Ultimate 707 Conversion Guide

The Definitive 1:72 707-120 (B) Conversion Guide

by Jodie Peeler, Rob Sherry, and Merlin Woodman, credit to Bill Matthews for VG measurements. For corrections or issues on clarity, please PM user LH707. For questions about steps, please search past threads or start a new one.

This is a guide to help you build a 1:72 scale 707-100(B) or 720(B) from one of the AMT C-135 kits. Some of the information in here can also be used to modify or improve 1:144 kits. Let’s begin with a look at the different basic members of the Boeing KC-135/707 family:

367-80	“Dash 80” prototype, first flew in 1954.  Only one built (N70700).
367-80B	“Dash 80” after re-engined with JT3D turbofans.
KC-135A	Basic Air Force tanker.  Longer and wider fuselage than 367-80.
C-135A	Transport (non-refueling) variant of KC-135.
C-135B	C-135 re-engined with JT3D turbofans.
VC-135C	“Plushed-up” C-135B used for VIP flights.
KC-135E	KC-135A re-engined with JT3D turbofans.
KC-135R	KC-135 re-engined with GE/SNECMA CFM-56 turbofans.

Several variants of the C-135 have also been used for testbed, electronic warfare, weather investigation, and other assorted purposes.

707-120	Original production version, with JT-3C engines
707-120B	New-build and re-engined 707-120s with JT-3D turbofans.  All new-build and retrofits included the B720 wing glove mod.  Additional leading edge devices (Krueger flaps) added. Taller vertical fin and small ventral fin added, horizontal stabilizers lengthened by 20″ per side.
707-138	The first shortened fuselage version, built for QANTAS, with JT-3C engines. Based on the 707-120, with the same wing and the fuselage shortened fore and aft of the wing.
707-138B	707-138s built or retrofitted with JT-3D turbofans. All new-build and retrofits included the B720 wing glove mod, leading edge devices, and lengthened horizontal stabilizers as well.
707-220	707-120 airframes with JT-4A engines ordered by Braniff for “hot and high” operations.
707-320	The first long-range “intercontinental” version of the 707. Increased fuselage length and a new wing with increased span and revised trailing edge plan form. Enlarged horizontal tail. Used JT-4A engines.
707-320B	707-320 fuselage and horizontal tail married to a further revised wing featuring a new trailing edge plan form and a new wing-to-fuselage fairing. Revised low-drag wing tips added 20″ at the tip of each wing. Equipped with JT-3D engines. Increased leading edge Krueger flaps.
707-320C	707-320B wing and fuselage modified with a main deck forward freight-loading door, stronger gear, and strengthened floor. Some built as pure freighters, and some built as convertible freighter/passenger aircraft.
707-420	707-320 equipped with Rolls-Royce Conway turbo-fans; the first turbofan powered 707-320s.
707-700	One-off 707-320C refitted with CFM-56 engines, used as basis for military derivatives
720-020	707-120 fuselage shortened fore and aft of the wing. Introduced the leading edge “glove” to the -120 wing, inboard of number 2 and 3 engines; later used on 707-120B and 138Bs. Additional leading edge devices (Krueger flaps) added. Except for the first aircraft, all were delivered with the 40″ vertical fin extensions and shallow ventral fins.
720-020B	720-020s built or retrofitted with JT-3D turbo-fans. All built with tall vertical and shallow ventral fins, and lengthened horizontal stabilizers.
VC-137A	USAF Special Air Missions variant of 707-120 used for VIP flights.
VC-137B	VC-137A after 707-120B conversion
VC-137C	USAF Special Air Missions variant of 707-320B used for VIP flights.

