Drift

January 07, 2018

‘Reverse engineering’ is a ten dollar way of saying ‘copying someone else’s work’. It sounds technical, it sounds professional, and it disguises the fact that we’re doing something our parents and teachers told us not to do. Nevertheless, when it comes to automotive service parts it is generally legal as long as features not essential to proper function aren’t duplicated — that would be illegal counterfeiting.

If an item is ‘reverse engineered’ from a good sample — typically a single piece, rarely the number required to establish median dimensions or tolerances — and the person or persons hired to do the work knows the intended use and functional requirements, parts made from ‘reverse engineered’ prints can be useful and cost effective. Frequently, once the original equipment manufacturer has discontinued their version, they are the only service alternative.

However you look at it, though, the truth remains. A copy is never the same as the original. It’s always a little ‘fuzzy’. At Auto Gear we call that ‘drift’.

Which brings us to ‘Muncie’ four-speed gears, for half-a-century some of the most frequently purchased, and consequently most frequently ‘reverse engineered’ items in the North American manual shift aftermarket. On these items, as new vendors enter and copies are made from copies, ‘drift’ has been significant and, in some cases, function or durability have been compromised.

Some time ago, we noticed that the synchronizer cones on our ‘Muncie’ mainshaft gears weren’t where they should be. We checked our prints — ‘reverse engineered’ like everyone else’s — and our Italian vendor’s prints, and confirmed that manufacturing was delivering good, accurate gears according to our specification. The problem was ‘drift’. And we knew that the only proper way to proceed was to get back on the ‘gold standard’, the specifications on the original equipment drawings. So we called in some favors, got the General Motors dimensions, revised our prints, and ordered new ‘cone corrected’ gears.

At the same time, we noticed that ‘drift’ was also present in our synchronizer rings, purchased parts then not under our direct engineering control. To put these — the most technically complex and least appreciated item in the gearbox — where they belong, under our control and back on the ‘gold standard’ to assure proper synchronization, it was necessary to obtain General Motors prints. We did this, also.

The new synchronizer rings, our 18-091-002, are now available. They are ‘cone corrected’ to match the new ‘cone corrected’ gears, and are made exclusively for us to our prints in Poland from CW713R bronze, the standard for high-performance European production. They feature the correct pointing (for faster shifting), strut pockets with generous corner reliefs (to prevent cracking in the heavily-loaded corners), nine equally-spaced wiper slots (to initiate synchronization sooner), and three pilot lands (to reduce ‘cold ring rattle’).

Here’s where it becomes interesting. The design gap between the mainshaft clutch ring (the 36 teeth over which the clutch slides to engage the gear chosen) is, using the General Motors dimensions — the ‘gold standard’, remember — .062″. This is the same dimension we obtain when our new ‘cone corrected’ gears or new old stock General Motors gears are used with our new ‘cone corrected’ 18-091-002 synchronizer ring.

Unfortunately, because the end user community commonly believes that a larger gap is better (it’s not usually, but that’s for another time), most aftermarket ‘reverse engineered’ gears have ‘drifted’ toward higher cones. On these, our 18-091-002 will sit ‘high’ and necessary strut float may be reduced or even eliminated entirely. To accommodate high cones without strut ‘pinching’ aftermarket synchronizer rings have ‘drifted’ toward compensating oversize cones. Observing this is, in fact, what sent us back to the ‘gold standard’ originally.

Conversely, using our WT297-14C (1963-65 design) or WT297-14D (1966-70 design) synchronizer rings on ‘cone-corrected’ gears will reduce the gap between the clutch and synchronizer rings.

So, for best performance, our older ‘pre-cone-corrected’ gears should be used with our old ‘pre-cone-corrected’ synchronizer rings, and our new ‘cone-corrected’ gears should be used with our new ‘cone corrected’ synchronizer rings.

To make this easier to understand and the required items easier to purchase, we will be adding one (1) each of 18-091-002 to each ‘cone-corrected’ mainshaft gear shipped. Kit numbers and pricing to follow.

We apologize for the confusion, but this is the only correct way forward. Thank you for your understanding and continued support.

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