S2000 diffs: Why they break, and how to stop it from happening (again)

These last few weeks we’ve been working on finding ways to improve the S2000 diff. I’ve always loved these cars, even though they are convertible and I’ve never been a huge rag top fan. But there’s something about a 9,000 RPM redline that appeals, and combined with what I think is a fantastic looking, light, rear-wheel drive platform, it’s a car that I wouldn’t mind owning.

A customer recently approached me saying that he was having trouble holding his diff together on his track-driven S2000. I did a little searching around and found similar murmurings on online forums around the world. The pinion gear is usually the thing that breaks first, which can cause pretty serious collateral damage, often leaving owners with repair bills in the thousands. People often make the assumption that the gears are weak, but after a fair bit of investigation, I think that is misinformed – although there’s always room for improvement, particularly through Isotropic Super-finishing (ISF), a service that Neat Gearboxes are proud to offer.

Presently there’s only a few (fairly unattractive) options for S2K owners in Australia and the rest of the world who want to put more power or traction through their diff:

  1. Swap the Honda diff out for known strong options such as a Nissan 300zx or Ford 9” rear end. The mounting issues and cost is heavy, and so is the extra weight. It’s also not Honda, which irks purist owners, understandably. It is a flagship vehicle, after all.
  2. Send a core to the States for the Comptech bearing cap upgrade, or have one sent here for the cost of the work plus a core charge. Freight/customs is a headache on a used item. Those boys clearly do top work, and only a few of their diffs have failed in high power applications. But to be frank, looking at their design, we here at Neat Gearboxes started to wonder what, exactly, a bearing saddle does (more on this below).
  3. See the diff failing every now and then as cost of ownership. Just put a wrecker one in whenever things go bad. Not only is this expensive, it’s also potentially very disruptive. You may miss out on a track day race, or your daily drive could be off the road for a considerable amount of time while you change the diff over.

We decided to do some investigations ourselves, to see where things go wrong. After examining the case, we see a glaring weakness in the S2K diff: The bearing cap fastener bores measure just over 10.5mm, while the fastener shank measures just under 10mm. This leads to the cap being able to laterally “walk” over 0.6mm – see the picture.

Image

What this means is that the bearing cap depends wholly on the under-head tension of the bolts to “stay put”. Heat, which causes material expansion, and the lateral forces from a helical gear set driving themselves apart, both make this a tough ask for those two M10 bolts that have to stop the diff centre from exiting the rear of the vehicle. As soon as the caps come out of alignment, so does the carrier bearing. Once that happens, the gears are out of alignment, and, BOOM! It doesn’t matter how strong your gears are – if they come away from each other enough, they’ll eat each other in a hurry, and the smaller guy (the pinion) always loses.

Taking an example from engines, the main bearing caps for the crankshaft are not only held down by bolts or studs, but located by dowels so that the bearing caps can’t go walking. So, we’ve decided to come up with a way to accurately and cost effectively introduce bearing cap dowels and stop those pesky bearing caps from walking away.

Stay posted, more to come.

And remember, to get you up and running we have repair kits and final drives in stock. Bearing kits start from AU$249, and the 4.77 gear set is just AU$429.

See http://www.neatgearboxes.com.au for details.

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One Response to S2000 diffs: Why they break, and how to stop it from happening (again)

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