Converting Automatic to Manual Transmission 68-72 BUICK A Body
By Mark O’Leary (moleary)
Several preliminary tasks must be completed in preparation for the project to convert the vehicle from an automatic to a manually shifted transmission. Before tackling any major undertaking on your vehicle, have access to a factory assembly manual. The next prerequisite is to allocate the funds for the project. Depending upon your resources and mechanical ability, the budget can range from $2000 - $4000. If you decide to involve a professional mechanic shop, be ready to pay more. With all the parts and handy assistants with the will to do so, this swap can be accomplished in as few as two long days. Do not hurry; budget the time necessary depending on your ability, helpers, etc. Haste makes waste, so have some fun, do it right as it is an investment.
Below is an overview that I prepared based upon converting a 455 BUICK A Body factory equipped with a TH-400 to a Muncie 4 Speed. The process is relatively the same steps for all 68-72 A Body vehicles but the details vary between year make and model. Assuming you have the necessary mechanical skills and tools, it is a rewarding process to embark upon.
The first order of business is to thoroughly inspect the vehicle to assure it is a viable candidate for the swap. A manually shifted vehicle depends much more upon a solid body and frame in existence within the factory tolerances than a vehicle configured with an automatic transmission.
1) Confirm the chassis and body is within factory tolerances. If the vehicle has ever been in a wreck and not repaired properly, there could be alignment issues later with the fitment of the manual transmission and associated components that otherwise go un-noticed with the automatic transmission.
2) Prior to putting the vehicle into operation with a manually shifted transmission, procure and install either a factory set of rear control arm frame braces or an aftermarket set. Manually shifted vehicles place a much higher stress factor on the frame at the rear box that GM factory remedied utilizing the stiffening value of additional frame braces. See the assembly manual for details. This photo shows a factory set on the bench.
3) Confirm the block is drilled for Z-bar (Bell Crank) ball stud. Photo shows the factory work.
and with ball stud installed...
4) Confirm the engine mounts are not worn or broken. Replace as necessary.
5) Procure all parts
68-72 Manual Transmission Swap Parts List
Manual Transmission Floor “hump” sheet metal.
A pattern of the transmission tunnel is very helpful. Ask around they’re out there to borrow to mark out the position close to a factory layout.
Correct Clutch & Brake Pedal assembly with pedal pads
Clutch pedal rod to Bell Crank (Z-Bar) & fire wall boot
Firewall steering column plate for manual transmission
Manual Transmission Speedometer
Manual Transmission Speedometer cable & grommet
Manual Transmission Steering column. A floor shift column works but is not correct appearing to a trained eye. Each year varies from the others with key in column, reverse lockout, and manual shift vs. floor shift automatic.
70-72 utilizes a neutral safety switch & wire harness extension
70-72 utilizes a reverse lockout. A 70-72 steering column is required for this all to function. The frame bracket & frame : transmission linkage rod, spring assembly from the automatic is re-used. Reverse lock out transmission linkage is required
TH-350 drive shaft yoke & correct length drive shaft.
BBB Frame Cross member.
Manual Transmission emergency brake cable and hooks
Flywheel –Surface Prepared and match balanced to the flex plate from the engine
Crankshaft Pilot bushing
Clutch assembly- pressure plate, clutch disc, throw-out bearing
Clutch fork & dust boot
BOP Bell Housing & correct bolts
Clutch Rod (Bell Crank / Z-Bar to clutch fork). Small block and Big Block are different
Bell Crank (Z-Bar) frame bracket including the (2) coarse thread screws
Bell Crank (Z-Bar)
Bell Crank (Z-Bar) block stud and dust boot
Clutch Pedal Return Spring
Shifter-Hurst Competition Plus
Floor consolette is optional
HURST Shifter arm PN#5335.
Shifter knob & adjustment set nut.
• Black ball for non-consolette,
• white ball with consolette,
• HURST t-handle
Shifter boots- upper and lower boots for consolette equipped vehicles. Non consolette use only one
Trim boot rings as required for consolette or non consolette option
Trans control switch (tcs) & wire harness from switch to carburetor
6) A couple photos of parts
7) Have access to the factory assembly manual for the vehicle being worked on.
8) If using a new oil-light bronze pilot shaft bushing (recommended), dry fit it onto the input shaft of the transmission to assure a proper fit. There should be no more than .001” clearance. Using a micrometer, measure and record the O.D. of the pilot bushing. Place the pilot bushing in the freezer 24 hours prior the installation into the crankshaft to contract it to aide installation later.
