Spring and T-arm Retainer Cups

On mid-year Corvettes, the ends of the main leaf of the rear spring and the rearward ends of the trailing arms each has a large metal retainer that is inserted into a hole and peened into place. These retainers have a peripheral circumferential lip, the purpose of which is to constrain a large rubber bushing that is part of the outboard mounting system for the leaf spring. During the 1963-1966 model years, as well as the early portion of the 1967 model year, these retainers were GM part # 3831588. This part was discontinued from service in the summer of 1967. The 3831588 retainer was superseded by GM part # 3910701, which still remains available from GM. The earlier part number was used exclusively on 1963 through 1966 cars and sporadically during the early part of the 1967 model year. I have seen the early style retainers on 1967 cars with VINs up to approximately 7,000, corresponding to build dates in late December 1966 or early January 1967.

People restoring their rear leaf spring or trailing arms for 63-67 cars have to choose between restoring original 3831588 retainers or buying new 3910701 retainers. Unfortunately, the original retainers were made from steel, that was neither plated nor painted. As would be expected, original 3831588 retainers are often rusty or worse, often making restoration impossible. Typically, restorers opt for using the 3910701 service replacement retainers. The 3831588 and 3910701 retainers are, however, noticeably different in terms of the geometry of the peripheral lip. On the currently available 3910701 part, the peripheral lip is 0.325” tall and oriented at approximately 80 degrees from the base plane. On the earlier 3831588 part, the lip is noticeably shorter and is oriented at approximately 45 degrees from the base plane. This difference in lip height and angle is easily detectable by a knowledgeable and observant chassis judge and can and does lead to a points deduction during judging. Typically, such a deduct will take place at a regional or national event, although the differences in the retainers has been pointed out in an article in the Corvette Restorer from a few years ago and one should not be surprised to get a deduct at a chapter judging event if either of your chassis judges is an avid reader of the Restorer.

Some years ago I have developed the tooling to modify the 3910701 retainer to mimic the earlier 3831588 retainer. In the photo below I show a new 3910701 retainer on the left, an original (and badly rusted) early style (3831588) retainer at the center and my modified retainer on the right. Notice how the angle of the lip in the modified retainer is a close match to the original retainer shown at center. This correct lip angle is what most judges look for.

This photo shows a side view of the currently available GM 3910701 retainer on the left, a used original 3831588 retainer at the center (minus a portion of the bottom section that was rusty and disintegrated during removal, from an original 1966 leaf spring), and my modified retainer on the right. The modified retainer closely approximates the lip height and angle of the original mid-year retainer shown at the center, although the modified retainer is slightly larger in outer diameter (about the thickness of a dime). The cost for a set of four modified retainers is $50, including shipping (US destinations only).

Note that the original GM 3910701 retainer cup is only available as an NOS item; it was discontinued by GM in early 2014. I have a limited supply of the orginal, uinmodified NOS GM retainser cups and I sell a sell of four for $35, including shipping (US destinations only). The NOS cups typically sell for $10 to $15 each on ebay, so $35 with shipping for a set of four is a bargain. Also be aware that the reproduciton version of these cups sold by all of the Corvette catalog companies differ from vendor to vendor but none is identical to the original GM version. Why take a chance uisng repro cups if you plan on having your car judged?

I also have a custom-made tool that is used to swage the 1” diameter collar of the retainer cup that inserts into the spring or T-arm. The tool is described in an article in the Corvette Restorer (Vol. 36, No. 2, Fall 2009, p. 38-40; If you have Adobe Reader, you can read a copy of the article by clicking the following link: RestorerArticle.pdf). The tool can be used even if the T-arms and spring are still attached to the car. Of course, the large bolt at the end of the rear spring needs to be removed in order to do the swaging step, so you will need a safe and secure system to constrain the spring once that bolt is removed. Such a system is clearly described in the Restorer article. If you are not confident that you can perform the swaging safely with the spring mounted on the car, do not try it. A rear leaf spring under load has a tremendous amount of stored potential energy in it. I rent the tool to perform the swaging for $11. I ask that you keep the swage tooling for no longer than 2 weeks before returning it to me using a USPS Priority Mail Small Flat Rate box. So you’ll need to have your ducks lined up before sending me a request to rent the swaging system.