Probably the most annoying...
Probably the most annoying part of the brake bleeding procedure on a C4 equipped with the second of the three brake modulator assemblies used from '86-96 is bleeding the modulator valve. This is the rig we use. We put the 10mm wrench on the bleeder first, then slip the bleeder jar hose over the bleeder nipple. You need a pressure bleeder to do this, and even taking the best of care can result in some brake fluid spillage. To catch that, we usually stuff the area under the bleed fitting with some rags.
For race track use, the stock C4 front brakes are marginal in their ability to deal with heat. I figure the combination of grooved rotors, Ford fluid, the better brake cooling from Mid America's air dam, and, when the time comes, a more aggressive brake pad, would give us enough additional brake performance to be able to run occasional time trial events with the car and not have to worry too much about brake fade.
At some time in the future, if our race track adventures become more frequent and our budget gets larger, then I'll consider an aftermarket brake conversion.
Another six months went by and one day, I noticed a little spot of oil on the floor. After a week or so, this became a bigger spot below the car's ZF S6-40 six-speed manual transmission. I got underneath the trans with a bright light. The transmission was leaking at the part-line between the extension housing and the rear case.
The Service Manual provided the torque specification for the extension housing bolts. The first bolt I put the torque wrench on was the lowest one, and it turned but didn't tighten. Obviously, the threads in the case were stripped.
The business end of the driver's...
The business end of the driver's side of the Mid America Designs front air dam. The MAD piece is highly desirable for a car that sees both street and track duty because it maintains factory ground clearance and mimics the OE air dam's appearance.
The ZF was used in all stick-shift Corvettes from 1989-96. Generally, it's a robust design, but there are a couple of caution areas. First, the synchronizer assemblies were designed for low shift effort in spite of the massive gears necessary to meet GM durability schedules with a 450 ('89-early-'93) or a 400 (late-'93-96) lb-ft input torque rating. As a result, the synchros, especially the multi-cone units on the lower gears, are not tolerant of power shifting. This makes the ZF a poor choice for hard-core drag racing. If you power shift a ZF, expect short synchronizer life.
Secondly, based on my experience and that of other Corvetters I've been in contact with using Internet mail lists such as VetteNet, the ZF seems to have occasional trouble with extension housing bolt hole threads in the rear transmission case getting stripped. It's hard to imagine those threads stripped themselves, so I suspect ZF had a problem with overtightening of those bolts during assembly. The C4 powertrain consists of the engine, transmission, torque beam and the differential bolted together as a rigid unit. Because the powertrain mounts to the car at the front, with the engine mounts, and at the rear, with the differential mounts, torque loads are transmitted from the torque beam to the transmission through its extension housing. If the lower bolt is in partially damaged threads (due to overtightening), the fluctuating loads from drive axle torque may eventually pull the bolt out of the hole.
DIY maintenance of the ZF is not easy. Parts availability is restricted by a peculiar legal agreement between General Motors and ZF Industries that makes most of the S6-40's parts unavailable in North America. Whatever ZF and GM were thinking about when they made that agreement in the mid-'80s, it clearly was not the interests of Corvette customers. For major repairs, in most cases, practical choices open to most '89-96 owners are either a new transmission, an overhaul at the ZF Industries Service Center in Indiana, or one of several independent rebuilders around the country. Both choices are very expensive. Certainly, forcing a new gearbox or an overhaul on someone needing only new synchros is a bit of a rip.
Mounting the air dam pieces...
Mounting the air dam pieces is easy, but routing and attaching the ducts is a little more complex. Shown is our passenger side duct. We had trouble with MAD's cable ties, so we substituted our own "industrial strength" items. Use care when mounting the ducts so that they do not interfere with suspension or steering movement. Also, no matter where you run the hoses, with the steering at lock, you will probably have the inside of the turned-in wheel slightly rubbing its respective hose. For that reason, view the hoses as expendable over the long haul, because the wheel may rub a hole in the duct.
Minor repairs, such as gaskets, output shaft seals, the reverse light switch, skip-shift solenoid, replacement extension housings, and front bearing retainers are serviced by GM Service Part Operations (SPO).
My leaking gearbox was going to require repair of the lower extension housing bolt hole threads in the transmission's rear case. In a situation like this, some C4 owners would have either an extension housing replacement or a transmission overhaul forced upon them by an ignorant or greedy repair facility. We found a simple and inexpensive way to fix this problem, and the repair can be done with the trans in the car for minimal cost and several hours work.
To start, the gearbox must be drained. Some DIY's are baffled by the metric socket-head drain plug. The easiest solution is a Volkswagen transaxle plug tool, which also fits the ZF. Next, the driveshaft and torque beam must be removed. I found the easiest way to get the torque beam out, since the extension housing had to come off, anyway, was to remove the bolts and take the housing and the beam out together. For more specifics, read the manual.
A durable fix for most any stripped bolt hole is the "Time-Sert" Thread Repair system sold by Time Fastener Company. A Time-Sert is a thread insert that is locked in place during installation. Because it's not a coil-type thread repair, it is a permanent and more durable way to fix a damaged bolt hole. I ordered a Time-Sert kit for M8x1.25 threads.
The kit contains half a dozen or so inserts and the tools necessary to install them. First you drill the damaged hole out to a larger size. Then, you use the counterbore tool to cut a recess at the top of the hole. Next use the kit's tap to cut the threads for the insert, then blow the chips out of the hole with shop air. Lastly, screw the insert onto a special insert driver, lubricate its threads, then install the insert. As the upper part of the insert bottoms in the counterbore, the driver expands the bottom of the insert, locking it in place.