Tech Tips

'Bolters helping 'Bolters is a beautiful thing!


          As any of us who own one know, the 1.5 or 2-ton Advance Design trucks aren't very practical for hauling or driving on modern highways. And if you want to take one to a show that's out of county, you have to haul it. The good news is that diesel upgrades aren't as hard as you think. In fact, most of the components are available -- all you have to do is find the right donor vehicle. Generally, the best donor for this swap is the ubiquitous P-30 step van. Be advised -- This is not a swap for everyone, but if done right it is a big improvement and almost necessary if hauling much of a load at today’s speeds on the highway. Here's Grigg to explain the basics of ...

The P-30 Diesel / Driveline Swap
By Grigg Mullen III
Bolter # 6590
1948 Chevy 2-Ton 5-Window
1952 Chevy 1-Ton Longbed
  December 3, 2008
The ubiquitous P-30 step van -- Finding the right P-30 donor vehicle is the first step in transforming your AD "Big Bolt" into a more capable, diesel-powered hauler. The good news is that after you remove all the needed parts, you either have a garden shed or you can sell the aluminum body for scrap and get a lot of your cost back!

A fellow 'Bolter recently posted this question:

       "I'm now the new owner of a 48 1.5 ton. I'm planning on putting a 6BT Cummins in it and using it as a hauling truck. I have an opportunity to possibly get the axles and rims (with radial 19.5" rubber) from a P30 Frito-Lay truck. I was thinking this would be great, as I would end up with much easier to find tires, disc brakes in front, and still have the 10-bolt style bolt pattern. Will this work ok? I plan to leave the front suspension as a leaf system, but the rear suspension will be a custom self-leveling air ride system (like semi-tractors have)....

       "I was worried about weight capacity as the P30 is only a 1-ton chassis, but then I found that the gross weight of my 1.5-ton is only 12,500 so I should be ok with the P30 axles I would think..."

       This plan is reasonable and achievable. The axle swap described is even what I have done with my truck. I am planning a full air-ride rear suspension, similar to a Hendrickson model HAS-120 or the Kelderman kit offered for a F550.

       Although not a complete step-by-step guide on how to do a diesel engine and driveline upgrade, here is some background on this swap that appears to be gaining in popularity.

The Goal

       The goal, or reason for considering the use of later model parts on our old 47-53 Advanced Design (AD) trucks is to make them more useful and safer to use on today’s roads where 45 mph is not as fast as it once was..  Original trucks are great to play with, use for chores around the farm, and fun to take  to local shows.  But if you want an AD truck to use as an everyday work truck a little more HP, economy, speed, and better brakes and steering will be welcome improvements.  You can achieve these improvements by using commonly found parts from a GM P30 chassis, often seen as bread trucks, RV’s, UPS trucks and similar.  There are three main desirable parts on many (but not all) of these chassis. 

  • Engine, most desirably the Cummins 4BT or 4BTA, essentially 2/3 of a Dodge Cummins engine.  It will give you more power (which will allow for a higher road speed) and better fuel mileage than your old Stovebolt six, and is of a size that easily fits the engine compartment of an AD truck.
  • Front axle, brings large disc brakes to the truck, which will help to quickly and safely slow and stop your loaded truck.
  • Rear axle, also brings matching disc brakes, and more importantly several choices of easy to find faster gear ratios.  

The Engine Choices

        There are a lot of diesel engines available that fit within the confines of the Advance Design engine compartment. Most will require varying degrees of fabrication to deal with engine mounts, radiator clearance, steering box, electronics, etc. In my opinion,  the simplest and most bang for the buck diesel engine swap is the readily available Cummins 4BT, found in P-30 step vans among many other applications.

       The 4BT is a good choice primarily because it is nearly a bolt-in application (if you use one with the GM mounts and bell housing) and has no electronics. If you use one with the GM bell housing and front motor mount, it bolts right in. Some slight modification of the front cross member may be required. As well, the radiator might have to be moved forward slightly, depending on your specific application. Generally, though, you can even use your original radiator with the 4BT, but you should probably make sure its the heavy-duty (thicker one) one and have it cleaned, inspected, and repaired or re-cored as necessary.

