“Resto moding” a gearbox is not the same as a simple rebuild. While it might involve reusing some key components and even go back into service alongside unmodified examples, the goal is to provide improved performance and a long service life.
In most instances, those key components are limited to the housing, inspection covers, and bearing retainers. All of which need to be thoroughly cleaned, inspected, and refurbished while new and improved revolving elements are manufactured. Pay special attention to the bearing bores and do not handicap the final results with sloppy fits or misalignments.
It is tempting to reuse shafts, especially if they appear to be in good shape. Unfortunately, no one can tell how much of a component’s fatigue life remains. New revolving elements need new shafts as well.
If the “new” gearbox is going back into service alongside existing units, it may be important to retain the same “exact ratio.” This does not necessarily mean the exact same number of teeth on each component, or even the same number of total gear and pinion teeth.
The “optimum” number of teeth has changed over the years, as has the “best” combination of ratios by stage. In previous postings I went over the way rating standards used to treat sets differently based upon where they were in the drivetrain. The basic formula has changed too, and it no longer encourages a higher numerical ratio late in the sequence.
The “old way” would put a 6- or 7-to-one reduction in the output position and have very low (say 1.5 to 2.5 to 1.0) in the input stage. A “modern” design would have no more than 4 to 1, as the output with a higher ratio as the input stage.
Center distance combinations have changed as a result. Classic designs would have an input stage center distance of half the output; modern designs have increased that to three quarters of the output centers.
When “resto moding” you are stuck with old-style center distance combinations, so you cannot automatically change to modern ratio combinations. Think of it as a gearbox Sudoku puzzle. You must juggle the tooth numbers and face widths around to obtain the highest overall power capacity.
The process gives you a new appreciation for hunting tooth arithmetic if you have to match the ratio exactly. Juggling the variables while keeping helix angles “reasonable” will tell you just how talented a gear designer you are.