I'm working through a transmission rebuild for a SA and have noticed some differences in machining of the replacement synchro hubs vs the original. The original hubs are machined flat at the same height in the valley between the cogs, the replacement hub is machined in a way where it is flat for 1/3 of the way, and then is tapered down about .005" for the rest of the width with a transition point in between. This is for the 1-2 hub, but I've seen the same for the 3-4 hub. Is this taper machining beneficial? Is this an improvement/updated design?
See pictures below. Look out for optical illusion in the pictures, the arrows point to the valley between the cogs.



Old hub part number 0437-17-261

New part 0820-17-261B. Replacement per Mazda Competition.

 

Interesting observation. I'm no expert in transmissions, but it's possible this is done to help the shift collar engage the synchro more easily, as it has more wiggle room to line itself up with this smaller diameter feature. I could be wrong though.

 

I could see how that might be the case. Hoping someone knows for sure.

 

I'm going to start off by saying I don't know for sure, however I have some insight into what you're looking at from a machinist's standpoint.

The surface finish at the root of that spline on the replacement part looks like absolute crap compared to the part it's replacing. It's quite possible you may be measuring a "peak" on one side and a "valley" on the other.

Does the major outside diameter have any taper?

 

Given that these parts are made in low volume, vs literally hundreds of thousands per year when the car was new, I am going to assume that it's simply inaccurate machining/looser tolerancing, and not look for any deeper meaning.

You ought to have seen the new GSL-SE rotor housings that I bought in 2021. They had numerous casting bubbles that were filled with epoxy as a repair, and one of them had the exhaust insert clocked wrong. I had to extract the insert from an old rotor housing in order to make it work. Very much something that would have not passed inspection for the production line!

(They were definitely not new old stock, they also had rougher looking casting on the outside, and they had QR codes on them)