Since my pinion gear recently broke I was wondering the differences in the pinions of rx7's. Do the non lsd pinions have more teeth or is it just the ring gear that is different? Are all the pinions compatible with the rearends as long as the matching ring gear is installed?
In other words, can I use a 3.909 ring and pinion from a junkyard rx7 in my gsl-se?
Are the pinions in the 3.909 the same as the pinions in the 4.10, are there more teeth or are the teeth cut at a different angle?
I do not want to do this, but I have not found any pinions for the 4.10 yet.

 

Ring and pinions are machined as a set. You can probably get away with a pinion from a different but same ratio set but it will most likely never be quiet and will probably wear much quicker. You are better off replacing the whole third member from a good rear end or using a matching ring and pinion (even if used) on your carrier. You would want to do the latter if your rear end came with an LSD and the ring and pinion you found is on an open differential.

Carrier type (open, LSD or spool) has nothing to do with the ring and pinion. From one set of gears to the next for the same application, the lower gearing (higher numericaly) will have less teeth on the ring (where tooth contact area permits) and the teeth will have a bigger pitch (because the ring gear diameter must remain constant and there will be greater load on the gears in the lower gearing so they need to be bigger) and the pinion will have bigger pitch gears to match and have a reduced circumference (to make up the slack from an eliminated tooth). The problem that arises with very low gearing is that the ring gear diameter needs to remain the same AND bigger pitch gears need to be used on the ring gear AND the pinion diameter is reduced (for overall lower ratio) AND the pitch is bigger on the pinion (to match the ring) For any given ring diameter, there is a minimum nuber of teeth you can put on the pinion (ever see a one tooth gear? Or even a 3 or 4 tooth pinion? Never!) so that you have decent gear contact. To go smaller on the pinion (to get lower gearing) you need smaller pitch gears that are not as strong as the bigger pitch and guess what happens to torque multiplication when you lower your gearing? Of course it goes up thus the need for bigger pitch gears. But gear contact goes out the window on lower gearing... you see the dilema?

Let`s look at real life numbers.These are typical gearing selections.

3.909 gears:
-43 tooth ring
-11 tooth pinion

4.10 gears:
-41 tooth ring
-10 tooth pinion

4.56 gears:
-41 tooth ring
-9 tooth pinion

4.88 gears:
-39 tooth ring
-8 tooth pinion

5.29 gears:
-37 tooth ring
-7 tooth pinion

You can clearly see that as the gearing goes down, so does tooth count on the ring. The ring is a constant diameter so pitch (size of gears) goes up to take the slack. In addition, to get the desired lower ratio, the pinion diameter must be reduced. One can clearly see there is only so low you can go for a given ring diameter before the pinion is too small to be able to handle the torque load.

Ok sorry to wander off like that. The answer to your question, if you cannot clearly see it above is no the pinion from a 3.909 is not the same as for a 4.10 set and yes you can use any matching ring and gear that you can find for your application.

Toughguy

 

Thank you, your post has really cleared things up for me.

 

while kinda on the topic can i use a lsd from a gsl on my SE
i think that the SE's axles are large but im not sure and since my cars one leggin like a bitch lately i was wondering if i could grab a lsd from a gsl and put it in while im home on break.