And here are the basic measurements for each variant:

Variant	Wingspan	Fuselage	Cabin Width
367-80	130′ 39.62m	128′ 39.01m	132″ 3.35m
707-100/200	130′ 10″ 39.88m	138′ 10″ 42.25m	148″ 3.76m
707-138	130′ 10″ 39.88m	128′ 10″ 39.20m	148″ 3.76m
707-300/400	142′ 5″ 43.40m	145′ 6″ 44.35m	148″ 3.76m
707-320B	145′ 9″ 44.42m	145′ 6″ 44.35m	148″ 3.76m
720	130′ 10″ 39.88m	130′ 6″ 39.78m	148″ 3.76m

Before you Begin

Decide which variant of the 707 you wish to build, for this will dictate the kit you start out with. Obviously, you will not want to convert a KC-135 into an “Intercontinental” 707, for that’s what the Heller kit is for. However, there are several differences in engines and other details, and these will dictate which kit you start from. It’s recommended you read David Hingtgen’s “Ultimate 707 Guide” to familiarize yourself with some of the variations of the 707. If you have pictures of the subject you’re trying to build, figure out which configuration it had, as some of these differences will be described later in this article. In this article, parenthesis around a B, e.g. “720(B)” indicates that this change is for both the turbojet and turbofan versions.

There are two primary variants of the AMT KC-135: the basic KC-135A, with J-57 turbojets; and the EC-135C, with JT3D turbofans and the proper extended stabilizers. There are other variants available with the two different engine types; however, stay away from the RC-135V kit, which has huge SLAR pods molded into the forward fuselage, making it useless for this project; and the KC-135R, which has CFM56 engines.

If you plan to build a turbojet 707 or 720, note that the KC-135A kit’s J57s lack the organ pipes and turbocompressor inlets, so you might want to pick up a set of resin engines from HaHen, which also have four turbocompressor humps. For a 720, you can use the complete HaHen engines for 2 and 3 and the HaHen engines with the AMT pylons for the non-T/C 1 and 4. Below is a list of conversion models and which kit you’re best off starting out with:

707-120/VC-137A/-138/720: KC-135A and resin JT3Cs
707-120B/VC-137B/-138B/720B: EC-135C

Now that you know which kit you want to build from, let’s look at the fuselage dimension changes that are needed:

Variant	Fore	Aft	Cabin Width
367-80	-.14″ -3.6mm	ok	-.17″ -4.3mm
707-100/200	+1.11″ +28.2mm	+0.556″ +14.1mm	+0.056″ +1.4mm
707-138	+0.278″ +7.1mm	-0.278″ -7.1mm	+.056″ +1.4mm
720	+0.833″ +21.2mm	-.556″ -14.1mm	+0.056″ +1.4mm

Construction Phase 1: The Stretch/Shrink

There are two ways to add/remove length and move the wing to body fairing (henceforth WBF) on this conversion, namely the “donut method” and the “WBF-move method.” The donut method involves making a vertical cut ahead of the WBF and immediately aft and adding or removing sections (donuts), while the WBF-move method involves cutting around the WBF and moving it. The donut method makes it easy to preserve the rear fuselage taper when stretching but involves resculpting the back of the WBF, while the WBF-move method spares you the puttying while forcing you to carefully carve and re-align or recontour the fuselage taper. Look at your kit and figure out which approach you’re more comfortable with before proceeding. Next, glue the tanker crew hatch and cargo door in.

Option A: The Donut Method
Make the first cut inline with the rear edge of the opening for the wing center section, and add or remove the plug at this point. As you reattach the rear fuselage, make sure to line up the cut edges over the top of the fuselage; the bottom isn’t so critical, as this area will be covered in putty anyway.

Once the rear is done, make a cut anywhere ahead of the wing to add the length. There are two methods of adding length; both work well. The first is to stack styrene sheet together to form a sandwich of appropriate thickness. While this makes a strong plug, it also makes a semi-bulkhead inside the fuselage, which will look odd if you have open windows. You can cut this out with a knife. The second method used a strip of .040″ or 10.2 mm styrene cut to the appropriate width, and super glued into place. This also works well, and doesn’t waste as much plastic as the sandwich method.

