[Roger Bettoni (Roger)]

This has probably been asked before, but here goes anyway.
How do you tell what case type you have, if you don't have the tag on the diff or the build sheet, and is this a crucial factor in changing gear ratios? I'm assuming mine to be the less strong (not the 489 case - what's the other one?) as it's behind the original 318/904 combo.
Why I ask, is that I want to transplant some higher gears into it, to replace the 2:73's which I find sluggish at best (I assume that is what's in there now). I would like to go for 3:23's, 3:55's or 3:91's. Is this at all possible? Any advice readily accepted.

[Andy Neal (Tweety)]

there's a casting number on the casing itself Roger, just use the last 3 numbers of that to identify the case you have '742' '489' '741' etc
Then you'll need to get the gears to suit the case style you have, they are cheap enough from Summit and those guys. MR. Robson is fitting some as we speak so he'll be able to fill you in on exactly what's involved.
If you have about a 28" tall tyre then the 3.55's are just about a perfect compromise between acceleration and top end

[Dave-R (Roppa440)]

Yeah. Andy is spot on. The casting number is on the drivers side of the pinion near the UJ. To check what you have in there first jack the car up so both whels are off the ground.

Rotate one wheel. If the other wheel rotates in the opposite direction you do NOT and a LSD (suregrip) diff.
In this case drop one wheel back on the ground to hold it still and count the number of times the driveshaft rotates for every complete rotation of the wheel. Just under 2.75 turns will be the 2.73 gears, just under 3.25 turns will be 3.23 gears etc. 3.23:1 is by far the most common.

If both wheels rotated in the same direction when you spun them off the ground then you have a LSD (suregrip) diff. In this case you should NEVER rotate one wheel with the other on the ground as this wears the LSD. You can just rotate both wheels as above to find the ratio.

No matter which diff you have they are all very strong and you can get a good selection of gears for them. Mine had 3.23 gears from the factory. I fitted a set of 3.91s and just this weekend a set of 4.30s.

[Dave McBride (Bluewhale)]

Remember that for an open diff you divide the turns by two. i.e. 5.5 turns will be a 2.75 gear. For a posi 2.75 turns will mean 2.75 gears HTH

[Dave-R (Roppa440)]

That is why i said put one wheel back on the ground if it is an open diff! Then you can directly counts the turns as on a lsd/posi/suregrip.

And don't use that "posi" term on a Mopar board young man!

[Roger Bettoni (Roger)]

Thanks for that guys. I'll check it out today and let you know what I find. No doubt I'll be posting again for advice when I install new gears too.

[Dave McBride (Bluewhale)]

See, I've learnt something new already, cheers Dave. Wish I had known that a year ago when I was trying to find out if the gears in the axle I had bought were what the bloke said they were

[Roger Bettoni (Roger)]

Okay - here's my news. I definately have an open diff (this i knew anyway) and the casting # is 741. My wife did the rotation of the wheel while I held the opposite one firm and pre-marked the uni-joint with white-out to see the number of rotations. We did this several times as I could not quite figure out if I was doing something wrong, but only counted about 1.7 turns of the drive shaft for each revolution of the wheel. Did Chrysler make such a tall gear ratio? Am I stupid or something?

[Dave-R (Roppa440)]

Mmm. Maybe the holding one wheel still trick does not work after all? Try it Dave McBs way and let the wheels spin in opposite directions and then divide the result by two.

[Dave-R (Roppa440)]

Did you ever sort out this problem Roger? It would be nice to know the result.

I just realised I have a good picture of the diff casting number. Here it is. You can see this is a '489' casting which is the 1 7/8" pinion with the crush sleeve.

[Phil Raynor (Philth)]

Dave, reckon you need a few more bolts on there...

[Dave-R (Roppa440)]

A gasket might be nice too!

Once I get my propshaft back (driveshaft) I have to check all my angles again before tack welding the axle to the spring mounts. Then the whole thing has to come apart again for full welding and a drop or two of new paint on the housing.

[Phil Raynor (Philth)]

What rear UJ straps will you be using, Dave?

[Dave-R (Roppa440)]

The stock straps that have been on the car for 33 years mate.

I know the accepted thing is the strengthen this area. But my propshaft bloke who is making my new shaft said something which made a kind of sense to me. He said the prop (and the rear uj area in particular) should be the weak part of the chain. Which would you rather have go on the start line at the track? The trans? The diff/axle? or a cheap part that is easy to get replaced anywhere?

He said the driveshaft should be considered as a fuse in an electrical circuit. I thought that was interesting.

However it could be that as he is in the drivshaft repair business he is just looking after his trade!

