1st gen rx7 question

Can you change the input flange on the front of the diff without changing the gear mesh clearances inside?

I know there's a crush washer in there. Not sure if removing the flange nut, swapping the flange and then reinstalling the nut would affect the gear setup.

This is for a roadrace car, so high stress usage.

Is this ok to do?

I've seen it done with good and bad results.

I've actually had that nut come loose. I torqued it to about 80ft-lb and crossed my fingers.

Yes.

I bought a S model back in '84 (all I could afford at the time), but swapped in a used LSD/disk rear about a year later. The input flange bolt circle was different, so I had to swap in order to bolt up to the driveshaft. Soon afterward, I was stationed in Germany (Army). The car was given plenty of exercise on the autobahn for the next couple of years. About three years after that, bearings started to go, and the diff was replaced with a junkyard part. Again, I had to do the flange change. The diff cocked 100-150k miles following the flange swap. The second diff had 75-100k miles. YMMV

Per the post above, I did replace the nut both times.

Knurled, can you elaborate on the bad results you've seen?

Was it just the nut coming loose or other problems?

Thanks

yes, just note the gear last between the ring and pinion before you swpa the pinion flange. If you over tighten the nut then the lash increases possible causing bad stuff to happen

If a diff uses a crush sleeve, you don't want to over torque the pinion nut, as you will crush the sleeve more, and add too much preload to the pinion bearings. Just don't get nuts- torque to 80 or so foot pounds, and loctite the nut.

Um, the pinion nut must be at least 95 foot pounds. Mazda diffs don't have collapsible spacers so the backlash won't change.

But what does happen: once the flange is installed and torqued, you need to grab the flange and see if you can move it up or down any amount at all. If it does, you will feel this as a 'click'. That means there is no preload on the pinion bearings which is A Bad Thing. Don't confuse this with turning the pinion which tells you what the R&P backlash is. If you feel a click, rotate the pinion several times, then retorque and check again. (This happens on 2nd gens too.) The other thing I have seen: you need to torque the nut, drive the car a day or three, then retorque.

I have also redrilled and tapped a large pattern flange to accept a small pattern driveshaft, that's easy because all 1st gen and N/A 2nd gen diffs use the same diameter locating 'snout' on the driveshaft, the only difference is the bolt pattern.

sevenracer wrote: Knurled, can you elaborate on the bad results you've seen? Was it just the nut coming loose or other problems?

Nut coming loose resulting in the diff self-disassembling after a trip around the block (quite literally). This on a late rearend that someone'd put an early style flange on, and sold before using it.

Same thing happened AGAIN to that same car.

Me, I just use whatever driveshaft it takes. They changed the driveshaft flange for '83, and the axle splines in '84, so there are three different flavors of rearend: non-square (rebuildable!) driveshaft/small axle, square (staked joint) driveshaft/small axle, and square/big axle.

Putting one driveshaft on the other's flange isn't as simple as redrilling, as the pilot register is different.

Well, that didn't go so good.

I bought a new lock nut and swapped the flange, torqued the new nut to 95lbs. Didn't notice anything unusual during the process.

But, now I have a "rough spot" when I turn the wheels accompanied with a loud metal clunk. It happens roughly 6 times per wheel rotation - happens in either direction. It's a 5.12 gear is that matters.

I can't say for sure that the clunk wasn't there before, because I didn't spin the wheels any significant amount beforehand. Last time I ran the car, after my race I noticed noise coming from the rear of the car. Thought I had found the issue as a rear caliper had come loose and was contacting the rim.

Now, I think I am hosed. Based on the sound, don't think I'll take it on the track. I don't have a lot of good options for a different rear end for this weekend.

Is there anything that could have happened while the nut was off that caused this?

Thanks, Neil

Actually found the issue. There are two sheet metal pieces - one on the flange and one on the diff. They were bent and contacting significantly in one spot. A little work with a hammer and a drift - problem solved.

Amazing how things are solvable after walking away and starting fresh the next night.

So, back in business, heading to Savannah tomorrow afternoon.

Have fun!

As an aside, I did the diff-flange swap on my 83 GSL with no issues, including after I sold it and it was turned into an IT track car.