Option B: The WBF-Move
Make a vertical cut on the underside half of the fuselage right after the WBF and a second one ahead of the center wing gap using a razor saw or hacksaw. Next, make a horizontal cut along the panel line that runs along the fuselage, using a hobby knife to score the plastic before cutting through. Once your cuts are complete, clean up the gaps. If you’re doing a regular-length 120(B) or a 720(B), make a third vertical cut in the middle of the cut section and add length (1.385″ for 120(B), .277″ for 720(B)). You can do this by securing the two sections and adding a .010″ styrene piece overlapping on the inside and an .040″ styrene piece in the actual gap, then sand smooth. For a -138(B), the fuselage length remains the same. If you’re doing a 707-138(B) or a 720(B), you’ll need to remove a section aft of the WBF (.277″ for -138(B) and .554″ for 720(B)). AMT conveniently provides a cut guide in the form of the tanker reinforcement strip as seen at left (second one for 138(B) and fourth for 720(B). Cut along this line until it meets the horizontal cut line, and remove the sliver you’ve just separated. Finally, glue the WBF back in using .010″ styrene overlap pieces inside, reinforced by scrap sprue sections. Make sure the fore and aft gaps correspond to the dimensions at the top of the article (no aft gap for 720/138 because you chopped it out).

Construction Phase 2: fuselage widening and pre-join preparation

To add the appropriate fuselage width to the 707s, add 0.056″ or 1.4 mm to the upper portion of the fuselage (i.e., in a strip between the halves). Start adding width from the forward edge of the main deck cargo door, and stop about 0.5″, 12.7 mm ahead of the tail fin. Make sure you taper the added section at both ends to prevent it from having a pinched look once joined. For the 367-80, remove 0.085″, 2.2 mm from the same areas along each fuselage half; this will keep you from having to saw off the tail and reattaching it later (as a certain Fine Scale Modeller article prompted many years ago. Ahem).

Once the fuselage pieces are reassembled, check them over for thin spots. There may be some around the tops of the wing fairings; reinforce them, especially if you decide to cut in a lobe crease. It’s also wise to make sure the insides of the joints around the shortened/lengthened areas are adequately reinforced before proceeding. If you did the donut method, you can also either add the putty to recontour the rear of the wing fairings here, or save that for later. It’s probably easiest to put that off until just before attaching the wings.

While you’re at it, you may decide to add open windows to your model. You could do this by inlaying a piece of clear acrylic, or by drilling and filing out each individual window; however, the easiest way is with the photoetched window belt strips included in the Flightpath 707 Intercontinental Detail Set. This requires you to cut out the areas of the fuselage along the window line and to carve away an area in which to inlay the strips; however, this is much easier, and is more likely to produce pleasing results, than other methods.

For the lobe crease, start by drawing a horizontal line along the side of the fuselage roughly even with the bottom of the cargo door, or 3/16″, 4.8 mm below the top of the wing fairing. The crease will run from about 1″, 25.4 mm from the rear of the cockpit windows, disappear into the wing fairing, reemerge from behind the fairing, and end about 3.75″, 95.3 mm from the tip of the tailcone. The crease does not begin or end abruptly, but “fades” in and out at the nose and tail. As always, a good photo reference is ideal for getting this right. The cut doesn’t have to be too deep; on a model of this size, you really only need a fine line to give the appearance of the crease. It is probably best to use a Dremel tool set at about 10,000 RPM with a pointed (conical) steel cutter to make the line. It takes a steady hand, but if you go slowly and follow the drawn-in line, it will look fine. Once the groove is cut, carefully sand down either side of the line to give the required rounded-over appearance. Again, don’t go overboard here; it’s surprising how small this detail can be and still look good. The sanding can be a bit tricky close to the wing fairings, but a small needle file will work in the tight space immediately ahead of and behind the wings.

