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Discussion Starter #1
Howdy! Newly registered but have been following this forum for a couple years. It has saved my behind a time or two and its great to have such an awesome community to look to!

I am sitting on a 1987 GMC P30 bread truck. Its in need of a conversion from the 3-speed manual to a durable automatic trans.
Admittedly, I am a novice when it comes to anything past the firewall. I have spent hours on the google and 4BTSwaps threads and it seems like there are alot of opinions out there.
Whats the bottom line? I need the most affordable option, easiest swap, and reliable. OD would be nice, but not mandatory. This is for hauling, not performance. It will do some highway miles, but mostly around the city (Food Truck).

Should we go with a TH400, a727, 46rh, 700r4.....? Shoot it to me straight!


You guys are the best.
 

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I suspect that transmission is a 4 speed manual - 3 speed is unheard of in a 1 ton truck. Does that 1987 GMC P30 have a Cummins 4bt in it now? If indeed that is a GM 4 speed on a 4bt, you have a a very rare (and desirable) combination. Rare enough that most of us have never seen one. If so, you should be able to trade the setup (adapter plate, flywheel, clutch, transmission, etc) for a decent automatic setup).

Conversion to an automatic involves an adapter plate, flex plate, wave ring, torque converter matched to the 4bt torque curve and a automatic set up to handle the high torque from a 4bt at lower ROMs. I'm a Ford / manual guy, so I don't have the specifics at hand.

Feed us some details, the day crew has more experience.

Is your engine slightly tilted (most of the 4bt / TH400 were tilted for clearance in the engine compartment)?

Russ
 

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Discussion Starter #3
Thanks Russ -
I believe it is a 4-speed, but since the low gear is a “granny gear” or crawler gear I’ve always heard it referred to as a 3-speed .
yes is has a 4bt in it currently. I will get specs on the existing tranny tommorow And update post.
 

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47RH would be simple and reliable. No extensive wiring, came behind the Cummins already, seem pretty durable and you get OD.
 

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Discussion Starter #5
Current manual transmission Looks to be an Eaton , it’s very hard for me to tell, no discernible stamp...
if it makes a difference this is technically a GMC P3500
 

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How many speeds, location of shift ball, picture of trans., pad on one of lower sides of trans should have data on it. Some rebuilders remove tag, to make it hard for normal people to ID!

Ed
 

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Discussion Starter #7
Ed- i don’t know if you can make anything of these numbers, I definitely don’t see a tag of any kind.
it has 4speeds .
if I understand what you mean by location of “shift ball” it’s basically top-center .

126913
 

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Current manual transmission Looks to be an Eaton , it’s very hard for me to tell, no discernible stamp...
if it makes a difference this is technically a GMC P3500
If its an 87 its not an Eaton transmission. Newer Eatons arent 4 speeds. Possibly a Clark but doubtful. If I was betting money, id bet its a SM465. Cast iron top cover, not aluminum. Data tag would be upper rear pass side if there is one.
 

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Discussion Starter #10
I’m learning this is not the original motor , and in all likely not the original transmission.
 

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Cummins was the repower. Originals in most of these vans were gas engines. The repower was mated mostly to vans original transmission.
 

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Really need to know what you have before you make a decision. Is the transmission adapter plate on the engine a Ford, GM, or SAE? All Fords used in vans were manual 4 speed units, either a T19 or NP435. Only a scarce few of the GM vans were manual with the SM465. Almost all GM's had the TH475 automatic. With the SAE housing you'd find an Eaton 5 speed manual or an Allison automatic of some type. I this vehicle is ever going to travel more that 50-55 MPH you'll need some type of transmission with an OD gear. About the only GM some of our guys have used its the 4L80e which can be kind of expensive to add since you don't have the parts. Of course you can sell the flywheel assembly to recoup some cost. A P3500 would have likely either have had a Gas 350 V8 or a Detroit 6.2 or 6.5 diesel as stock.
 

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I'm pretty sure you have an SM465 from the looks of it. Can you take a pic of the drivers side of the transmission? Not a close-up of anything, but an overall pic. I will be able to tell you for certain if it's a 465 that way.
 

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If its an 87 its not an Eaton transmission. Newer Eatons arent 4 speeds. Possibly a Clark but doubtful. If I was betting money, id bet its a SM465. Cast iron top cover, not aluminum. Data tag would be upper rear pass side if there is one.

Bingo
 

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Thanks Max! I cant really get in from the side. Does this one help at all? View attachment 127018
Yup. It's an SM465. It's a 4 speed with a granny low gear. IMO it's the best 4 speed truck transmission in it's size range. Gear ratios are 6.55, 3.58, 1.70, 1.00. Reverse is 6.09. It's a good trans, but the iron top units can become sloppy shifting. It's easy to fix this issue though. I ran one behind my 12V Cummins and it never gave even a hint of trouble. I'd keep it unless you absolutely have to have OD.
 
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My stock 1986 Ford E350 bread truck (Grumman) would hit the governor at 55 MPH. I expect that your 1987 GM P30 has the same maximum speed (The national 55 MPH speed limit was in effect then).

I had the governor spring changed out for a higher RPM spring. The bread truck would cruise at 65 MPH (with the resulting loss of MPG for pushing a cinder block through the air with the engine going beyond it's efficiency curve). The truck would go faster, but, 65 MPH was "good enough".

And a warning about tires. Check the date codes. If there are no date codes, the tires were make before 1990. In a moment of willful ignorance (make that magnificent stupidity) we ran undated tires for over a year. Eventually pealed a tread at 65 MPH - Tread slapping against an aluminum wheel well DOES get your attention. At this point, the 4bt was removed and the bread truck is a storage shed
 

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If you have an SM465 then you have the fairly rare GM manual flywheel setup. Most of the GM vans were TH475 automatics. Those were only so so. If you want to stay with a GM transmission with OD the 4L80e might be a possible choice. It won't be what one would call a cheap install, You'll need the GM/Cummins flex plate and wave ring assembly plus an outboard controller for the transmission. Some other misc parts will be needed as well. Most of our guys lean toward the Dodge automatics since they came behind the Cummins. A few have done the Allison 1000/2000 but that isn' t cheap either.
 

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Discussion Starter #19
Yup. It's an SM465. It's a 4 speed with a granny low gear. IMO it's the best 4 speed truck transmission in it's size range. Gear ratios are 6.55, 3.58, 1.70, 1.00. Reverse is 6.09. It's a good trans, but the iron top units can become sloppy shifting. It's easy to fix this issue though. I ran one behind my 12V Cummins and it never gave even a hint of trouble. I'd keep it unless you absolutely have to have OD.
Thanks Russ - It does appear to be the SM465. The OD is not my major concern, as much as needing an Auto. I have trouble finding employees these days who can operate a manual trans :-/ .
I'm looking for the most affordable option for swapping.
 

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Discussion Starter #20
Thanks Char!
My mechanic and I are leaning towards a beefed-up TH400, however with the adapter and TC its looking to be about $4200. This is before the purchasing the correct wave ring (which I can't find a source for currently, and modifying the drive shaft.
With parts and labor, we are planning on about $7k, which makes my groin hurt a little.
Would the 4L80e be more compatible with the existing set-up?
 
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