, check that the lower bolt that holds the arm to the box is secured and a cotter pin is in place.

[Chip Flor]

Thank you Geoff for responding.

-checked tire pressure 24 front 28 rear (tubed "X" Michelin 155 R15 radials)
-The pitman arm mark matches the ZF box.
-there is no lumpy feel at the steering wheel (only by hand)
-have marks on ZF box aligned as well as steering wheel centered.
-have been bouncing and rolling the car on my super flat garage floor.
-set two parallel pink serveying strings equal distance for the rear and front from the center of the wheels on jack stands.
-adjusted both sides several times to obtain a toe in on the tires as well as the wheels exactly 1/16" per side = 1/8" toe in overall.
-repeat rolling and bouncing the car several times back and forth and remeasured.
-same 1/8" toe-in overall for the front as well as the rear.

[Chip Flor]

Have to still ask.

Does your typical ZF box have increasing play when you turn past the center marks (40 to 60 degrees)?

What is the proper amount of adjustment for the lump (on center) you feel turning your box (by hand past the center marks)?

Is my box bad? with this play?

According to the builder this is absolutely normal. Has anyone else had this (lose play) experience at about 40-60 degrees past center (left or right from center)?

Again it is rock sold (no play) for those 80-120 degrees (overall from center 40 -60 degrees per side) on the steering wheel where most of your turning takes place.

[Jim Clement]

If the top is off it is easy to line up the marks.
the picture is a bit fuzzy, but you can see the marks on each side.
for the record, although I rebuilt mine with new bearings and seals, and although it did not leak, it still did not drive well.
I currently have a new TRW unit in my car, but want to have my old / original box mechanically refurbished, and reinstalled.
sorry for the large picture.. I do not know how to adjust the size.

[Chip Flor]

thanks for the pic Jim.
Also, if you take out the oil fill plug the peg lines up exactly on center to the hole.
This too is another way to double check your marks are aligned.

[Jim Clement]

oh.. that would be much easier..
if you had to have your unit rebuilt again, any suggestions as to who could rebuild an early steering box ?

[Vic Skirmants]

Do you have the correct ADJUSTABLE left tie rod? If not, there is no way to center your ZF box.

[Ron LaDow]

Have the still ask.
Does your typical ZF box have increasing play when you turn past the center marks (40 to 60 degrees)?
What is the proper amount of adjustment for the lump (on center) you feel turning your box (by hand past the center marks)?

Yes, the centered position has the least 'slack' and it increases off center. This is a result of the geometry of the tapered rolling pin in the tapered 'worm gear', and since, in use, the road wheels keep the pin loaded against one side while turning, there is no problem.
At the center, however, there should be no lump; merely the lack of play. Sounds like yours is a bit tight, which will cause wear.

[Chip Flor]

Yes I have the double adjustable links on both sides (long and short) Both tie rod links came with the ZF box.

I agree the lump was/is a concern this is why I called the builder immediately. He explained that if you could turn it with two fingers that is is not too tight. I too think it is too tight, however he loc-tited the adjusting screw with the same 547 case sealant by loctite. He also loctited the oil fill plug with the same loctite 547 case sealant.

So without heating up the adjusting screw it is cemented into place. I guess I have to live with this.

[Chip Flor]

Jack Staggs of San Clemente CA rebuilds these early boxes (949) 492-9606.
I have never done and business with him. He was one of the speakers at the 2019 North meets South tech session.
He is well respected from people the SoCal 356 community.

[Chip Flor]

Ron thank you for pointing out the play increases and that I'm OK. It makes sense because from what I can see the peg rolls on one side of the worm gear (going in one direction and then rolls on the other side going the opposite direction).
If there was no clearance the wheel would hit on both sides and scrape its way around the worm gear. My guess this is why they are silky smooth rolling along the worm.

[Chip Flor]

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