Main Landing Gear Mounts
The KC-135 kit comes with a cargo deck that runs most of the full length of the model and is attached to the gear well. If you plan to build your model with the AMT cockpit and gear wells with wheels down, you will need to move the gear bay mounting structure forward (367-80) or aft (all 707/720s) as needed so that it aligns with the moved wing-to-body fairings and wing roots. For the 367-80, you can cut the .14″ off of the front to match the front fuselage shrink, and for the 707/720s, cut just fore and aft of the wheel well and add/remove the same amounts as prescribed at the top of the article. To add length (720 and 707s), you can cut a sheet of .030″ styrene to put on top of the deck to stabilize the three sections. For a 707-138, you can cut a .277″ section from the back and move it forward as seen at left. Make sure you dry-fit all of the deck and the three gear bay parts inside one of the fuselage halves to ensure alignment before gluing the deck together. The trailing edge of the WBF has a small lip that the rear end of the gear well sits on, align these and then glue the deck into place. You don’t need to worry too much about the appearance of the underside of the deck inside, as it is not visible from outside the model.

Nose Gear Doors
If you’re doing a gear-down build, look at a picture of the subject you’re planning to build, and see if it has the older trailing-spade nose gear design, or the later barn door style introduced around 1963. In the pictures above, the left one has the trailing spade, while the one on the far right has the barn doors. The AMT kit gives you the trailing spade, if your subject does as well, you’re set. If your subject has the barn doors, you’ll need to scratch-build them. Widen the gear door gap by cutting back from the gear well door panel line visible in the upper picture at left while the fuselage halves are still separated, and then cut two pieces of .030″ styrene that fit into the slots you just created. Glue these new gear doors into the edges of the slots so that they resemble the picture at far right (a Heller 707 molded with the barn doors). Note that the doors angle out by about 10 degrees from the vertical.

Air Conditioning Packs
The C-135 has only one air conditioning pack in the wing roots, while the 707 has two. Take a look at the AMT part for the wing center section, and you’ll see the single inlet on the left side. For a 707, you’ll need to add an identical inlet to the right side. There are two ways to approach this, one way is to add putty and carve, the second is to cut and move the plastic beneath the intake.

Carve Method: Start by applying a thick layer of epoxy putty to the inside of the part, on top of where the new inlet will be. This is necessary to provide a new surface into which the inlet will be cut. You can gauge the thickness required by looking at where the plastic is molded for the molded-in inlet. Now start carving. Using the molded-in inlet as a guide, draw or cut a guideline for the front of the new inlet; remove all the plastic ahead of this line. Don’t worry when you start exposing the epoxy applied to the inside of the wing piece, as that’s what it’s there for.

Cut and Move Method: Cut the trailing edge and two sides of the intake to mirror the left side using a hobby knife, then bend it in and glue it in place using segments of scrap sprue.

To complete the inlet, sand the area smooth using a needle file, or a small sanding stick. Be careful to preserve the contour of the fuselage next to the inlet, and the thin strip alongside the opening. The lip of the inlet should be slightly rounded to match the left side.

Construction Phase 3: Fuselage join

Once you’ve got your fuselage halves set up, go ahead and glue them together. Make sure you add enough nose weight if you’re building gear down. You may want to glue the cargo deck to one side and the center wing section in the bottom, then add the second fuselage half, or join the halves with the deck in the middle and then add the center wing section. Figure out what you think will work best and then go with that. It’s prudent to use scrap sprue pieces as alignment and reinforcement tabs, especially on the upper fuselage. Now you’ll need to slice off the pieces of the refuelling boom blister, and fill in the resulting hole. The easiest way to fill the space is to glue scrap sprue and a piece of sheet styrene in the opening, then lay putty over the area. It will probably be much easier to apply fairly thin layers of putty, and do some sanding as you go, building the area in thin sheets. This should be easier to work, and should use a lot less putty. While you’re sanding, also remove the molded-in reinforcement strips around the fuselage, which were used only on the C-135 series. Once the second A/C inlet is done, install the center section.

