RB25 Gearbox Swap

[sailaholic]

I thought the rx7 gearbox was the same as the mx5 5 speed but with some bigger bearings?


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[corners]

This is the fault with the S15 boxes, explained to me by my mechanic:
Issue is with thrust washer are that when you shift in to 4th or 3rd pressure is put on the grove and reverse gear on the shaft causing the washer to climb out of the groove and chip the reverse gear. This allows the washer to separate and move around in the groove. When this happens it fails completely and just moves around which is not great.
Options to repair that he was against was welding to the shaft as it could weaken the material but would stop the thrust washer moving around. You'd have to weld on the reverse side. He also said if you don't want reverse he could have a solution.....
He said straight cut gears would put bugger all load on the thrust washer and that the nismo stuff could possibly help as the gear angles reduced. Also food for thought would be getting a better circlip setup so there's less movement under load. Another option that would help significantly would be machine a couple of mm off the shaft and getting a custom thrust washer made so it sits deeper. This is not recommended as it weakens the shaft and the coating is damage.

[hks_kansei]

Sounds like a wear and tear thing.

The thrust washe gets old and with years of wear/heat it allows more movement to the point that parts clip each other.
I'd think that replacing/upgrading the washer should do plenty.

And yeah, helical gears tend to try to force each other apart sideways, unlike straight cut which just tend to try and climb each other.


5 Speed Lada Nivas have a similar issue with 5th gear, the gear faces don't fully mesh (instead of being 100% face contact it's below 50% if I recall)
Under load the helical gears force each other apart a bit, and if the load is high enough they begin to only mesh on the ends of the teeth, where they then snap the teeth off and destroy 5th.
Granted, it's also worsened on that box since the oil level is below 5th, so 5th tends to also run pretty dry and unlubricated. (which is why you shoudl over-fill the box on a Niva and never engage 5th below 90kph)



So, the moral here is that if you want strength in a gearbox, maybe don't use the box from a Lada.

[corners]

I think its a lot to do with the additional power and torque as I don't imagine Nissan had the box designed to factor in a 50% increase in the factory output

[hks_kansei]

I think its a lot to do with the additional power and torque as I don't imagine Nissan had the box designed to factor in a 50% increase in the factory output

Sorry, I thought you meant it was an issue on S15 boxes in general (stock or not)

I can see how putting a decent chunk more power and torque into a box might cause issues

[corners]

I think its a lot to do with the additional power and torque as I don't imagine Nissan had the box designed to factor in a 50% increase in the factory output

Sorry, I thought you meant it was an issue on S15 boxes in general (stock or not)

I can see how putting a decent chunk more power and torque into a box might cause issues

Haha it only seems common on s15s when people wind the wick up more.
Its very rare for standard s15s to break.

[hks_kansei]

Lol, it's vary rare for a standard S15 to exist at all these days.

[corners]

Another common swap these days is Z33 to retain 6 speed but I don't know how hard of a job that would be in an mx5

[lightyear]

I wonder if very solid engine mounts and a gearbox crossmember would help the 6 speed.
Would be good if there is an economical way to make a high horsepower MX5 be reliable.