1961 C30 4sp Transmission: swap choices?

[MichaelNicholas]

Hello again.

I drove my new truck today for the first time (1961 C30 1 ton, 261 straight 6, 4 sp manual trans) and found a change I want to make right away: the transmission. As mentioned it's a four speed, 1st gear a granny gear, not worth using in normal driving, 2-4 are very short gears with a top speed of 55mph.

As of now, I'm planning to keep the inline 6, but am wondering if there is a common tranny swap that gets your truck a better range and a top speed closer to 80-85mph.

I'm having trouble identifying the trans based on the charts I have found on this forum and a google search. The Saginaw sounds like it might be common, but my inspection plate on the drivers side has 6 bolts instead of 7 (photo attached).

Thanks
Michael

[Blue666(2)]

Your existing trans might be a Saginaw SM 420. ( https://www.novak-adapt.com/knowledg.../manual/sm420/ )

If you want to keep with the strong hauling/towing abilities of the C-30, a great upgrade would be a NV 4500. It's five speeds with a granny low and an overdrive. I'm not sure of the direct bolt in but it should be pretty close. https://www.novak-adapt.com/knowledg...manual/nv4500/

[andyh1956]

That's not an inspection cover, it's a PTO opening.
Your trans is prolly an SM 420.
Do you want an overdrive? I don't know if a 261 will push your truck 80 MPH. You may need a double over & a tailwind to get that fast!
A Camaro five speed would prolly be your cheapest way to go, it would bolt to your existing clutch housing with a little modification. A NV 4500 would be a better truck trans but you would need an adapter clutch housing of some type to fit it.
If you want better shift ratios a BW T19 would be good w/ a 4 - 1 first gear but your top speed wouldn't change.

[Captainfab]

If you are going to use your C30 as a truck, then the NV4500 would be the best choice. However, the NV4500 bellhousing will not bolt up to your 261. What you would have to do is buy or make a 1" thick adapter plate top adapt the trans to your bellhousing and run a Dodge NV4500. Also the NV4500 as most all OD transmissions have an electronic speedo versus a mechanical speedo as your truck has.

Here is a thread showing using an adapter plate.

http://www.67-72chevytrucks.com/vboa...d.php?t=613597

Another option for you would be to change the differential to a later model such as a 14 bolt or a Dana 60 or 70 with 4.10 or 3.73 gears.

[MichaelNicholas]

Originally Posted by Blue666(2)

Your existing trans might be a Saginaw SM 420. ( https://www.novak-adapt.com/knowledg.../manual/sm420/ )

If you want to keep with the strong hauling/towing abilities of the C-30, a great upgrade would be a NV 4500. It's five speeds with a granny low and an overdrive. I'm not sure of the direct bolt in but it should be pretty close. https://www.novak-adapt.com/knowledg...manual/nv4500/

Originally Posted by andyh1956

That's not an inspection cover, it's a PTO opening.
Your trans is prolly an SM 420.
Do you want an overdrive? I don't know if a 261 will push your truck 80 MPH. You may need a double over & a tailwind to get that fast!
A Camaro five speed would prolly be your cheapest way to go, it would bolt to your existing clutch housing with a little modification. A NV 4500 would be a better truck trans but you would need an adapter clutch housing of some type to fit it.
If you want better shift ratios a BW T19 would be good w/ a 4 - 1 first gear but your top speed wouldn't change.

Nice! I will see what I can do to confirm the SM 420 suggestion! From the photo I just saw, it looks like a match.

I found a reman Chevy/GMC NV 4500 on Ebay and they say "TRANSMISSIONS ARE BUILT TO YOUR VEHICLE SPECIFICATIONS". I'll contact them (or someone like them) to see what they say about mating to a 261.

Thanks
Michael

[MichaelNicholas]

Originally Posted by Captainfab

If you are going to use your C30 as a truck, then the NV4500 would be the best choice. However, the NV4500 bellhousing will not bolt up to your 261. What you would have to do is buy or make a 1" thick adapter plate top adapt the trans to your bellhousing and run a Dodge NV4500. Also the NV4500 as most all OD transmissions have an electronic speedo versus a mechanical speedo as your truck has.

Here is a thread showing using an adapter plate.

http://www.67-72chevytrucks.com/vboa...d.php?t=613597

Another option for you would be to change the differential to a later model such as a 14 bolt or a Dana 60 or 70 with 4.10 or 3.73 gears.

Differential was my first thought, but a mechanic friend didn't think it would make that big of a change. He suggested the tranny change, but I have since read of both approaches to the problem. When I drove this truck on the expressway today the engine was wound all the way up! I was barely doing 55. It seemed REALLY hard on the truck. BTW, it broke down on that expressway on the way home from picking it up. $355 to tow it the rest of the way home . Put a new coil in.

[Captainfab]

How big of a difference a differential change will make depends on exactly what gears are in your differential and what gears to change to.. A C30 likely has 4.57 or 5.14 gears. Changing to 41.0's or 3.73's will make a noticeable change.

Bolting a NV4500 up to a 261 has to do with the bellhousing, not the trans. Unless you are talking about going with the adapter plate as in the link I gave you last night.

Originally Posted by MichaelNicholas

Differential was my first thought, but a mechanic friend didn't think it would make that big of a change. He suggested the tranny change, but I have since read of both approaches to the problem. When I drove this truck on the expressway today the engine was wound all the way up! I was barely doing 55. It seemed REALLY hard on the truck. BTW, it broke down on that expressway on the way home from picking it up. $355 to tow it the rest of the way home . Put a new coil in.

