Repairing the Model T Ford Rear Spring

The suspension of any Model T Ford consists of a transverse leaf spring mounted above each of the axles. It is a pretty simple arrangement, which Henry Ford called his “three point” suspension system. This was superior to the common semi – elliptical setup used on most cars of the day because it allowed a Model T’s suspension to pivot significantly on the fore / aft axis, allowing the car to have both traction and control over very uneven terrain. Roads in the days of the Model T Ford were generally bad, unpaved, muddy, and full of deep ruts.

The rear leaf springs used on the Model T evolved over the years as most parts of the car did. Here is a brief (but not comprehensive) overview of the rear spring changes over the production run.

1909 – 1912

Eight  leaves (all body styles except runabout / torpedo / roadster)

Seven leaves – roadster, runabout, torpedo. End bushings either brass or bronze.

All springs were “taper leaf” design. This was a carry over from springs designed for horse drawn carriages. All but the main two leaves were ground both on a radius and to a graceful end for appearance sake.

Springs were clamped at each end with a bolt securing each the clamps. Several clamp designs were used depending on the spring manufacturer.

1913 – 1917

Springs continued in the same general design but were less carefully ground and somewhat less dainty appearing because they were not ground as much. End bushings were changed to less expensive and better wearing steel. In 1916 model year the spring oilers were deleted from the spring shackle, and a hole drilled through each main leaf and bushing to allow oil to be added (it never was by most owners).

1918 – 27

The taper leaf design was eliminated in favor of leafs that were simply cut off on the ends. A 9 leaf spring was added for sedans. Runabout springs became 6 leaf in 1920. Beginning in 1925 the new Roadster Pickup body style also used the 9 leaf spring.

In this issue of Model T Ford Fix we disassemble the rear leaf spring and rebuild it for best handling and ride quality.

To begin any repair of the rear spring requires disassembly and rust removal. In our case we had parts of two rear springs from 1909 – 1912 tourings. To get to this point requires removal of the bolts from the spring clamps, and removing the center bolt from the leaf spring. You will need two very large (at least 6″ capacity, made of steel) C clamps to remove the center bolt safely.  With our spring already disassembled we threw the parts in the back of the pickup truck for a ride to the local sandblaster.

The next day our sandblaster reported that all the parts were ready to go. We picked out the best looking leaves to make an 8 leaf touring assembly. For clarity’s sake we will identify the lowest leaf as #1 and the shortest leaf on top of the spring as #8, with all the others falling in line with this order.

We use a graphite paint to promote easy slipping between the spring leaves, and reduced wear of the steel parts. Our local farm store sells quarts of Slip Plate for $15. Another similar product is called “EZ Slide”.

When you get home with your Slip Plate and are ready to use it, the first thing after opening the can you will need to stir. And stir some more. And keep stirring. All the graphite flakes will be packed at the bottom of the can. It takes considerable effort to get the product to its desired thick, smooth consistency.

The #1 leaf (furthest from the camera) is painted with the graphite paint only on the top side, while #8 (closest to the camera) is only painted on the bottom side. We used a cheap 2″ brush to apply the product. The photo above is part way through the process, eventually the entire bottom of leaves 2 – 8 was coated.

Leaf #1 is painted only to about the spot where #2 covers it. We leave the metal bare where the gloss black paint starts.

After all the leaves are coated on one side we put the lid on the can of paint and put the brush in an old coffee can with an inch or so of mineral spirits in the bottom. The paint is dry within about an hour, so we flip leaves 2 – 7 and set #1 and #8 off to the side.

On most Model T springs you will be able to see marks where the spring leaves have been rubbing each other over the years. You need to paint only the areas where there is contact with the graphite paint. We painted the top of #2 – 7 and let them dry overnight.

We had a little rain overnight which caused a bit of surface rust on our freshly sandblasted bare surfaces. That will have to be removed before painting. Lesson learned!

A couple of 8″ steel C clamps are used to bring the leaves together enough to start the center bolt. New bolts are available from all the reproduction part vendors; ours happened to be a NOS Ford original. They are 6″ long which makes spring assembly much easier. With the center bolt through all the leaves we tighten the clamps together and run the nut down.

The square head of the bolt must be aligned as shown in order that the spring can fit into the corresponding square hole in the frame’s rear cross member.  We used a high speed motor with a fiberglass wheel to remove the light surface rust prior to painting.

The bolt is cut off with about 3/4″ protruding below the spring.

A 3/32″ hole is drilled in the spring for a cotter pin. Ford did not originally use a washer here but we like a lock washer in this case.

Early spring clamps were secured with unusual hardware that is totally unlike any you can buy today. The bolts in this case are 1/4″ diameter, with 20 threads per inch, but if you were to try and use a modern tap or die on the threads you would damage them because the threads are not cut on the same angle as used in SAE threads today. The nuts are 1/2″ wrench size, again not like standards used today.

The bolt heads are also uniquely shaped, and require a 3/8″ wrench, again unlike any modern fastener.

After the nuts are tightened we place a bucking bar under the bolt heads and peen the ends of the bolts so they cannot loosen.

After all the surface rust was removed we wiped down the spring, primed it and painted it gloss black.

This spring is ready for another 100 years of service.

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