Ive worked on VR4, RVR, GSR, EVO I, EVO II and EVO III gearboxes over the last couple of years and found no difference between the tooth helix of the centre diffs between RVR, GSR, EVO I, II and III's.
The EVO III gearboxes I worked on were EVO III GSR with viscous LSD, and EVO III GSR without LSD. I have not worked on an EVO III RS gearbox yet.
I can tell you that there is definitely no difference between GSR and Evo I-III centre diff tooth helix.
Even Mitsu parts database lists the centre diff part numbers for the GSR and EVO I-III as MD741919 = one part number for all of them.
Now part numbers aside, think of another physical and logical way of proving that the GSR, EVO I-III all have the same centre diff housings or more importantly, tooth helix.
The centre diff lives inside the gearbox and its teeth mesh onto the intermediate shaft.
As far as the actual intermediate and input shafts go, the intermediate shaft from the GSR, EVO I-III are all the same.... its only the input shaft that differs between GSR, EVO I and EVO II/III boxes.
Therefore if the intermediate shaft is the same between GSR and EVO I-III, then why would the tooth helix on the centre diff of the EVO III be any different when they are all mating to the same intermediate shaft.
I dont know what experiences Blackvr and Brisvr4 were having with their gearbox builds, but it shouldnt have been the case unless there was a difference in your early VR4 gearboxes that doesnt mate to GSR/EVO I-III internals.
Im concerned that Blackvr said about the EVO III centre diff didnt fit onto the EVO II gear clusters because EVO II-III gear clusters are the same.
The differences between EVO II and EVO III gearboxes are that the EVO III front diff crownwheel has 63 teeth and the EVO II has 64 teeth. EVO III also has the angle of the roll pin on the reverse/5th gear shift fork on a slightly different angle, thus requiring an EVO III specific 5th/reverse shift fork.
But there is another difference also... early EVO III boxes were identical to EVO II boxes apart from the crownwheel of course. This means that early EVO III boxes possessed solid cast steel selector forks for 1/2, 3/4 and 5th/R same as the EVO II boxes, but the EVO III still required its own specific 5th/R fork due to the different roll pin angle as mentioned above.
Then sometime during production Mitsubishi commenced producing the EVO III gearbox with the updated selector forks with nylon pads. Probably from lessons learned from the EVO II and early III competition cars as the solid forks may have been producing excessive heat > friction on the hub sliders and wore out prematurely under extreme operating temps
. Im thinking if there is a difference it'll be with the VR4 box or a certain run of VR4 boxes.
