Problems with 1961 rear differential...need advice

[par4tom]

I've looked through some of the other threads, but can't find the correct starting point.

I have a 1961 Chevy truck, that has an L48 out of a 67 Camaro Supersport. Anyway, the truck still has the SM420 tranny and the original rear differential with posi...I think it's a 4.11 ratio.

I"m looking for what I should do, either to the tranny (less preferred) or to the rear differential (More preferred).

The truck makes about 330hp with a mild RV cam, and I'd like to use the truck occasional to pull a small travel trailer, but I'm concerned about getting the gearing wrong. I've seen some guys change the rear ratios, but opinions seemed varied.

Again, I'd like to keep the tranny (I realize 1st gear is of little use) if that is possible.

Would like to have some decent pulling power, but more importantly, be able to cruise down the California freeways doing 70, without the motor being so wound up. Can somebody help me with where to start?

TC

[Juspassinthru]

I had mine changed to a 3:07 but mine is a 250 motor and a 3speed, most of my driving is hiway commuting and I don't pull anything .I think a 3:56 gear or a 3:42 would work well for your application based on the info you have given, also taller tires help as well in the overall equation.
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[m455sx]

believe it or not put in a 700r4 and a tall rear tire you can have overdrive plus towing.

[par4tom]

I want to keep the clutch...the taller rear tire is a great idea though...

T

[MusicMan70]

http://www.classicjunkyard.com/new_process_a833.htm

http://www.customclassictrucks.com/t...n/viewall.html

[61_FL_Apache]

I have a 3.08 with 215/70/15 tires and the rpm is around 2800-3000 at 70.. Too fast for me. Im planning on a taller tire next time.

[par4tom]

Hi 61 Fl Apache,
What rpm range would you target for 4th gear? The 3.08 gear ratio sounds easier to change, but Music Man sent me a link on another gm transmission that looked really interesting.

[Captainfab]

The gear ratio you likely have is the 3.90's. That seems to be the most common in the Eaton HO32. The only other gears available are 3.38's. These are available from the aftermarket. I believe Yukon Gear makes them. You can find them at Randy's Rind and Pinion, and possibly Toms Differentials.

Depending on your tire size, you can go with a taller tire, or maybe an OD trans as suggested above.

[61_FL_Apache]

2000 in 4th gear at 70mph.
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[MikeS.]

Hey par4tom, welcome from an old Fresno boy. I'm sure missing the fog right now, it's better than 15* and snow.
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[Juspassinthru]

61flApache, what eng and teams are you running?
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[61_FL_Apache]

305 V8 and TH350 tranny.

[LVPhotos]

How about a Gear Venders od? I run a T5 out of Camaro .65 od 3.73 posi with 295/45/18 rear or about 28"
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[par4tom]

Thanks for some good ideas. I'm thinking the starting point here is going to be the transmission. The truck is already apart and the sbc removed. I'm going to hunt around for that early 80s 4 speed... I ll have a pristine sm420 for sale here soon in California. Hello to the other California fellas here...I hope to be on the road within the year! May take the 61 for a drive to see Captain Fab up on Idaho... Beautiful country!
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[Brad54]

I went through all of this with my Suburban. Had a Granny 4spd and Dana 44 rear.
I found all the gear ratios for the trans, and made a chart with different rear gear ratios using the tires I have. I narrowed it down to what gear ratio I wanted with that trans.
The Granny 4-spd is, for all practical purposes, a 3spd... you don't use 1st unless you're towing.
So going with that, your final drive ratio will be 1:1... you'll use 1st to get off the line while you're towing.

I ended up with a 4spd manual OD trans, with .73:1 final drive, and 3.42 gears, because I had one of those transmissions.

I think with the Granny, you could run 3.23 or even 3.09 gears, treating it like a 3spd, and still be able to tow with it.
Ask yourself this: What will you be doing more--driving it with or without a trailer?

Now, to put one more fly in the ointmnet, if you swap gears, you will probably have to get a new differential carrier, because 3-series gears don't fit on 4-series cases, low 3-series gears don't fit on mid-3 series cases, etc. A new dif is going to be about $400, plus the rebuild kit, plus the gear set. You're looking at $700 or better, plus having it set up.

-Brad

[par4tom]

Following up:

Gentleman, thanks for the earlier advice. I've decided on putting in the NP440 transmission and now am trying to source one out. Everyplace I've called has sold them already.

I've got a friend in Fresno, who runs a transmission shop and can get me one completely rebuilt for $1,100 bucks with 6 month warranty on his work. I'm leaning this direction. It has the aluminum case and is a GM product, not the 833 Chrysler one.

Currently, this truck has a 350 with the original sm420, and the bellhousing holds the hydraulic clutch as well as the rear drivetrain mounts (I have the old horseshoe motormount in the front). The engine and bellhousing are out of a 67 Camaro SS.

Question 1

Is the SM420 bellhousing bolt pattern the same as the Np 440?

Question 2

Is the sidemount shifter going to clear the stock bench seat? The SM420 is a top mount. (I understand I may have to do some work on the transmission hump cover...

Question 3

I've gone to the trouble to pick up Captain Fab's brake and clutch kit and have it installed, can I use the hydraulic clutch with this transmission? What other problems am I going to have to deal with to get this transmission in?

Thank you for the input...it is a lot of fun working with everyone here.

Tom

['63GENIII]

Hey Tom, I'm not sure how difficult it would be to put an NV3500 in there? I just put in a 4500 and it was fairly straightfoward and the overdrive is awesome. The 3500 is a 1/2 ton unit and would be worth looking into.

[MusicMan70]

Following up:

Gentleman, thanks for the earlier advice. I've decided on putting in the NP440 transmission and now am trying to source one out. Everyplace I've called has sold them already.