       The 6BT, commonly found in Dodge pickups and described above, is also an option, but being a six-cylinder engine it is reasonably heavy, and fairly long. Because of the length and weight it usually requires more fabrication and modification to fit than the four cylinder 4BT. Also, the later 6BT's ('94 and newer) require electronics.

      Other choices are possible if you are willing to modify and fabricate. For example, I am modifying a '48 2-ton with a Detroit 4-53T. This is a little more involved than the Cummins 4BT swap. It will require minor firewall modifications, and a lot of fabricating for mounts and such.

       When it comes time to decide which diesel engine is best for you -- Pick an engine you understand, admire, respect, and are not afraid to work on; anything less and you won't be happy.


       With the GM-outfitted 4BT, installing your original SM-420 4-speed transmission or the later SM465 is as easy as bolting it in.
Other transmissions will work too but may require different adapters for the back of the engine and then the appropriate bellhousing that bolts to the adapter.  The most straightforward transmission installs will be ones that were available in Dodge trucks with the Cummins, all the adapters, transmissions, and flywheels will interchange between the 4 and 6 cylinder with minimal effort, but some expense in finding the parts. One of the better choices is the NV4500 5 speed overdrive.
       Even more transmissions can be used from medium duty trucks, they will have a round SAE style bellhousing to mate with a compatible engine adapter on the Cummins.  One economical option is a Spicer 3053A from a deuce and a half military truck, it’s a 5 speed OD with a SAE # 3 bellhousing.  Other transmissions with the next size up bellhousing, a SAE #2, will work but start to get pretty big and heavy.  On the large end of the possibilities is a small (at approximately 425 lb) Roadranger  10 speed, either direct or overdrive.  I have the overdrive model RTO-6610 in my truck.

       For Cummins engine info as well as ideas and tips on engine and transmission conversions with both the 4BT and the 6BT spend some time browsing and asking questions at the forum

Overall Gearing and Speed

       One of the most important parts of an engine, transmission, and or axle swap is proper gearing.  Rule of thumb assuming you have a suitably powerful engine for the vehicle is to gear the vehicle so the engine is running at it’s peak torque rpm when in high gear on the highway at your comfortable driving speed.  This will give the vehicle good driving characteristics and good fuel economy.  For example, my Detroit Diesel 4-53T makes peak torque at 1,800 rpm, and I want to travel 70 mph, I know my tire size is 38” outside diameter. 

       One variable is the transmission high gear ratio, either 1:1 for a direct drive model, or in my case a 0.80 OD is available.  That leaves just the rear axle gear to decide on, so I plug all the numbers into a gear calculator and come up with needing half way between a 3.54 or a 3.73 rear axle gear.  From prior experience with an almost identical setup I think the 3.54 will be ideal, and if not then I’ll have to later swap to 3.73.  More gear ratio calculators and advice can be found in this Tech Tip.

The Axle Choices

      Just because the P-30 you found has a 4BT in it doesn't mean you should buy it on the spot! The P-30's come with a variety of axle set ups. I have seen all of the following on P-30 chassis, or on chassis very similar to (I did not read all the tags):

  • A -- 5-lug front 10-lug rear I-beam front, Dana 70HD, Dana 80, or Rockwell rear.
  • B -- 5-lug front independent suspension, 10-lug rear, similar rear choices as above.
  • C -- 8-lug front and rear with I-beam front, very few of these, and probably Dana 70, 70HD, or a 14-bolt rear.
  • D -- 8-lug front and rear with independent front suspension (one of the most common 8-lug P-30 chassis) and probably the same rear axle choices as the I-beam front axle version.

       The one of interest for an AD 1.5 or 2 ton is Choice A -- an I-beam front with 5 lugs and a Dana rear with 10 lugs. The Rockwell is not such a good choice as fast gears are either not available ever, or hard to get/expensive. You can quickly spot a Rockwell by the square axle "tubes."

The 5/10-lug axle pairs should have disc brakes all the way around, as I have not seen them any other way (other than our antique trucks).  These axle combinations come from trucks with an approximate 14,000 lb GVW, so more than your 1.5 ton AD, and slightly less than your 2 ton AD truck. Here is an example of axles ready to be installed; note the large disc brakes front and rear.

