GMC Motorhome Steering Box
Presented at GMC Motorhomes International Fall 2006 Convention 9/25/06
THE GMC MOTORHOME STEERING BOX
By Don Wirth
Theory of operation
The Saginaw power steering box is a hydraulically assisted mechanical rack and pinion
steering system different only in that the rack drives the pinion rather than normal where the
pinion drives the rack. The rack is also the hydraulic
piston that moves the steering geometry. It is
mechanically driven by a 7/16” pitch lead ball screw.
This ball screw is mechanically driven by the steering
wheel. Between the steering wheel and the ball screw
is a complex hydraulic valve with many ports for both
left and right movements. When turning left or right
the hydraulic oil is directed to the piston (rack) and at
Torsion bar from inside the stub
shaft
the same time a small torsion bar inside the stub shaft is
winding up. As the steering wheel is relaxed the
torsion bar unwinds and closes the hydraulic valve to relieve the force on the piston and the
box will hold in the curve position until you move the steering wheel to a new curve or
straight ahead position. The valve ports are held in a new port closed location when in a turn.
All boxes have this torsion bar.
For additional, and more detailed, information on the operation of the steering box refer to
the service manual.
Identifying the “proper” steering box
The power steering box used in the GMC motorhomes was built by
Saginaw and was used on most General Motors vehicles in the
decades of the 60’s, 70’s and 80’s as well as some Ford and Chrysler
vehicles.
There are several versions of this gearbox. For clarity, I will address
only those that are usable in the GMC Motor home. In the 60’s,
70’s, and 80’s there are only two castings that fit our requirements.
The truck box that I will call left
hand is mounted on the outside
of the frame. The car box is
right hand and is mounted on the
GMC motorhome
inside of the frame. Although
steering box
they look very similar they are
left and right castings and the car
box casting cannot be used in the GMC motorhome.
However all the inside components of the car box are the
same as the truck and can be used.
Motorhome end plug with
extended boss and spacer to
limit movement
All trucks 1/2-3/4-1&1-1/2 ton GMCs also several Dodge and a few Ford trucks.
All full size GM cars.
Trucks 3/4 and above have no internal added stops and
yield 4-1/4 turns lock to lock. When these same boxes are
used in the 1/2 Ton truck and our GMC a spacer is inserted
at the top of the stroke and the bottom aluminum pug has a
extended boss. These two items restrict the movement to
3-1/3 turns lock to lock as in our GMC’s. Also, truck
boxes may have a stub on the input shaft. No problem, just
cut it off.
Stub on the input shaft on some
truck boxes. Just cut it off.
Some muscle cars in the late 70s offered 2-1/2 turns lock
to lock. These car parts can be used in our MH. However,
It gives stiffer steering and more effort to park.
This change in steering ratio is done by
changing the pitch of the lead on the ball screw
from 7/16” to 9/16”. I will cover these
mechanics later.
Metric components started in 1984. While these
late model steering boxes will bolt into the
chassis, you need to change the way the
power steering fluid connections are made.
Adapters to convert the metric fluid connection
lines to earlier standard fittings are available.
The newer boxes use an o-ringed metric end
while the older boxes use the common 3/8" flare
end. NAPA sells the required adapters. They
are:
The Lead screw on the top was used for
quick turning muscle cars. The 7/16”
pitch lead screw on the bottom is what is
used on trucks and GMC motorhomes.
Weatherhead #1445: 3/8" is 5/8 18 - 14x1.5
Weatherhead #1446: 3/8" is 5/8 18 - 16x1.5
Weatherhead #1447: 3/8" is 5/8 18 - 18x1.5
An alternative and simple way of dealing with the metric hoses if you get the box in a
wrecking yard is to keep about six inches of the metric power steering hoses with the box.
On installation cut off six inches of your original hose and then connect your original hose to
the metric hose with a tube coupling. The tube diameters are close enough to do this. I use
Swage-lock fittings and it will pull down just fine.
This information about metric threads applies to the late model steering gear like that being
sold by Cinnabar.
In Summary, To find the correct box
Any GMC truck 1960-1980. If you are away from home the big truck box can be used as is;
just be careful in a tight turn to stop turning when you reach the suspension stops.
The 3/4 ton and above, to conform 100% to the MH, will need the internal spacer
from your box installed and your extended boss bottom aluminum plug used in place of the
truck plug.
If you just wish to rebuild your present unit you can
use internal components from any full size GM car
1960-1980.