[61K10]

find a clark 5sp trans with the (over drive) 5th gear. They don't shift real well but should bolt up. might have to change the clutch disc?

[andyh1956]

There was a company down in Lumberton or Purvis Mississippi that made adapter clutch housings, transfer case adapters & output shafts for the Clarks but I have lost the contact info for them.
The Clark has the O/D up front where it belongs, the 4500 in the rear like an overgrown Toyota transmission...

['63GENIII]

Originally Posted by 61K10

find a clark 5sp trans with the (over drive) 5th gear. They don't shift real well but should bolt up. might have to change the clutch disc?

Those Clarks with the OD are a little tough to find. Maybe a school bus?

I had one in a '57 3/4 ton Napco truck. It actually shifted really well once I shortened the shifter by a foot.

Just my personal opinion.... the Clark will take more abuse than the 4500. IIRC, it weighed quite a lot more than a 4500 as well.

[MichaelNicholas]

Originally Posted by Captainfab

How big of a difference a differential change will make depends on exactly what gears are in your differential and what gears to change to.. A C30 likely has 4.57 or 5.14 gears. Changing to 41.0's or 3.73's will make a noticeable change.

Bolting a NV4500 up to a 261 has to do with the bellhousing, not the trans. Unless you are talking about going with the adapter plate as in the link I gave you last night.

I'm going to start with a rearend swap since that's on the list anyway. Found a 14 bolt Chevy that came out of a " heavy 30". The guy says it was a ramp truck, but doesn't know what gears are in it. I've searched and haven't come up with anything yet. Anyone familiar with this rearend?

[MichaelNicholas]

Originally Posted by '63GENIII

Those Clarks with the OD are a little tough to find. Maybe a school bus?

I had one in a '57 3/4 ton Napco truck. It actually shifted really well once I shortened the shifter by a foot.

Just my personal opinion.... the Clark will take more abuse than the 4500. IIRC, it weighed quite a lot more than a 4500 as well.

There are several on Ebay

-Michael

[Captainfab]

The only way to know for sure is to pull the cover and either count the teeth or simply find the tooth counts stamped on the edge of the ring gear. My guess would be that it has 4.10's or possible lower. Have you pulled the cover on your truck's differential to see exactly what you have now?

Originally Posted by MichaelNicholas

I'm going to start with a rearend swap since that's on the list anyway. Found a 14 bolt Chevy that came out of a " heavy 30". The guy says it was a ramp truck, but doesn't know what gears are in it. I've searched and haven't come up with anything yet. Anyone familiar with this rearend?

[63burban]

1980s C20 and C30 trucks sometimes had 3:73 or 3:42 gears. My brother has a 1988 single wheel K30 with 3:42 gears. Really nice all around ratio. Also keep in mind you are asking 50 year old technology to go 80mph.. As my father would say "your nuts"

[dieseldude4bt]

3.42 gears would not happen in a square body K30 because there is no 3.42 gears for the dana 60 front.

[63burban]

2wd square body do have them and 3:42 is a lot easier to find in the 88 and up trucks

[dieseldude4bt]

Originally Posted by 63burban

2wd square body do have them and 3:42 is a lot easier to find in the 88 and up trucks

K30 is not 2wd.

[69 C10 LWB]

You could always get a gear vendors overdrive and keep your factory trans and rear axle.

[whateverpratt]

You've got an sm420 in there right now.

If you have the H072 rear axle, you probably have a 5.14 gear set, but you might have 4.57 in there. There's a 4.10 gear set that's the highest GM manufactured for that axle. 4.10's will definitely drop your rpms. Also, the GM literature mentions a 5.83 ratio for that axle. If you happen to have one, sell it to me They are super rare

If you can find a cheap t5, that's what I would look into for your overdrive because it's an easy swap. Most negative posts about them make them sound like they are built out of plastic, but my brother put one behind the 283 in his '65 Malibu and has thrashed on it for 5 years now. He has a spare that he rebuilt since he assumed the used one he had swapped in wouldn't last, but it's held up fine.

I did the nv4500 swap in that link above, and I love it. If you go that route, the most difficult thing will be the adapter plate between the bellhousing and the tranny. I was fortunate to know a friendly machinist that turned mine for cheap.

As far as top speed goes, my c10 would do 70 with the 230, nv4500, 12 bolt with 3.73's and 30" tall tires, but that poor little 6 didn't like it much. However, I had woefully neglecting tuning the engine, so that might have helped. A properly tuned 261 should do fine on the freeway with an overdrive and higher gearing.

I did swap in a 350 finally, and boy that made all the difference in the world. I loved my old six, but the power increase from the 350 was amazing.

[MichaelNicholas]

I did the wheel turn, count the drive shaft rev thing and it appears that I have a 5.13 gear. I'm able to find plenty of dually 4.10 rears, but looking for something smaller yet. BTW, all old parts will be kept for potential new owner.

Thanks so much for the info
michael

[whateverpratt]

You aren’t going to find anything smaller than 4.10 for that axle (unless you can find one of the 3.90 sets that was custom made). You don’t need a whole rear axle. The H033, H052, and H072 axles were all banjo style with a removable third member like the Ford 9”. Take out the 8 axle flange bolts that hole the axle to the hub, slide the axles out ~6”, take out the 10 bolts holding the third member in and grunt. Place your 4.10 third in the housing, bolt up the axles, and sell your 5.14 third to a rock crawler on pirate.