The Rear Axle Swap

            The rear is a simple swap -- You only need to move the rear spring pads out approximately 1" each and at the same time adjust the pinion angle to suit the drive-line, this assuming you use leaf springs like original.  Just measure the original center to center distance on the springs and duplicate that measurement when you reattach the spring pads.  To set the pinion angle   It is best to get the truck as complete as possible and with the weight on the springs, then follow the normal procedure for measuring and setting the angle as found in this Tech Tip.   Finally weld the spring perches in place.

Narrowing the Front Axle

      The front axle swap, unfortunately, is not so simple. The P30 front axle will need new springs, and the big thing is it needs to be narrowed approximately 10". The original front axle for an AD 1.5 or 2 ton should have the following measurements:

  • Center to center of spring center pin holes; 26 and 13/16 inches
  • Wheel mounting surface to wheel mounting surface; about 69.5" (within a 1/2")          

      The first axle I narrowed was from a Chevy 3500HD truck, which is almost identical to a P30 axle other than the steering, I narrowed it to match the dimensions of my original axle.  Having learned a few things from that process, in the future I will narrow a P30 axle only enough to nicely fit the wheels and tires in the front fenders, this measurement may be an inch or two wider than the original axle.  By only narrowing it enough to fit nicely should save some trouble with the spring pads and U-bolt holes, and also allow for better turning radius.

      This is a picture of the front axle from a P30 chassis, used for step vans and RVs.
      To narrow the front axle, cut about a 10-inch section out of the center of the axle, discard it, and weld the two halves back together. The first one I did took several days of measuring, layout, and prep, then a few hours of actually welding, and another day or two on finishing. That process can be seen here, although not very detailed or complete The next one I did took about a day total. That one can be seen here in more detail although it is not a P30 axle the process is exactly the same.

The Front axle is forged steel, probably forged cast steel, and definitely not cast iron. Proper pre and post heating is the only real secret to welding them, that and having a welder that thoroughly understands the process and the consequences of a poor job.
Here are step by step pictures of narrowing an I-beam axle (note this is not a P30 axle, but same process):

Step 1 --Measure the original axle, new axle, spring locations, wheels, tires, and fenders.  Then decide how much the new axle needs to be narrowed to fit nicely.

Step 2 -- Cut the proper amount out of the very center of the new axle to give you the desired wheel and tire placement. This is easy to do on a horizontal bandsaw. Image One. Image Two    

Step 3 -- Grind a proper weld prep on the ends of the I-beam to allow for 100% weld penetration at the joint.  A 45 degree bevel on all surfaces down to a sharp point works well, so long as both pieces  match up nicely.  To make the welding easier do most of the bevel on the top and bottom surfaces where it is easy to weld. Image One. Image Two    

Step 4 -- Clamp the two halves to a sturdy steel table or a section of heavy I-beam.  Clamp it down tight with as many clamps as you can fit.  You are trying to keep the two flat surfaces where the springs sit in the same plane.  Also align the joint in the middle and keep the axle straight, usually a straight edge along the front or back of the flat pads is sufficient.   Image One

Step 5 -- With a rosebud tip on an oxy acetylene torch preheat the I-beam at the joint and at least 12-18” away from it on both sides.  Consult with your professional welder, but generally the temp will be hot enough to quickly boil a drop of water, but not red hot.  You want to maintain this temp through the rest of the welding process using the torch as necessary. Image One

Step 6 -- Weld the axle together.  Start with a good pass on all that you can reach while it is clamped down, then unclamp and continue to weld evenly by alternating sides.  It’s a good idea to clamp it down flat again at every opportunity.  Be sure to give yourself plenty of weld to grind off later, you don’t want to miss a spot only to have to reheat and reweld later.  Image

Step 7 -- Once welding is complete clamp it down tight one last time.  Then post heat with the torch to a nice even heat for a 2’ to 3’ section around the weld, again it will be hot but not to hot, consult with your professional welder. Image

Step 8 -- Once it is evenly heated insulate the axle to allow it to cool as slow as possible. I used a ceramic wool blanket, you could also use buckets full of wood ashes.  The axle was still slightly warm over 8 hours later. Image

Step 9 -- Grind the weld down smooth all over.  Image

Step 10 -- Use a needle scaler to texture the area so it matches the rest of the axle.  Image

Step 11 -- Finally you are ready for a coat of paint and no one will ever know what was done (unless you tell them)

       If you are not very comfortable with your welding skills have a professional weld it. Actually both axles above I had a friend do the welding, I just set everything up, chipped slag, and wire brushed for him. Now that my welding ability has improved I will do the next one myself. Again, proper pre and post heating is the only real secret to welding them -- that and having a welder that thoroughly understands the process and the consequences of a poor job.