I've got a friend in Fresno, who runs a transmission shop and can get me one completely rebuilt for $1,100 bucks with 6 month warranty on his work. I'm leaning this direction. It has the aluminum case and is a GM product, not the 833 Chrysler one.

Currently, this truck has a 350 with the original sm420, and the bellhousing holds the hydraulic clutch as well as the rear drivetrain mounts (I have the old horseshoe motormount in the front). The engine and bellhousing are out of a 67 Camaro SS.

Question 1

Is the SM420 bellhousing bolt pattern the same as the Np 440?
It should be; here are some pics. Does your friends tranny come with the specific Hurst shifter/rods as well? *Classic Trucks article

Question 2

Is the sidemount shifter going to clear the stock bench seat? The SM420 is a top mount. (I understand I may have to do some work on the transmission hump cover...
Maybe and here is a photo of the new shifter location. You may have to change out the shifter arm or cut and weld something; I went to buckets.

Question 3

I've gone to the trouble to pick up Captain Fab's brake and clutch kit and have it installed, can I use the hydraulic clutch with this transmission? What other problems am I going to have to deal with to get this transmission in?

Yes you can use the hydraulic set-up. That's what I have behind my GMC V6. Again reference the article but be advised that some of the captions in the article do not match the photos. Also you will need to address the drive shaft (I had one made-Inland Empire Driveline) and transmission crossmember fabbed up (Photo from Ebay vendor)and turn down the front main bearing on the trans to match

[Captainfab]

I'm pretty sure the bolt pattern on the NP440 is the same, but the issue you mak have is the diameter of the front bearing retainer. The front bearing retainer needs to fit snugly into the bore in the bellhousing. The SM420 trans and corresponding bellhousing have a diameter of 4-13/16". Starting in 1968, when GM went to the SM465 trans, the diameter of the front bearing retainer and bellhousing increased to 5-1/8". I do not know if the NP440 is the larger or the smaller diameter. I am thinking it is the larger 5-1/8". I'm not sure if it is possible to turn down the larger bearing retainer to 4-13/16". You may have to change the bellhousing to a '68-'72 bellhousing. Then you will need a bracket to mount a hydraulic clutch slave cylinder to the new bellhousing.

[hop sing]

Originally Posted by par4tom

I've looked through some of the other threads, but can't find the correct starting point.

I have a 1961 Chevy truck, that has an L48 out of a 67 Camaro Supersport. Anyway, the truck still has the SM420 tranny and the original rear differential with posi...I think it's a 4.11 ratio.

I"m looking for what I should do, either to the tranny (less preferred) or to the rear differential (More preferred).

The truck makes about 330hp with a mild RV cam, and I'd like to use the truck occasional to pull a small travel trailer, but I'm concerned about getting the gearing wrong. I've seen some guys change the rear ratios, but opinions seemed varied.

Again, I'd like to keep the tranny (I realize 1st gear is of little use) if that is possible.

Would like to have some decent pulling power, but more importantly, be able to cruise down the California freeways doing 70, without the motor being so wound up. Can somebody help me with where to start?

TC

if u have a posi correct 61 i'll buy it ,,does ur rearend look like a 9' ford ie 3rd member style? let me know thx hop

[par4tom]

The rear end is not the original 61 rear end, it appears to be out of a 64 or 65. Interestingly, my dad told me he bought if from a Mr. Frey here in Fresno (he's deceased) who was the father of Amber Frey, the girl that was involved with Scott Peterson (Peterson was the guy who killed is wife and dumped her in SF Bay).

Not sure why I shared that but sometimes the story is better than the rest.


Anyway, I followed up with the guy on the NP 440, and it is out of an 82 4wd, he is going to rebuild and remove the transfer case. I guess the transfer case goes with the sale so if anyone is looking for one of these, let me know.

Does this mean this transmission casing is cast iron.

Also, I have found out that the search for answers is more robust usine MY6 in the search rather than NP440.

At this point, I am going to pull the truck out of the other shop building and look at the bolt patterns here. Thank you for the photos, they were very helpful. I will reciprocate in kind as I go forward on this project...

[Captainfab]

Since that trans is a 4x4 version, is the builder going to change the output shaft and add the tailshaft housing, so you can use it in a 2wd application?

[par4tom]

I posed these questions to the builder. He is going to call me tomorrow with the answers. Including the size of the retainer. We think it has the mechanical speedo...

Regarding the transfer case, he is going to pull that off and put on the correct tail housing, go through the transmission completely, replace all the seals and other problems (if any). He is going to throw in the existing bell housing and the transfer case at no charge. I'll have to get rid of those. It looks like the bolt pattern is the same as the 420.

This is a cast iron transmission, with the heavy shift arms. Something I wanted. No shifter. I'll have to fabricate a bracket (or find a complete shifter) somewhere else. I'll also have to fabricate a cross member.

I suspect I'll have about $1,600 to $1,700 in the entire deal...but a truck that should be a good runner!

I do have one question...the rear end is a 12 bolt posi, out of a 64 or 65. Can someone tell me what the most likely gear ratio would have been? And...if I regeared, how low I could go?

[Captainfab]

If you stay with the original bellhousing or one like it, you will not need a trans crossmember. The trans will hang off of the bellhousing just fine. Now if you change the bellhousing to one that does not have the mounts built in, then you will need a trans crossmember. Of course you will then also need to change your motor mounts to side mounts instead of the front saddle motor mount. Otherwise there is nothing to keep the engine and trans from flopping around.

In my experience the most common gear ratio in the 12 bolt in these trucks would be the 3.73's. I don't know why you would want to go any lower. If anything you might go a little higher to the 3.42's. I would just drive it as is before changing the rear gears.

[MusicMan70]

Post #18 shows one that's been turned down.