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Fiat 4-Spd transaxles had minor variations to the gearsets over the years, and although similar in appearance, they used different gear ratios between 128, X19, and Yugo. It is common to find the internals from a good running 128 inside of an X19 as a cheap transaxle rebuild. As a result, servicing and ordering parts for X19 4-Spds can be challenging.
Always count the teeth on every component you are looking to replace. Also use the following notes as guidelines:
X19 4-Spd transaxles always had 49 teeth on the 4th gear final drive. Fiat 128 and Yugo 4-Spd transaxles had 47 teeth. The driven gears and the cluster shaft cannot be mixed and matched between these variations.
Up to 1977, 4-Spd transaxles used 65mm synchro rings on 3rd and 4th gear. In mid '77, they changed to larger 72mm synchro rings and bigger sliding sleeves. This design was also carried on into the 5-Spd transaxles. As a result, 77-78 gearboxes are considered the most robust of the 4-Spd units. These cases can usually be identified from the outside by their 13mm head bolts holding the end cap in place, as opposed to 10mm head used on the small synchro boxes.
As a result of the larger synchros, 1977-78 transmission use different shift forks (5-Spd style), and place the cluster-shaft reverse gear in a different location than the earlier boxes.
Yugo and 128 internals can be swapped into an X19 4-Spd case as long as all of the components are swapped. You cannot mix-and-match!