I'm building an S5 NA motor and I have S4 and S5 parts in my garage. The S4 stationary gears have useable bearings but my S5 set does not. I know the S5 gears are hardened, and I want to make sure I won't damage my engine at 8k rpm with S4 stationary gears before I install them.

Shawn Landers at Rotary Resurrection seems to think it will be fine but I want to double check. I figure Mazda made them hardened for a reason. I am happy to replace the bearings in my S5 set if I need to. To make matters worse, I'm not even 100% confident which set is which and apparently they are visually identical.....

 

Yes they are interchangeable.

 

Yes they are physically interchangeable, but the S5 stationary gears are supposedly hardened by a nitriding process.

 

This is correct. I'm curious whether that's necessary for longevity, reliability, or some other reason when I'm building an otherwise completely S5 engine

 

Even if you do use the better s5 gear you can still damage your engine at 8k. All this info is first google hit.

 

Any rotary can sustain damage at 8,000 RPM if it isn't put together or tuned properly.

Just to clarify, gas nitriding is only a surface treatment. It's not very thick, (maybe a couple thousandths of an inch) and the base material on the stationary gears should already be hardened beneath this nitriding. Chances are that the S5 gears have already worn through the nitriding if they have an appreciable amount of miles on them.

High RPM failures are usually from a lack of proper clearances and a lack of proper balancing. If everything meets or exceeds the stock S5 clearances and/or balancing specs, I'd run the S4 gears.

All that being said, I would be more afraid of the bearings having proper clearance to the eccentric shaft since they're from two different engines. I don't understand why rebuilders are so scared of replacing the bearings in these engines. I replaced mine with new mazda bearings and I clearanced them myself. I have had zero bearing issues after almost 40k miles and countless redlines and 8K RPM rev limits.

It's time consuming to lap new bearings in but it's a very simple, straightforward process.

Edit: please note I can't verify just how deep mazda nitrided these gears. If you want to remove all doubt, run a new set of RX-8 stat gears.