Transaxle transmissions - Servicing

Transaxle transmissions are a standard in modern vehicles, as many cars today are front wheel drive.  We did our work on a transaxle transmission from a Mazda 323.  It's fairly old, and gives us a very good view of what a typical transaxle manual gearbox would look like.

Before we dismantle the gearbox, we need to take some pre-checks, as this job can take plenty of time and there will be blood.  Check for missing parts, mainly bolts, nuts, etc.  check the free rotation of the input shaft, check the free-play of it and notice anything that may be wrong.  Use the gear shifter shafts to change the gears, and see how smooth it is.  Make sure it is locking into each gear.  Check if any bolts are missing, they could compromise integrity.  Check the side ways movement of the shafts.  See how easily the selectors engage a gear. We found there were a few missing bolts and a fair amount of free play in the gearbox.  Otherwise, gears engage fine and smoothly. (aside the fact that the gearbox is not in the car anymore).

Dismantling the gearbox can be a long procedure, and I recommend that one person does it with full focus of what they are doing.  We need to make the habit of organizing parts as we remove them, and keep a memory of what everything is and where everything goes - otherwise there can be left out parts and may have to disassemble the gearbox again while in the process of assembling it.  I took plenty of pictures just in case.

We made some calculations of the gear ratios.  We know that the 4th gear ratio is 1:1, so we can leave that one out for now.  This is done easily by counting the number of teeth on the 'input' gear, (gear which is on the input shaft) and counting the number of teeth on its partner gearbox. For example, input gear has 30 teeth, output gear has 10 teeth, this gives a gear ratio of 3:1.

We then checked the clearances of the syncro rings.  This is done by using a feeler gauge and inserting the biggest one possible between the ring and the gear.  We would need to refer to specifications to see if it passes, other wise if it is crunching or grinding during gear changes, it will need replacing..

We also checked the thrust clearances, which is the clearance between the gears.  It is fairly hard to explain, so I have asked my friend YouTube to help me explain it:

Here is the procedure that we took when disassembling the gearbox.  (of course it's a lot harder with gearboxes that have come out of a car, I've done it before, about four of them)

A friend actually got to me a while ago, was complaining that the car he had just built was not moving while in gear, and he narrowed it down to the gearbox.  The car would not move at all when a gear was engaged, while engine off.  I had figured that the shift lock system might have an issue, and that two gears were engaging simultaneously.  I do not have much experience with repairing gearboxes but I could only imagine that the issue might be that.