9) Preferably situate the vehicle in a climate controlled shop / garage environment. Securely lift the vehicle level and sufficiently off the ground to safely access the underside of the vehicle to allow transmissions to pass under.
10) Remove the drive shaft, Trans cross member, automatic shift linkage, Speedo cable and kick down (detent) cable or wires depending upon TH350 / TH400. Depending upon the vehicles exhaust system, this too may need to be disconnected and out of the way.
11) Confirm that the correct drive shaft yoke is installed on the drive shaft. Most manual transmission output shafts require TH-350 short yoke. Verify Drive shaft universal joints are within service tolerances, replace if needed.
12) Remove the Trans inspection cover.
13) Remove the six bell housing bolts. Store for reuse.
14) Remove flex plate bolts from torque converter.
15) Use proper jack and remove automatic transmission from engine. Utilize a transmission tail shaft plug to prevent fluid spillage.
16) Remove flex plate. Inspect the rear main seal on engine and repair if leaky to prolong the life of the future clutch disk.
17) Prepare flywheel. Machine shop time. If a used flywheel, assuming it is serviceable, re-surface it. Next step is to balance match the flywheel to the flex plate removed from the engine.
18) Remove front seats, carpeting and sound deadener.
19) If replacing speedometer, remove dash components and old cable as required & install new components.
20) If replacing Steering column, remove all components as required & install new components.
21) Remove bolt for brake pedal and install new brake & clutch pedal assembly, clutch pedal rod through the firewall and the boot.
22) If applicable to the model year, install transmission neutral safety switch & wiring.
23) With the use of a pattern as mentioned in the parts listing, carefully mark out the location on the floor to cut the opening. This is a crucial step so be certain it is in the correct location. If you do not have a pattern of a factory floor pan to mark out the hole.
24) Use the cutting device of your choice to cut the opening in the floor. Factory cut was rather crude.
25) Align the new floor hump and spot weld to the body.
26) Good time to install some Hush Mat or other new sound deadener.
27) Use a quality seam sealer on both sides of the floor and apply protective coating.
28) Remove driver side front wheel and the inner fender hard plastic dust shield.
29) Locate the two holes in the frame and mount the Z-bar (bell crank) frame bracket.
30) Install Z-bar (bell crank) ball stud into block.
31) Install the Z-bar (bell crank) and connect clutch pedal rod.
32) Remove any oil and grime from the pilot bushing opening in the crank shaft using a little bit of brake cleaner on a rag. Using a micrometer, measure the O.D. of the pilot bushing opening in the back of the crank shaft. Compare that measurement to the measurement recorded earlier of the new pilot bushing O.D. Both measurements should be very close to each other with the pilot bushing being ever so slightly larger (.0005”+/-) .Also measure to assure the opening is concentric (centered) on the crank shaft.
33) Using a deep socket that measures slightly smaller than the measured O.D., make one wrap around it with a small piece of 80 grit sandpaper to create a snug fit into the crank opening and twist the socket around to clean up the opening to bare metal. Sometimes this process is needed to enlarge the O.D. very slightly in the crank to accommodate the new bushing. To do so, simply continue to build up layers of 80 grit sandpaper and utilize an air or electric driven until the opening is ready to accept the new pilot bushing. DO NOT oversize the opening.
34) Remove the new pilot bushing from the freezer. If the bushing did not have enough time in the freezer, use the canned compressed air for cleaning computers and blast the bushing until it is frozen white in color. Watch the fingers!
35) Measure the O.D. comparing to previous measurement of the bushing prior freezing and the O.D. of the crank shaft opening. If all measurements agree and it appears the bushing should fit the crank shaft, it is ready to install. If not, take the necessary measures.
36) With the bushing diameter confirmed, proceed to install into the crank shaft opening. Place the bushing squarely into the crankshaft opening. It should just grab the opening. Using a ½” drive impact socket that is the same diameter as the bushing, gently hammer the bushing into position. The bushing should be seated flush to the transmission side face of the crankshaft opening. Avoid installing the bushing too deep and avoid excessive hammering on the crank shaft.
37) Install the prepared flywheel onto the crankshaft. The flywheel will only line up with the bolt holes in only one position. Utilizing a thread locking compound, torque the flywheel bolts to the required specifications. Always torque using a star pattern alternating between bolts until the specified torque is achieved. Do it twice.
