Manual Transmission Disassembly and Inspection
One of the main advantages of a manual transmission over an automatic transmission, beyond the more-involved driving experience, is that they are very simple to use, fix, and rebuild. There generally are very few things you can do to make them better other than make them stronger if you intend to race them.
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For street-driven vehicles, there may be a few minor material changes to improve strength and/or durability, but many if not most of the procedures are similar to factory service procedures. As a result, I won’t show every step for rebuilding a typical manual tranny. I highlight important areas that need to be given some extra attention when rebuilding such a tranny, in this case the Toploader that was available for our 1968 GT. I show some of the major steps involved in taking this transmission apart and putting it back together again. We again relied on Big 4 Transmissions of Paramount, California, to share some of their expertise with working on this type of transmission. This helped me address some issues that aren’t normally covered in the shop manual, but which have become commonplace over the years.
MUSTANG RESTORATION: MANUAL TRANSMISSION DISASSEMBLY- STEP #1
One of the most useful tools needed when rebuilding a manual transmission is a pencil magnet, such as this one. It is invaluable when retrieving small springs, detent pins, and the like from small passages where oil and/or dirt conspire to keep them stuck out of reach. As you remove these small parts, make a note of where they came from, so they go back in the right place.
MUSTANG RESTORATION: MANUAL TRANSMISSION DISASSEMBLY- STEP #2
Many of the larger external parts of the transmission can be removed at the beginning of the process. These will include the shift linkage, the tranny mount, the top cover, and the input bearing collar, as is shown here. Inspect it for excessive wear or damage on the surface where the throughout bearing slides. Remove minor scratches with emery cloth, but any significant flaws mean replacement of the part. The shaft seal on the inside of the collar will be replaced; these have a tendency to harden with time and can thus develop a leak.
MUSTANG RESTORATION: MANUAL TRANSMISSION DISASSEMBLY- STEP #3
During the disassembly of a manual transmission, many small parts, such as detent pins and so on, need to be removed for access to other parts. There are also situations in which parts such as shafts need to be oriented to a certain position before they can be removed. The factory service manual generally describes these procedures in detail. However, the general rule is that if a part seems to be difficult to remove, don’t force it. Look for of any pins or retaining rings that may have to come out before it can be removed. Also try rotating the part to different positions to see if that solves the problem. Lots of force should not be needed to remove the vast majority of components (except for bearings).
MUSTANG RESTORATION: MANUAL TRANSMISSION DISASSEMBLY- STEP #4
Look for any signs of wear or surface damage when removing the various support shafts. While it is rare, pitting or galling has occurred under some circumstances. Minor imperfections can be removed with a fine emery cloth or pad, and the parts can then be rinsed in the parts washer to remove any residue. Reversing the retaining pins is a good thing to do when reinstalling these shafts because the opposite sides of the pins are less worn and provide a more secure fit.
MUSTANG RESTORATION: MANUAL TRANSMISSION DISASSEMBLY- STEP #5
The shift forks are one of the main components to examine for signs of wear. This usually occurs at the tips where the forks contact the sliding collars over the synchros. A small amount of polishing is fine, but any excessive wear can cause problems with incomplete engagement and/or the transmission coming out of gear. Also look for any cracks or excess wear in the bore for the shaft, but these are fairly rare unless the transmission was abused.
MUSTANG RESTORATION: MANUAL TRANSMISSION DISASSEMBLY- STEP #6
The sliding collars must also be examined for any signs of excess wear on the outer surface where the shift forks make contact and on the inner teeth as well. These generally are reused with only a trip to the parts washer needed before they’re reinstalled in the same place.
MUSTANG RESTORATION: MANUAL TRANSMISSION DISASSEMBLY- STEP #7
The main shaft assembly gets removed and installed/ loaded from the top, hence the name. Be careful lifting the assembly out, so the shaft surfaces and the gear teeth don’t get nicked or scratched. These components are petty stout, but if one of the gear edges scrapes the shaft, damage can occur. When disassembling the main shaft, look for any galling on the shaft or on the inside of the gears. You want everything to be nice and smooth and free of flaws.
MUSTANG RESTORATION: MANUAL TRANSMISSION DISASSEMBLY- STEP #8
The main case normally will have some degree of sludge in it just because of the type of gear oil that was used. The main indicator of potential problems is how much material is on the magnetic plug, seen to the left of the reverse gear assembly. If there are a lot of metal filings on this plug, then you can be sure you’re in for an unpleasant surprise in the near future. Larger particles are especially problematic because they can be an indicator of chipped gear teeth or a broken retention or detent pin. Any significant accumulation of material on the magnetic plug should be taken seriously to the point of making sure the source of the material is positively determined.
MUSTANG RESTORATION: MANUAL TRANSMISSION DISASSEMBLY- STEP #9
There are generally two different sizes for the synchro “dogs”: the shorter ones are for the ¾ synchro, while the longer ones go with the ½ synchro. These normally do not exhibit much wear but are usually replaced anyway as precautions against wear and fatigue. The spring clips that these contact, however, generally do show signs of wear at the points where they touch. The dogs have a tendency to wear grooves in the clips, and thus the clips are always replaced in a rebuild. While not often, these clips have been known to wear through and break.
MUSTANG RESTORATION: MANUAL TRANSMISSION DISASSEMBLY- STEP #10
A rebuild kit for a manual transmission includes the usual assortment of gaskets, seals, and bushings, but it should also include new bearings/ rollers, synchro rings, plus the various spring clips, retaining clips, and other small parts. While the bearings do not need to be packed with grease, they should be pre-lubricated with gear oil after they are lightly blown off with air.
Written by Frank Bohanan and Republished with Permission of CarTech Inc