So I bought a 1.8 Housing and Im almost ready for my RX7 LSD
 

Alright guys, I purchased a 1.8 differential housing, driveshaft, and the half stubs and now im getting the rx7 lsd rebuilt and what not and getting ready for install. Quick question. Will my ring and pinion gear from my 1.6 open differential work with the 1.8 housing? and also Is it really worth it to change it to 3.9 ?

the 1.6 and 1.8 have differnet sized r&ps's. The gears are the weak link anyway.

no it will not work. they are the whole reason we do the swap the ring and pinion gears are way to weak. yes a 3.9 is worth it if you are gonna make 250+whp then you will have longer 1,2,3 gears making driving the car and not hitting the revlimiter easier.

so the 3.9 gear only came on 1.8 models right? All 3.9 gears that i come across should work fine.

see my faq, all your answers are already there.

NA 1.8's had 4.10, NB 1.8's had 4.30 (some had 4.10 and 3.9), only the auto FC RX-7 had a 3.9 ratio, but you cannot use their pinion gear. The only way to get a 3.9 is either get one from a non-US 6 speed, or get a pinion gear from a late model 1st gen RX-7 with the short pinion gears (like an 85) and a ring gear from an auto FC.