38) Install the clutch disc and pressure plate to the flywheel following the directions provided with the new clutch. Utilizing a thread locking compound, torque the pressure plate to the flywheel using the spline alignment tool provided with the clutch kit to assure the clutch disc is centered while the pressure plate is being installed. Always torque using a star pattern alternating between bolts until the specified torque is achieved. Do it twice.
39) Inspect the shifter and linkage for excessive wear. Rebuild / replace if necessary.
40) Using a vice or transmission stand (preferred), bench fit the shifter and linkage onto the transmission. Follow the procedure in the vehicle’s assembly manual to set all the linkage up and adjust correctly. Be sure to set the shifter stop bolts to the correct positions. Below are pics of stock linkage set up on M-20 trans
The second photo shows how to set up an AutoGear M-21 Steel Mid Plate Overdrive unit. Note that the stock 3-4 linkage lever & rod needs to be flipped over as the 3rd gear is actually 4th in the transmission, 1.0:1 and 3rd is 0.87:1 so the linkage flip allows for the correct shift pattern to be retained. Also, for 70-72 with correct back drive application, as shown in both photos, the stock 3-4 linkage rod will not work with the Autogear set up. I pawed through piles of linkage rods I had available and found one the right length to bend up just right to clear the reverse / Back drive 70 -72 and operate flawlessly. The factory 3-4 linkage rod will not work without hacking and welding which I did not want to do in the event I switch back some day so that I will have the correct rod.
41) Once the shifter and linkage operate freely and easily on the transmission while on the bench, remove the shifter handle and set aside careful to not lose the nuts and bolts.
42) Dry fit the bell housing onto the retainer hub of the transmission. This fit should be snug, not tight. Using the sandpaper wrapped socket method, gently hone around the opening in the bell housing until a smooth fit onto the transmission is achieved. Set bell housing aside.
43) Apply a light coating of grease to the surface of the transmission hub for smooth throw out bearing movement. Dry fit the throw out bearing onto the transmission to assure proper fit and smooth movement.
44) Install the transmission mount and torque to specifications. Always use a new mount.
45) Fill the transmission with fluid. Typically two quarts are used. The use of proper fluid is essential to proper operation and long life.
46) Be sure the correct speedometer gear is installed in the transmission for the rear end gearing of the vehicle. The trans builder can install the correct gear with the rear dif gear and tire diameter information
47) Inspect the clutch fork to assure it is a serviceable unit. Replace if necessary.
48) After the dry fit of the throw out bearing on the transmission, install the throw out bearing onto the clutch fork. Take special care to install the bearing properly into the clips of the fork. The instructions provided with the new clutch should address the correct installation of the throw out bearing onto the fork.
49) Install the clutch fork with the bearing installed onto the bell housing. Be sure the ball stud is not worn; replace if needed. A dab of grease between the ball and the fork is required.
50) Install the bell housing onto the engine using the correct bell housing bolts.
51) I like to install 2-1/2” precut pieces of threaded rod into the two bottom holes and passenger side top of the bell housing that the transmission mounts to. These should be threaded in just past the inside face of the bell housing and protrude approximately 2” +/- out. These studs will aide in installing the transmission into the bell housing. Pic shows one installed…
52) Assure the alignment is correct of the clutch disc and pilot bushing. Utilizing an old transmission input shaft, gently slide it into the clutch and seat into the pilot bushing. Note the geometry of the input shaft to the hub opening of the bell housing. Adjust accordingly until the geometry is equal and the input shaft slides in and out of the clutch and pilot bushing easily.
Pic of clutch alignment tool and a used input shaft which I prefer
Pic of bell housing and aligned clutch; ready for trans install.
53) Note the position (clocking) of the clutch disc spline. With the transmission in neutral, rotate the input shaft of the transmission approximately into the same orientation as the clutch disc.
54) Using a jack, and the tail shaft plug again, lift the transmission into a position such that the input shaft is aligned with the throw out bearing and temporary studs. Gently advance the transmission noting carefully the input shaft sliding through the throw out bearing and onto the temporary studs until the input shaft slides through the clutch disc. NEVER ALLOW THE TRANSMISSION TO REST ON THE CLUTCH PRESSURE PLATE. At this point the input shaft retainer hub of the transmission should be very near or touching the opening in the bell housing. Continue to gently advance the transmission until it is seated into the bell housing. NEVER USE THE BOLTS TO DRAW THE TRANSMISSION INTO THE BELL HOUSING. If the transmission does not seat into the bell housing, continue to support it with the jack and temporarily install the clutch rod from the Z-bar (bell crank) to the clutch fork. Have an assistant depress the clutch pedal while continuing to advance the transmission until it is fully seated into the bell housing.