Once you have the axle narrowed there are still a few things to consider before you can bolt it in. The spring pads on the axle and the u-bolt holes need attention, there are two ways to resolve these problems.  The cleaner solution involves more welding to enlarge the spring pads enough for the springs to have a good flat place to sit.  Here is the additional weld needed, and also fill in the old holes: image . Then grind or mill the surface flat again: image . In the previous picture, you can also see where the new holes will need to be. This info is laid out after careful measurements on the original axle. The easier approach is to make adapter pads that bolt on where the P-30 springs did on the new axle and also have a place for the old truck's springs to be bolted to.  In this picture you can see that a pin pressed into the adapter plate locates it in the old spring center bolt hole, and the old inside U-bolt holes get plain bolts and nuts to attach it.  Then the new U-bolts have holes in the adapter plate that in this case share the old outside holes: image .

      The front springs need to be changed, I had new ones made with 2" more lift to account for the additional 2” of drop the new axle has in comparison to the original axle.  IF you use the plate adapter solution for the spring seats you may need less than 2” of lift in the springs.  If you have an engine swap planned, have the springs made to handle the different engine weight while you are at it.

      Some folks ask about using the springs from the axle donor vehicle.  I think you will find the newer model front springs are very long in comparison to the stock ones, and they also look rather flat and not curved so much. To use them, you would likely need to make custom spring hangers. Having custom, somewhat original, springs made is a better solution, less work, you get new (not used) springs, and they will be a much cleaner appearing solution. The downside could be a somewhat harsher ride because of the short length. Personally, I don't mind, or even notice for that matter.


       When swapping drivelines, you should get the hydroboost, master cylinder and proportioning valve -- pretty much the entire brake system from the donor P30.  The booster and MC can be mounted under the floor with either shop made or aftermarket brackets.  I like the idea of mounting the assembly to the frame behind the original MC location and using a long push rod from the pedal, I have seen kits for this but have not tried it.  I modified my original pedal assembly to hold the booster and MC like this, the aftermarket brackets would be simpler. The bracket to mount the booster to the frame is very simple, you can make it with some 3/16 or 1/4" plate, the picture here is all you need for plans: Look near the bottom of the page.You'll see that one change leads to another and another and another...


       Also involved is figuring out the steering, be it stock or newer PS. The P-30 steering gear box is not desirable. The P-30 steering box typically mounts outside of the drivers side frame rail, in front of the wheel and partly attached to the bumper or it's support. I have looked pretty close at the one I have on the shelf, and I see no easy way to use it in an AD truck.

       With the P30 axle you can likely use the original steering box on the truck, only requiring you to adapt or change the draglink to fit. 
I used the power steering gear box from an IH scout II, but it will only work with the steering setup on the 3500HD front axle, not the P30.  My future plans are to use a P30 axle, then to retain my IH steering  I’ll have to use part of the right side 3500HD steering arm and most of the P30 steering arms and tie rod.  Or I could use my complete original steering setup, but I like the placement of the P30 draglink behind the front axle and below the springs, so I’ll mix and match.

       This is the IH and 3500HD steering

       This is the P30 axle, it uses a draglink parallel to the frame.

Wheels and Tires

       The P-30's 19.5 wheels, even with 8R19.5 tires at about 33" tall, will look small and out of proportion on a 1.5 or 2 ton truck. There is a 22.5 option, but somewhat difficult to find the wheels. So you have four choices here:

  1. Live with the P-30's wheels and tires
  2. Find 10-lug 22.5 wheels (hard)
  3. Have custom wheels made to fit (expensive)
  4. Use original 20" wheels from some 1955 or later GM trucks, or enlarge the center holes of your older/original wheels to 5.25"

       For more info on wheel and tire upgrades for AD trucks, Check out this tech tip.


       If you want to use your Advance Design 1.5 or 2-ton truck for highway driving and hauling, the P-30 driveline swap is a great option. It does have some degree of complexity, though, so it is not a swap for everyone. But if done right, it is a big improvement and almost necessary if hauling much of a load at today’s speeds on the highway.



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