1962 THROUGH 1972 TACH DRIVE DISTRIBUTOR RESTORATION
By Dave Fiedler
This article covers both Transistor Ignition and points
style Corvette distributors from 1962 to 1972. Fuel Injection distributors are
not specifically addressed; however, many items will apply to these distributors
as well. My definition of restoration is to bring a distributor back to
its original appearance and function by reconditioning the original parts,
replacing parts only when absolutely necessary, and using the correct parts for
replacement. It is not a process of gutting the distributor of all its original
Delco components, substituting “will-fit” reproduction or aftermarket parts and
then calling it restored – a distributor must function properly to be called
restored. True restoration takes time, experience, patience and resourcefulness.
A general misconception is that distributors are sealed for life devices that never need attention with the exception of changing points, condensers, caps and rotors. This misunderstanding may stem from the GM service manuals which state that distributors have “permanent type lubrication and no periodic maintenance is required”. However the manuals do not adequately address long term wear issues, which leads into the first steps of restoring a tach drive distributor.
After the distributor is completely disassembled and cleaned with a solvent degreaser, then each part can be inspected for wear problems. Areas of typical wear and the solution are as follows:
Solution: If the main shaft and tach drive gears are sheared then replacement is the only solution. When the main shaft is replaced it is extremely important to use a shaft that has the same cam brazed onto the end. GM used (38) different main shaft cams in tach drive distributors from 1962 to 1972.This cam is fundamental to the centrifugal advance curve because it determines the shape of the curve as well as controlling other functional characteristics (see article “All TI Distributors Are Not Equivalent” THE CORVETTE RESTORER Vol. Twenty-One, Number One, Summer 1994). If an identical main shaft cannot be found, the original cam should be removed and brazed onto a good shaft. One should be careful to orient the cam in the same angular position (with respect to the roll pin hole) as it was on the original main shaft. Caution: It is possible that substituting a main shaft with a different cam will have a detrimental effect on the performance of the vehicle.
Solution: Main shaft wear is usually restricted to the area in contact with the lower bearing. Minor circumferential grooves can be removed from the main shaft by using fine grit abrasive tape and a lathe or similar means to rotate the shaft while polishing. An original main shaft is .489” in diameter- this diameter should not be reduced during polishing by any more than .001” in the area of the bearings. If the main shaft is heavily scored, then it will be necessary to replace it (see Solution A.) For bearing replacement, the lower bearing can be driven from the housing with a suitable drift. The housing should be measured to determine what size of bearing is needed. Delco machined the housings anywhere from .639” to .645” in diameter. A bearing with an outside diameter of .002” greater than the housing bore should be pressed into place and then reamed to the proper clearance- .0005” to .002” larger than the main shaft diameter. Bearing material should be bearing bronze not Oilite, brass or silicon bronze. Note: Upper bearings have not been mentioned because they generally see very little wear since there are no side forces acting on them. If an upper bearing needs to be replaced follow the same procedure outlined for the lower bearing.
Although there are many ways to repair this problem, most of the
solutions destroy the originality of the housing. One method that preserves the
originality involves the use of a custom-made spacer that is attached to the end
of the tach gear. The procedure goes as follows: The distributor housing is set
up in a Bridgeport-type mill so that the tach drive cavity worn area is
perpendicular to the axis of the tach drive gear. The damaged area is then
milled to produce an unblemished bearing surface. Next the tach gear, washer and
coupling are installed in the housing and the distance between the end of the
tach gear and the new surface that was generated is determined. The last step is
to custom make a spacer from bearing bronze to a thickness that will allow .010”
endplay in the tach drive gear. Since this spacer is held in place by the tach
gear, a hole must be drilled in the end of the gear so that a protrusion on the
spacer can be pressed into the gear.
Since the main shaft cam is so important, as previously discussed,
every effort should be taken to restore the original cam. Shallow indentations
can usually be removed by using a die grinder with a fine grit wheel to
uniformly remove material while maintaining the original contour. The surface
should then be polished with a sanding roll. Advance weight contact point
indentations can also be ground smooth, however if much stock removal is
required (which would alter the geometry of the weight) the result would be a
change in the advance curve. If that is the case, the advance weights will have
to be replaced using the correct style (four different part numbers were used.
