how many teeth are on a 3.90 ring gear? i have a few ring gears laying around and want to make sure i'm going to use the correct ring gear. also, how many teeth are going to be on the pinion gear that matches teh ring gear?

 

any thoughts???

 

anyone know?

 

I think a PM to wacky would answer your question.

 

i'll shoot him a pm

 

pm sent to wacky...

 

Post what he says, so the others searching can find out.

 

i'll definatly do that once i hear back from him

 

wacky said this "11 pinion and 43 ring = 3.909. But you have to remember that the early 84 had 3.933 also. I will double check these #s with my extra gear sets."

now last night i did a little math. i counted the teeth on the ring gear and the teeth on the pinion gear and taught myself a little something. what i did was divide the number of teeth on the ring gear i had laying around by the number of teeth on the pinion gear i have to go with it and what i got was 3.909091. the ring gear had 43 teeth and the pinion gear had 11. this supports what wacky said 100%. a big thanks to wacky to taking the time to respond to my pm! thank you. so that leaves me with some knowledge and something that i taught myself last night. i'm no professional mechanic, just a guy who has been working on 7's for 10years that just happened to stumble upon some really neat info. and with the help of wacky( based on his rotary knowledge i believe he may or may not be a decendent of kenichi yamamoto himself), was able to support my fluke finding.

 

So.. out of curiousoity, I wonder what the difference is between the 3.909 gears and the 3.933 gears...

Are less teeth on the pinion more desirable (stronger)? or is the difference negligable due to the fact that the teeth on the ring will change as well...


Just the other night I was flipping thought the competition manual and I noticed that a 4.625 gear ratio was possible.

The manual shows sevral combinations using 8, and 9 tooth pinion gears and 35, 37, 39, and 40 tooth ring gears.

My question is: Are all the pinions and ring gears interchangable, or do you have to have a matched set...
Seems like there could be a lot of different gear ratios available without buying the ring and pinion set. I would be very interested to know what is interchangable and whats not.

I know there has been talk of fine and coarse tooth gears but nothing has ever really made sense to me.

I am very interested in the subject, hopefully the discussion will continue here, but if there arent any responses I will create a new thread.

Just to peak your curiousity, here is a list of the KNOWN combinations possible:

Pinion - ring - ratio

11 - 43 - 3.909
? - ? - 3.933
9 - 37 - 4.111
8 - 35 - 4.375
9 - 40 - 4.444
8 - 37 - 4.625
8 - 39 - 4.875
? - ? - 5.125


Im interested to hear what you guys have to say!!! Someone can make sense of this all, I know it...YOU ARE OUT THERE SOMEWHERE!

 

I hate to bump this but I just stumbled upon an amazing gear set on accident. If you can inter change ring and pinion then I found another turbo friendly gear set. If the FC NA ring and pinion is the same 7" with the same axels as the 84-85 FB correct. well the 12a GSl rear comes with a 11 pinion and 43 ring giving it a 3.909 rear end. Well the FC uses a 10 pinion and 41 ring for a 4.10 r&p. So use the FC ring (41) on the FB pinion (11)and you get 3.727 gear set for building mad boost. Just thought I'd share that info. Now I may be wrong cause I have heard that the NA FC is actually a 7.5" ring, but I don't think it is. Someone correct me if I'm wrong.

 

thanks for the info!

 

If you can inter change ring and pinion



you can change ring and pinion but not just one.

the set must stay together or ka-booom

 

the FSM has the tooth counts in it.

the 85 book lists 11/43 for a 3.909 12A and 13/53 for a 4.076 13B

 

For FB, but I was talking about a FC rearend I had.

 

its in the FC book too, thats actually how i noticed it. i was going thru the car chapter by chapter, and i saw the tooth counts in the book. i was thinking it was kinda useless info, but who knew?

btw i think the miata guys buy a 3.7 R&P from somewhere.

i dunno how good an idea it is to take 2 used non matching gears and slap em together, although being a mazda it would prolly work.