Wes Duenkel
July 8, 2019
Contributers: Wes Duenkel

To paraphrase Forrest Gump, working on a high-mileage Mustang is "like a box of chocolates. You never know what you're gonna get." Often, a simple repair attempt uncovers worn components, mismatched parts, and shoddy old repairs. The stock driveline parts on 1979-1993 Mustangs were marginal at best. Add decades of aggressive driving and deferred maintenance, and you have a recipe for trouble. If your Mustang is feeling its age with every shift, we're going to cover some common Fox-body maladies and how to solve them.

Using new parts from Hays and Ford Performance Parts, we got a Fox-body Mustang shifting better and running smoothly. Read on to see how we did it.

Clutch Cable: Before getting too far, check the clutch cable. A worn clutch cable will increase pedal effort and the clutch will feel sticky, spongy, or notchy. Worn clutch cables are very common on Fox-body Mustangs. Replacing the clutch cable with a quality replacement is a quick fix for many aging Fox Mustangs.
Flywheel Surface: If you're replacing the clutch or changing clutch disc material types, it's generally a good idea to resurface the flywheel. If your flywheel's surface is warped, uneven, or has large discolored patches, the new clutch could shudder. Most machine shops perform this service at a very modest price. On the left is a recently resurfaced stock flywheel, while on the right is a Hays billet steel flywheel. We reused our resurfaced stock flywheel because it was still in good shape and we weren't changing clutch disc material types.
Flywheel Balance: If you're replacing the flywheel, be sure to choose the right flywheel for your engine. Pre-1981 302 engines require a flywheel with a 28.2-ounce imbalance and the 1982-up engines use a 50-ounce imbalance flywheel. Furthermore, pay attention to the balance side of the flywheel: It may have been modified to balance the rotating assembly (notice the drill marks on ours), so any new flywheel should be checked to make sure its imbalance factor matches the flywheel you're replacing.
Pilot Bearing: It's small, but a worn or incorrect pilot bearing can wreak havoc. The pilot bearing centers the transmission input shaft to the crankshaft's centerline. If the pilot bearing is worn or oversized, the transmission input shaft (and the clutch disc) will wobble and cause a serious engine vibration. Our test Mustang had this problem. Turns out, V-8 T-5 input shafts have a larger-diameter pilot bearing surface (0.668 inch) and require a National Bearing PN FC65174 or equivalent (left). Using a four-cylinder T-5 (0.590-inch-diameter pilot bearing tip) with a V-8 requires National Bearing PN 202SS or equivalent (right).
Rear-Main Seal: While it's technically not part of the driveline, when the flywheel is out is a great time to replace a leaky rear-main seal. This engine was recently rebuilt, and its rear-main seal was still good.
Clutch Pressure Plate: Like the flywheel, the pressure plate should be free of warpage, discolored spots, or major grooves. Sometimes the clutch fingers are worn or uneven. If you're replacing the clutch disc, go ahead and replace the pressure plate as well. The Hays Street 450 pressure plate (right) provides up to 30 percent more clamping force than the original (left).
Throwout Bearing: A worn throwout (release) bearing can squeak, grind, or chirp when the clutch is engaged, disengaged, or all the time. The "throwout bearing" nickname is well-earned. If you're replacing the clutch, it's cheap insurance to replace the release bearing. Our existing throwout bearing was bent from a ham-fisted installation.
Input Shaft Bearing Retainer: Along with the pilot bearing, the input shaft bearing retainer is often overlooked when servicing the clutch. The clutch pedal action will catch on the grooves of a worn release bearing sleeve. The original T-5 sleeves (above) were aluminum, which meant they wore quickly. The Ford Racing Performance Parts replacement (bottom) features a steel sleeve that's more durable.
Clutch Fork and Ball Stud: Over time, the clutch fork and ball stud can wear out. Our existing fork and ball stud (top) were heavily worn, so we replaced them with the new parts from Hays (below).
Driveshaft Slip-Yoke and Universal Joints: The slip-yoke should be smooth and free of any deep grooves. Our driveshaft's yoke (left) was so badly worn that it was undersized by 0.012 inch. The universal joints should operate smoothly without any slop or notchiness. If you replace the slip-yoke, joints, or rear flange yoke, it's a good idea to get the driveshaft rebalanced.
Tailshaft Bushing: The driveshaft slip-yoke is supported by the tailshaft bushing. It's usually pretty durable, but if your driveshaft slip-yoke is worn like ours, the bushing is probably worn too. We used a special tool to pull and replace the bushing without disassembling the transmission.
Driveshaft Balance: Even a perfectly balanced driveshaft will vibrate if the transmission output shaft runout and rear axle pinion shaft runout are not clocked correctly. A vibration felt in the shifter or in the floor (regardless of engine speed) can indicate an imbalanced driveshaft. To minimize driveshaft vibrations with our new Ford Performance aluminum shaft, we measured the point of maximum runout by using a dial indicator on the slip-yoke. We marked the spot of maximum runout (lowest number on the dial indicator) with a paint dot on the end of the output shaft. After aligning both points, we installed the Ford Performance Parts aluminum driveshaft. This way, any "wobble" at the front and rear of the driveshaft will cancel each other out as much as possible.
Aluminum Driveshaft: Rather than replace the worn slip-yoke and universal joints on our car's steel driveshaft, we just swapped the shaft with an aluminum unit from Ford Performance Parts. Ford's shaft (part number M-4602-G) is an easy way to solve several problems in one fell swoop!
Clutch Release Check: On Mustangs equipped with a T-5 transmission, a quick way to check clutch release is to perform this three-part test: (1) Select Neutral and depress the clutch. (2) Start the engine. (3) Select Reverse. If the transmission grinds going into Reverse, the clutch is not releasing enough.

Sources:
Hays
(866) 464-6553
holley.com/brands/hays

Ford Performance Parts
(800) 367-3788
performanceparts.ford.com

Photography by Wes Duenkel