When this surface is scored or worn unevenly, the only solution is to
machine the housing (using a lathe) in order it to get a good bearing surface.
If the centerline spacing is marginally out of specification, select fit
of parts (coupling, tach gear and main shaft) may solve the problem. Otherwise,
the only solution is to have the housing machined correctly. Since the position
of the tach gear is already determined, or at least very difficult to change,
the only option is to shift the main shaft centerline. This has to be done by a
very competent machine shop that can jig bore the bearing bores in the housing
to the specified center to center distance of .425”. Oversized bearings (on the
outside diameter) will have to be made to fit the bores. Ream the I.D. of the
bearings as in Solution B.
This procedure is very tedious and would significantly add to the cost
of a restoration.
G. Bearing misalignment- This is a condition where the upper and lower bearings are not on the same centerline. This condition can be detected by slowly sliding a new main shaft through a good top bearing and into the lower bearing. If there is misalignment you can feel the shaft hit the edge of the lower bearing and when fully inserted the shaft will not turn freely.
Solution: The housing will have to be jig bored in a fashion similar to Solution F; only this time the objective is to correct the alignment of the bearing bores while maintaining the correct tach gear to main shaft centerline distance. Again very tedious.
Solution: Replacing vacuum controls presents a dilemma, as GM/Delco stopped making vacuum controls in 1994. An outside source currently makes all the controls and they are packaged under the Delco name. Unfortunately, these controls do not look like and are not numbered like the originals. Originals were stamped with the last three digits of the seven digit GM part number. These three digits were followed by another number that indicated the total amount of advance the control would provide. Current controls are stamped with a B followed by a number that has no functional significance. Some reproduction controls are available which look very similar to the originals, but they miss the target on function. Best advise- look for N.O.S. vacuum controls.
I. Weight base/cam (on point-style distributors)- Inspect the cam for wear caused by the rubbing block on the points pivot arm.
Minor score marks can be removed from the cam by using a 3M Scotch-Brite
deburring wheel or equivalent abrasive. Replace the weight base/cam if the score
marks cannot be removed or if the cam lobes are worn below .987” diametric
reading from peak to peak. Note: If this part needs to be replaced use an
identical part- check and compare the three digit stamped-in code number. Ten
different part number weight base/cams were used.
After all parts are repaired or replaced and everything is
functionally correct (a trial fit of parts is a good idea at this point), the
cosmetic segment of restoration can begin. The housing usually requires the
greatest amount of attention. After thorough solvent cleaning, the housing may
have some paint left on it as well as some rust. Media blasting with glass beads
is probably the best means to prepare the housing for painting. Care should be
taken not to let any glass beads come in contact with the bearings or other
internal surfaces. Broken down glass beads are difficult to remove from this
area and they are abrasive in nature. Remnants of glass beads will quickly
destroy not only the bearings, but also the main shaft. After preparation, the
housing should be masked (no paint on the portion that goes in the engine) and
painted with a semi gloss black. After cleaning, other distributor components
can be carefully media blasted, cleaned again and then be either treated with a
rust inhibitor or replated, whichever was original for that particular part.
When everything is cleaned, painted and plated to satisfaction, the distributor is ready for assembly as follows:
For a restoration to be complete, the distributor should be
inspected using a distributor machine. Several functional characteristics can be
checked with this equipment, but the most important one is the centrifugal
advance curve. GM published advance specifications using three, and sometimes
four, data points (advance @ a given rpm) to establish a curve. It may take some
time and a selection of advance springs, but if the internal parts (main shaft
cam, stop bushing, weight base/cam or rotating pole piece and advance weights)
are correct, then the advance curve should meet the specifications. Each
distributor part number (53 different part numbers used from 1962 to 1972) has
specific internal components that work in combination to produce a centrifugal
advance curve that is optimum for that application. It is this advance curve
that helps generate maximum horsepower throughout the rpm range.
Owner, T.I. Specialty
1631 Pheasant Run
Richmond, Indiana 47374
Phone: 1(765) 962-4265
Hours: 10 AM to 6 PM EST