Motorhome end plug on the left, truck
end plug on the right
Using a ¾” 12 point socket to
identify proper input shaft for the
GMC motorhome
However, the
input shaft
must be a 3/4" 24point spline (use a
3/4" 12-point
socket to
confirm). And the Correct output shaft with tapered
spline, lock washer and nut
pitman shaft
(output shaft) must have a tapered splined end with lock
washer and nut as used on our GMC’s.
Most trucks and cars will have these features.
Removing the steering box from your GMC
Remove and cap the power steering hoses, remove the bolt securing the steering shaft to the
input shaft, remove either the pitman arm from the output shaft or the drag link from the
pitman arm. I recommend that the pitman arm be separated from the drag link rather than
from the steering box. Remove the four bolts holding the box to the frame. Don’t drop it on
yourself, it’s heavier than it looks.
Disassembly
The valve cartridge is a removable part but has very limited repair possibilities. It can be
removed by taking off the lock nut and with a
spanner wrench backing the cartridge out of the
steering box. The removable parts are the roller
thrust bearing in the bottom, the top bearing,
and the seal at the top. Earlier models also had a
dust shield, which is now not available except in Powercraft 7095 upper seal kit
a seal kit. Powercraft has a kit for these parts
Part # 7095. Seals are also available at bearing houses.
Normally the input shaft seal will be the only part needed. The seal and the bearing at the
output pitman shaft can be replaced, however the movement of the output shaft is so small
they hold up forever. Bottom line, for normal rework and setting up a gearbox only the top
seal needs to be replaced.
If you want to just replace the input shaft seal this can be done with the box still on the
coach. Remove the left front wheel and inner wheelhouse, the steering shaft at the input
shaft, the seal retaining snap ring, and the dust cover if used. This will expose the top of the
seal, which can be pierced and pulled out with probe tools or a small slide hammer.
If you do remove the rack piston and the balls fall out you will find that there are black &
silver balls, which are .0005” different in diameter. When reinstalling the balls, alternate the
colors. 18 balls will go into the rack piston; the other 6 go into the return guide. Hold the
balls in place in the return guide with Vaseline – do not use heavier grease.
Steering box adjustment
The only way to properly adjust the steering box is to remove it from the vehicle.
Tools required: Inch pound torque wrench capable of readings under 20 inch pounds,
Spanner wrench. 5/8” end wrench to loosen the pitman shaft adjusting screw lock nut, 3/16”
Allen wrench to turn the pitman shaft adjusting screw
The setting up of the gearbox is quite simple using an inch pound torque wrench. Note:
Most click type inch pound torque wrenches only work above 20 in/lbs – this is too high for
our purposes.
1) Adjust the valve cartridge preload.
The pitman shaft adjustment screw and lock nut must
be completely backed off by loosening the retaining
nut and turning the Allen screw counterclockwise a
few turns.
Pump the power steering fluid from the inside of the
box by moving the pitman arm back and forth.
Using an inch-pound torque
wrench to measure preload
Loosen the adjuster plug locknut and, using a spanner
wrench, screw down the adjuster plug cartridge until
you get about four (4) in/lbs of preload measured at the input shaft. Then lock it down with
the lock nut.
2) Find the center of the movement by turning the input shaft halfway through the total
movement. To verify, when centered the plane of the machined surface of the side
adjustment plate will be parallel to, and in the same plane as, the flat spot on the input shaft.
3) With the in/lb torque wrench turning the input shaft a few degrees from center in both
directions turn the pitman shaft Allen adjustment screw until you read another 10 in/lbs to
see a total of 14 in/lbs at top dead center. These numbers are nominal and could be slightly
more or less.
The alignment of these two machined surfaces is
mandatory (top dead center), and must be held when
installing the box on the vehicle. This is important
because the 10 in/lbs tight spot is only in contact for
about 8 to 10 degrees of steering wheel movement in
ether direction. If this true center is not maintained,
you will have magnified movement through the
steering system due to the machined clearance of the
teeth of the rack and pinion without preload. Having
the box on center and the wheels straight ahead is
Adjustable pitman arm
sometime
difficult to do in
an old system
unless you have
an after market
adjustable
pitman arm.
Alignment of the two machined
surfaces at top dead center
Summary
Well, there you have it. The steering box should no longer be a mystery and great steering
should be yours soon. Just find someone to do a good 6-wheel alignment for you. Make
SURE they keep the steering box centered!
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