Construction Phase 4: Wings

Wing Reinforcement
The AMT C-135s are infamous for their drooping wings. Before gluing the upper and lower halves together, reinforce the uppers, with carbon fiber rods, chicken skewers, or any other way you see fit. Then align all the parts and glue them together.

Vortex Generators
One annoying aspect of the conversion is adding the vortex generators to the wings. The C135s were delivered without them, while all 707s had two rows ahead of the high-speed ailerons. One row of 25 VGs begins .085″ forward of the rear spar line 2.45″ out from the wing root. The other row of 22 VGs is 0.75″ forward of this. The VGs are .090″ apart. You can either drill holes and insert wires as Bill Matthews did, or glue small pieces of .010″ styrene to the surface using a taped template. The VGs alternate orientation, being toed-in pairwise, as seen in the picture.

Construction Phase 5: Fins, engines, wing gloves, and other details

Depending on what configuration your subject has, you may need to modify some details. The following sections deal with these changes. Read through them to see which of them you’ll need to do. Some of these steps are independent subassemblies that can be done at any time throughout the build.

Engines
If you’re building a turbofan -120B or -138B from the EC-135C kit, you’ll need to come up with a third “humped” engine. The C-135s only carry two turbocompressor-equipped engines, so you’ll be one short. You may have to resort to scratchbuilding or casting a copy in resin.

The 720B only had two TC engines like the C-135, so if you’re building that version you won’t need the extra part. Military versions of the Heller 707 were sold as the Heller/Airfix E-3, with both CFM56 and JT3D (TF33) engines, so you may be able to find a spare set of Heller engines with the #4 turbocompressor hump. The Heller engines represent the earlier JT3D-1 or -3 engines with the longer spinner cones and the thin blow-in doors. If the aircraft you are modeling had the later JT3D-3B engines, the spinner cone is blunter and the blow-in doors are much larger. Check pictures to make sure, and then cut/fill/rescribe as needed.

Wingtip HF Antennas
For the VC-137(B), most 707-138(B)s and a small number of 720(B)s, you may need to add a wingtip HF aerial on the starboard side (VC137s had them on both wingtips). Check a picture of the airframe you want to build to be sure. The antenna juts *forward* from the wingtip. With the extra pieces in the EC-135C or KC-135 kit, this isn’t a problem. Take one of the overwing aerials (EC-135C) or the tail aerial with the refueling light (KC-135) and cut off the body of the antenna, leaving just the probe *and a bit of the fairing behind it.* Then carve a small chunk out of the leading edge and glue the antenna in place. A little putty may be needed to finish fairing the piece in. With the antenna installed, the right anti-collision light was moved to the center of the wingtip in a small blister, which is easily simulated with a drop of Kristal Kleer tinted with green food dye.

Vertical stabilizer The very first 707-100s (and one 720 for United) came with a short tailfin, which was later extended by 40 inches to improve lateral stability. The AMT kit represents the taller tail. If you’re building an early 707-100, you’ll need to cut .554″ off the top of the fin before mounting the antenna.

Ventral fins
Along with the taller vertical fin came a ventral fin, a full-size 40″ version on the 100s with the tall fin, and a shorter 19″ one on all the B models and the 720. The Heller 707 part represents the tall one, to get the short ventral fin you need to cut one down or scratch-build. The root chord is 3″ for both, and the height is .5″ for the tall one and .125″ for the short one. For the B models, take off about .125″, 3.2 mm from the bottom of the fin, and a little less from the leading edge. Align the trailing edge of the ventral fin with the trailing edge of the tailfin before gluing it in. If you’re using alignment pegs, you’ll need to drill holes into the fuselage in the boomer gap segment.