[Dave McBride (Bluewhale)]

Hmm, that's a good thought dave, however if youre putting out say 600hp and all the drivetrain can handle it, then so should the prop and connectors, otherwise you'll be under there every other month fixing it ...I suppose. Just my opinion.

[Dave-R (Roppa440)]

I think his point was that it should be almost up to the strength of the other parts but not quite. How you would quantify that I do not know. Trial and error/experiance? Sounds expencive to me.

I will keep an eye out for a spare set of straps etc. I am replacing the bolts anyway and will make sure they are up to the job (and I will get a few spares!)

[Roger Bettoni (Roger)]

Still haven't determined the gears in my diff. I asked another fellow who rebuilds diffs and he said to open it up and have a look inside for numbers. My casting number is 741 and I'll try counting the number of turns of the wheels for each rev of the tail-shaft to see if this works, once I have the back-end together again (hopefully tonight) and let you know what I find.
Apparently there is a code on the tag that lets you know what was installed at the factory - anyone heard of this?

[Dave-R (Roppa440)]

There is not a code on the fender tag. There is a code on the build sheet however. The factory also places a metal tag with the gear ratio stamped on it under one of the diff nuts. This tag is usually missing.

You can't just "open it up" easily either because you have to pull both axles out first and then remove the whole diff from the housing. But yes, counting the teeth on both the ring and pinion and them dividing them into each other will give you the exact ratio. Most people just spin one wheel and see how many times the prop rotates. My guess is it is a 3.23:1 ratio. In fact I would put money on it.

[Roger Bettoni (Roger)]

I have not found my build sheet "yet" and will make a concerted effort to find it one day. I am told that they can be in one of several places: behind the rear seat, above fuel tank, behind front seat etc. A fellow enthusiast told me that they usually put this in a place where they install any factory option ordered by the buyer. In my case it could be behind the bucket/bench seat panel/s as my car was optioned with them. Anyone heard of any other places they can be found?

[Phil Raynor (Philth)]

I know someone once said that the build sheet could be found tucked away on top of the glove box...

[Dave-R (Roppa440)]

They can be above the glove box liner, under the carpet etc. In fact when the car is built there is usually a few sheets in the car. It is usually the hard to get ones that survive. Different production lines tended to put them in different places.

[Joel Bennett (Joel)]

I thought that the standard gear ratio for the base line Challenger, slant six and 318,was 2.76. I'm I wrong or just miss informed?

[Jeff Maxwell (Redchalleng]

From the build sheet of my original 1971 318 car was the D31 904 AT and D51, 8.75" 2.76:1 Rear Axle. My guess is this is the correct one on this car since it's a 318. So Joel, I think you're right.

[Dave-R (Roppa440)]

I think you could opt for a 2.76 or a 3.23. I could be wrong though!

[Roger Bettoni (Roger)]

This has motivated me to search for a build sheet. My car still has the original glove-box liner and carpets too, so this will be my first area of investigation, then wherever is easiest.
Judging by the tone of the motor (no tacho) at cruising speed, and lack of acceleration, I tend to agree with Jeff and feel I have 2.76 gears, but the build sheet will prove the fact when discovered.
I'll also try the wheel turning exercise again too ..... when the sun comes up...

[Jeff Maxwell (Redchalleng]

Owner 4 was happy that he found most of the build sheet under a seat. I told him that I had another I found behind the back seat which I had forgotten about. Why it was back there I'll not know, but when I replaced the astro turf that graced my car when I bought it, it was behind that rear seat just ahead of where the top stores. Yes, my red car had green astro turf when I got it! But it wasn't door to door. It may have won the ungliest color combo - red body showing under spots of green turf!

[Phil Raynor (Philth)]

Did it have a penalty spot, too?

[Duane Phillips (Duane)]

My 318 Challenger has 3.23 gears, verified by a tag bolted to the 8 3/4 differential. My car originally came with a 3 speed trans and 2 barrel carb (now replaced with a 4-speed and 4-bbl). I do not have a suregrip, though that is a plan for the future. I may eventually go to a 3.55.

I have 2 build sheets; one the owner handed me. The other I found under the back seat, a location I read was likely. The 2 sheets are identical. The one from under the back seat is in rough shape though, with moisture damage and rust stains. Of course, the back window leaks.

Where would one find the rear gear ratio on the build sheet? Is it a code?

[Jeff Maxwell (Redchalleng]

Your build sheet would then show a D53, which is that 3.23 instead of mine D51 the 2.76. Yours is either a 8.75" or 7.25" axle, as the decoder listing the codes shows that both were on E bodies in 70 and 71.