Nissan Silvia S15 6-Speed gearbox “Circlip Mod”

We’ve been working with a few customers over the past few months to come up with a solution to prevent major gearbox failure in the S15 6-speed gearboxes.

Essentially the issue is the same every time: more-than-stock torque loads put excessive lateral force into the 4th gear “circlip” (actually a pair of spacer C clips retained by an outer snap ring) which causes it to chip the retention groove. This leaves 4th gear to freely float across the countershaft and inevitably come out of alignment with it’s drive gear and causes major failure.

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Even mildly modified vehicles are susceptible to the problem. In reality it is a design problem rather than a metallurgical one.

We’ve come up with a modification to ensure a stronger countershaft, and is an essential modification whenever one of these gearboxes is pulled apart.

Firstly a brand new genuine Nissan countershaft is machined to make the retaining groove deeper. As can be seen in the photos, the groove goes to the depth of the root of the teeth that it is cut through, and no deeper, to ensure that maximum shaft strength is maintained.

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The new shaft is then shot peened to ensure that any stress risers from the grinding process are eliminated and to put a compressive stress layer throughout the shaft to prevent crack formation from occurring.

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A new set of C clips is custom machined to match the new groove (lower set), giving about double the thrust surface of the original thrust shims.

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You can use this with the original retaining snap ring, and everything is “factory fit”. This system is far superior to welding the shaft and takes into account relieving stress in the countershaft after machining.

This mod is developed and sold exclusively by Neat Gearboxes for $849 including a new shaft, shipped to your door anywhere in Australia. Alternatively we can modify your existing shaft as long as it is undamaged. The price for this is $495 including crack testing and chemical hot tank cleaning. Other associated parts are available, such as rebuild kits, 4th gear replacement and so on – please contact us for any further info or if you’d like to place your order: info@neatgearboxes.com.au

We of course offer this mod in the context of a full rebuild, too.

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