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STEERING COLUMN BUSHINGS Revisited - SR-102A

At 05:29 PM 6/10/04 -0700, Grant, Michael wrote:
>" We are going to need some assistance developing standards for these bushings.
"260-160, 260-170, 260-190, Bush, felt_steering_MGA_too thick"


[Large part of response clipped, but pertinent info follows.]

I have in hand one of each 260-160 and 260-170. I do not have 160-190 (non-adjustable column top bushing). Measurement gives this:
260-160 --- 1-5/16" x 3-3/16" x 0.156" thick - Lower
260-170 --- 1-13/16" x 3-3/4" x 0.170" thick - Upper - [This one is too thick]

It has been nearly a year since I wrote about this issue on my web site, and mentioned that I need to install these parts. I suppose 17-1/2 years and 189,000 miles is enough for one set of felt bushings, as they are not too difficult to replace. With this little reminder, maybe today would be a good time to do it. If I manage to dress the felt parts to suitable thickness to make a snug fitting assembly (that works), then I might disassemble it and "try" to measure the finished thickness of the parts.

At 05:02 AM 6/12/04 -0700, Barney Gaylord continues:

I just R&R'd the adjustable steering column on my MGA to install the felt bushings. Only 3 bolts, and easy to handle on the bench. If I had known in advance that the bottom bushing was a good fit without any tinkering, I might have been tempted to do it without removing the tube. In fact it should be easy enough to do in situ, but may need to remove the steering wheel to clear the seat back when pulling the shaft.

I bought 260-160 and 260-170 directly from Moss Motors in early July 2003. See prior memo for the measured dimensions of the bushings. Measuring the tube and shaft I get these numbers:

Tube bottom end ID=1.154" and has the end curled inward slightly to prevent the bushing from falling out. Tube wall thickness is 0.048, so OD is 1.250". Shaft bottom end OD = 0.810". That makes the radial space 0.172". That bushing is only 0.156" thick, so it leaves about 1/32" diametrical clearance. The shaft can and does wobble around that much at the bottom end, but likely no one should care, as you would never notice that small lateral motion at the steering wheel. The 260-160 bushing is otherwise a perfect fit for width and length, so all is well as is. That's a nice improvement over the too-thick part I had to deal with 18 years ago. If you had a chance to specify a slightly thicker (maybe a non-standard material thickness) felt part, I would think that 11/64" (0.172") would be a better fit.

Tube top end ID = 1.314". Shaft top end OD = 1.090". That makes the radial space 0.112". My 260-170 bushing was too thick at 0.170" thick, and no way to assemble it even with great force on the shaft. It might be good at about 0.120" thickness, assuming we like a slight interference fit for snug operation at initial assembly. I will guess that 0.125" thick might allow it to be assembled but would likely leave it too tight for acceptable ease of turning with fingers on the steering wheel. But it wouldn't hurt to try 1/8" thick material if it's a standard thickness of felt stock. Maybe that was the factory's original intention.

So I pulled the same fix I used 18 years ago. I stapled the felt piece down on the workbench and touched off the thickness very gently with a wire wheel in electric drill. It only took a few minutes and one interim stop for a thickness measurement. I think I ended up with one edge a little thin at about 0.110", but at least half of the part was closer to 0.125" thick (or maybe not quite 0.125").

In assembly the shaft went in with a moderate push and some wrist action on the steering wheel hub. The wheel turns easily with one finger on the spoke near the rim, and there is no detectable radial clearance (like a nice small preload). The 260-170 bushing is otherwise a perfect fit for width and length. If you have a chance to specify a thickness tolerance, 0.125"/0.115" should be appropriate. As tradition stands this day and age, material thicknesses are often on the small side of tolerance to reduce cost at the mill, so you might expect nominal 1/8" felt material to actually be 0.120" +/- a bit anyway.