55) With the transmission now seated, replace the temporary studs in the bell housing with the correct mounting bolts & install top bolts. Torque to specifications. A thread locking compound is again recommended. Don’t forget to install the speedo cable clips with the bottom two bolts.
56) Install clutch pedal return spring. Be sure rubber bumper is installed on the clutch pedal bracket under the dash. This is the bumper the clutch pedal will stop on when the clutch pedal is pulled up (toward the driver)
57) Install the transmission cross member and bolt to transmission mount. Torque to specifications.
58) If so equipped depending upon vehicle model year, install reverse lockout linkage. This utilizes the vehicles automatic shift linkage of the vehicle was a column shift car. If not, procure the necessary components.
59) With the aid of a helper inside the car to operate the clutch pedal, adjust the clutch. There should be some “free play” on the clutch pedal. “Free Play” is the distance the clutch pedal travels before the throw out bearing engages the clutch pressure plate. Typically, ¾” is the distance the clutch pedal will have between the rubber snubber and the pedal when the clutch is fully disengaged or “up”. First, observe through the inspection opening in the bottom of the bell housing the position of the throw out bearing when the clutch pedal is up (disengaged). The optimum distance between the throw out bearing and the clutch pressure plate tangs is approximately ¼” with the clutch disengaged. This clearance must exist or premature failure of the throw out bearing will occur as the bearing will “ride” the clutch when driving. Adjust clutch by removing the cotter pin at the clutch rod trunion-block: bell crank (z-bar) and make one turn in or out as needed to effectively disengage the clutch disc from the flywheel surface and assuring the throw out bearing does not “ride” the clutch pressure plate tangs when disengaged. (If you have a small block style clutch rod set up, there is no trunion-block. Adjust by loosening the nuts at the end of the coupling and adjust in or out to achieve the desired throw.) NOTE: Aftermarket clutch kits vary between brands and sometimes require a taller or shorter throw out bearing than is supplied with the clutch kit. It is a good idea to ask around which clutch kits and throw out bearings work together so you know when ordering your parts you have the correct combination.
60) Once the clutch is adjusted, Install shift handle.
61) Have the helper dry shift through all the gears and observe from below to assure proper operation and no linkage is binding.
62) Assure the vehicle is setting level on the lift supports. Open the filler plug and allow the fluid to drain out. Usually about a half cup +/- will drain out leaving the correct amount of fluid in the transmission. Replace filler plug.
63) Connect new speedometer cable
64) Re-connect any of the vehicles exhaust system if necessary.
65) Install drive shaft. Depending upon what drive shaft you started with, you may need to have it shortened. With Vehicle sitting level on it’s suspension, measure center of differential u-joint yoke to back of output shaft. Subtract 1” inch. That is the correct length for the driveshaft.
66) Double check that all components are installed and secure.
67) With car still securely lifted level off the ground, assure area is well ventilated and start the vehicles engine and gently operate through the gears. Do not horse around! Keep rpm low.
68) If the vehicle will not shift through the gears, there is likely clutch adjustment required. Clutch adjustment was discussed back in step # 39. Re-attach and test again. With proper adjustment, recall that the throw-out bearing required clearance from the clutch pressure plate tangs with the clutch fully disengaged. If the clutch will not disengage and the maximum adjustment has been performed, there is a problem. Refer to the instructions provided with the clutch components.
69) If all operations run smoothly. Shut off vehicle and visually inspect all the components again including observing the clutch engagement and disengagement using the helper in the car to operate the pedal and looking though the inspection opening in the bell housing.
70) If all checks are ok, install inspection cover onto the bell housing.
71) Reinstall the driver’s side plastic dust flap and wheel.
72) Install carpet and seats & remaining interior parts as needed.
73) Lower vehicle, start engine and perform some gentle testing with the vehicle on the ground by engaging reverse then 1st gear, then back to reverse.
74) If all operations test ok, make a test drive. If minor clutch pedal adjustment is required, perform as needed.
75) Break in period of about 500 miles is recommended before fully placing the vehicle in heavy duty service. Re-inspect all components again at that time.