I'm using a Mopar 8 3/4 rear diff in 1974 Triumph Spitfire hot rod project. The case has the number 657 on it and I am told it is the same as the 741 case. The gears in it are apparently 3.91 ratio (or at least I was told so)but are very noisey and due for replacement. I would like to change them out but I've searched and do not see anyone who sells a 3.91 ring and pinion for a 741 case. What are my options? Does anybody sell a 3.91 for a 657 / 741 case? What would the 657 case have come in? Could they have been 3.91 from the factory or did someone actual make them at some point but they have since discontinued them?

If all else fails I know I can change out to a 489 case without much issue or I could cough up the big dough for a new aluminum 742 case. Anybody seen any really good deals on the aluminum 742 case? I can by it local (Canadian) for about $1100 but then I also have to add the costs of a new ring and pinion and related parts.

Thanks for the help.

 

741/742 and 489 cases are all i know. all of them should be availiable down to 4.56 ratio factory, and im not 100% sure but the "pumkin" should bolt rite in any front loading rear housing.

 

I know that I can swap in any series carrier into the 8 3/4 housing so I know I have choices to get my 3.91 ratio but they all mean buying either a 742 or a 489 housing. If it comes down to that I'll buy an aluminum 742 housing and go that way. My hopes were just to use my 471 housing and buy a new gear set.

I've tried adjusting the gears but the gears are worn bad enough it is time for replacement. The bearings are good, but I'll replace them at the same time I put in a new gear set.

I take it that knowone is aware of a 3.91 ring and pinion for a 741 carrier?

 

3.92? I have yet to find that listing for any carrier but that would be fine. I don;t remember the tooth count. It has been a little while since I took it apart and it's now back together. I'll have to double check but I'm sure that when it was originally put in we counted and double checked that it was a 3.91.

 

OG mopar would be 3.91 my local junk yard only charges 50 bucks for a center section. then you only need another 300 or so for new 3.91 ring and pinion plus some blue and xtra crush bearings too get the mesh right.