Wing glove The 720(B) and the 707-100Bs all have an inboard leading edge glove, extending from the inside of the number 2 and 3 pylons to the fuselage. The glove extended over the wing to the front spar. The leading edge was extended approximately 40″, 1.02 m at the wing root. Krueger flaps were added to the leading edge inboard of number 2 and 3, and to the leading edges of the outboard wing panels as well. One way to build the wing glove is to add a triangular piece inboard of the engines and a reprofiled root using the wing glove template. Cut parts WG1 and WG2 from .030″ styrene, then mount WG1 at the root and glue WG2 to the forwardmost part of the leading edge and pointing slightly up to the most forward point of the root addition as in the picture. One this is done, glue WG3 along the panel line on the wing and curve it around the leading edge. You can use MEK cement or other solvents to help bend WG3, or cut at the WG3/WG2 join and use putty for the underside.

Main gear doors
If you are building your model with gear down, you will have to bend the main gear doors to the correct shape. In the KC-135 kit they are molded in one plane so they can be built in the closed configuration. On the 707 the bottom and top bits angle out as in these pictures. Cut and score the gear doors, then bend them to shape and use tube glue to set the position.

Hushkits
If you’re building either John Travolta’s 707-138B (Q) N707JT or the QFM one (VH-XBA) in recent appearances, you’ll need to modify the engine bypass ducts to represent the Comtran hushkits (see light gray extensions at right). Please refer to this article on the topic.

Odds and Ends
Now that you’re done with the basic conversion, you can detail to your heart’s content. The Flightpath set includes an array of extras, such as cabin doors, antennae, pitots, windshield wipers, static dischargers and other such items for the particularly anal airliner modeler. In addition, True Details produces a couple detail items you may want to add to your model. One is a photo-etch detail set with landing gear doors, pitot heads, and antennae; another is the 10-wheel set of resin wheels and tires. The hub detailon these wheels is substantially better than what’s on the AMT parts. Note, however, that you will have to make other arrangements to represent the smaller main gear tires on the 720 (40″, 1.02 m diameter vs. 46″, 1.17 m on the -120s).

Markings / Liveries

Fortunately, the minor revolution that has come with the advent of the ALPS printer means that choices of markings for short-bodied 707s are much broader than they once were. If you know how to use a computer equipped with an ALPS and a suitable graphics package (or if you know someone who does), you can produce essentially any livery you want for this model. You can generate your own artwork, or even scan and enlarge the artwork from a 1/144 sheet.

For those who don’t have access to such a setup, however, here is a partial list of markings possibilities using professionally-manufactured decals. Keep in mind, however,that this list is several years old and that most, if not all, of these sheets have become very rare:

Operator	Aircraft	Scheme	Maker
American	707/720	Lightning Bolt	VFD
Braniff	707/720	Two-tone	Flight Path
Braniff	707/720	Jelly Bean	Flight Path, Liveries Unlimited
Pan American	707/720	(I have to tell you?)	Liveries Unlimited
QANTAS	707-138	Delivery	Roo Decal
QANTAS	707-138	Delivery	Hawkeye*
QANTAS	707-138B	V-Jet	Hawkeye
Western	707/720	Indian Head	Liveries Unlimited

*If you use this sheet to do a model of VH-XBA, you’ll need to cut out windows on the port side and fill them with Model Master Italian Red, print black lettering for the ship name, and add an eleventh window forward of the wing escapes on the starboard side (just use one that you cut out).

And there you are.

Sources:

1. Boeing 707 & 720, George W. Cearley, Jr., Privately Published, 1993
2. The Boeing 707, Barry J. Schiff, Arco Publishing Co, 1967
3. The Boeing 707 & 720, John A. Whittle, ed, Air-Britian Ltd, 1972
4. Classic Civil Aircraft:2 Boeing 707, Alan J. Wright, Ian Allen Ltd, 1990
5. Detail & Scale Vol 23, Boeing 707 & AWACS, Alwyn T. Lloyd, TAB Books Inc, 1987
6. VH-JET #1 and her Sisters, http://www.adastron.com/707/