I know Rick deOlazarra mentioned in a prior memo that his 260-170 part seemed to fit okay (except for a smidge on the length). Perhaps the thickness has been changed (corrected) from 0.170 to 0.120 sometime in the past year since I bought my parts. If so, that part may also be okay as is.

For the non-adjustable column, it seems both shaft and tube housing are equal diameter for the whole length, so the non-adjustable top bushing will be similar to the bottom bushing (except for the width).

Addendum, April 25, 2013:
Just a quick note. Nine years and 60,000 miles down, and the felt bushings with graphite oil are doing just fine, still generally snug, no reason to tamper with them yet.

Addendum, September 3, 2013:
Following is a chart of the felt bushes with their part numbers and design sizes, and a drawing illustrating the dimensions. Explanation of the design is below.
Part No.
BMC
AHH6027AHH60327H6255
Part No. Moss USA260-160260-170160-190
DescriptionFELT BUSH, lowerFELT BUSH, upper - adjustable columnFELT BUSH, upper - non-adjustable column
Size Imperial1-5/16" x 3-1/8" x 0.170" thick1-13/16" x 3-13/16" x 0.110" thick1-5/8" x 3-1/8" x
0.170" thick
Size Metric33.3mm x 79.4mm x 4.32mm thick46.0mm x 96.8mm x 2.80mm thick41.3mm x 79.4mm x 4.32mm thick
Design theory:
A thin felt strip is quite flexible and can stretched and compressed within certain limits. It is useless to attempt to apply formula for bending allowance to determine length of the strip (as would be done with sheet metal parts). If you stretch the part slightly it gets thinner. If you compress the length (or width) slightly it gets thicker. When the amount of deformation is fairly small the part acts rather like liquid, conforming to shape of the space allowed. If the volume allowed is smaller than the felt piece, then the felt can be compressed slightly, forcing out some air to reduce the volume.

If the felt strip is thicker than the space allowed it will be compressed to be thinner. In doing this the felt acts rather like a spring, because it has some resiliency. The more it is compressed the more force is found at the surface. Force at the surface results in friction on the rotating shaft, especially at initial installation. Friction in the steering is bad, so we try to avoid it as much as possible. Allowing for a lot of tolerances, thickness of the felt strip will be (nominally) about 0.002 thinner than the radial space allowed in the bearing journal.

To measure thickness of a felt pad, put it between two blocks, press gently together with your fingers, and measure space between the blocks with a caliper.

Length of the felt strip will be almost equal to circumference of the centerline of the installation space, allowing for a small clearance around the shaft. Since the felt acts somewhat liquid in a confined space, it is convenient to calculate volume of the space to be filled, and divide that by cross section of the felt strip to find the nominal length. When the width of the strip is same as the width of the retaining space, you can consider cross sectional area of the space and ignore the width. Then divide cross sectional area by strip thickness to determine nominal length of the strip. Calculating (ID+OD)/2xPI will be very close, as long as the strip is not being compressed.

When the felt is installed you can saturate it with graphite oil. This is likely to expand the felt a little to be interference fit. When assembled (with a bit of force), resulting compression force will squeeze some of the liquid out of the felt, and the felt will conform to the space allowed. When well lubricated, a little bit of initial preload could be good to have zero clearance in assembly (no radial play in the joint). Too much interference and compression on the felt will cause uncomfortable drag in the steering gear. If it only needs a small adjustment, then trimming a bit from the length of the felt strip and stretching it may suffice to make it thin enough. Due to the fiber construction of felt, it can have significant tensile strength, so it may be impractical to stretch it and thin it out more than about 5%. If the felt part is more than 5% too thick, you may have to resort to buffing the surface to remove some fiber material.

Addendum, February 15, 2016:
Nine months after the fact, On May 8, 2015 I replaced the upper felt bushing in a friend's MGA in Encinitas, California. I am happy to report that the Moss part was a perfect fit, so it appears that Moss Motors has it correct for both upper and lower felt bushings.

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