Toyota Hilux/Hiace Diff Conversion
The Toyota Hilux & Hiace Differentials - Written By John Alderson For The Works Escorts Web Board
The Toyota Hilux and Hiace differentials are all similar in design. They feature an 8" crown wheel, 31 spline axles and large bearings. They have ample capacity for an Escort and a converted Hilux diff will weigh about 60kg.
Common ratios to be found at wreckers in Australia are from 4.110:1 to 4.875:1. The taller ratio of 3.7:1 was delivered with V6 vehicles in the USA. Aftermarket suppliers in Australia have gear sets of 3.5:1, 3.7:1 etc. available new.
The closest fit for an Escort is the Hilux 2WD diff from RN10, RN15, RN20 & RN25 models manufactured during the first half of the 70's. Each of these has the same spring centres (940mm) and overall width (1350mm) as an Escort. The spring platform width is equal to the MkII Escort at 60mm but wider than a MkI which has 50mm width springs.
What needs to be modified?
Axle Shafts and Flanges
These axles have five M12x1.5mm studs on a 4.5" PCD and will require re-studding to match the MkII Escort pattern of four 12x1.5mm studs on a 4.25" PCD. The Hilux studs are longer than Escort studs. The MkI Escort has 7/16"UNF studs on a 4.25" PCD.
The drum/wheel location boss on the Hilux axle is Ø67mm compared to the Escorts Ø63mm. If Escort drum brakes are to be used then the boss must be machined down. If an upgrade to larger drums or disc brakes is chosen then the boss may need changing in size to suit the alternate drum or disc centre hole.
Some Escort mag wheels may fit over the Hilux boss but standard wheels and some mags will require the boss to be Ø63mm where it protrudes out further than the drum or disc and into the wheel.
If alternate discs are fitted over the axle flange this will increase the overall width of the diff by the thickness of the disc bell on each side of the diff. So if discs with a bell thickness 7.5mm are fitted then the overall diff width will be 1350+7.5+7.5=1365mm. Check your tire clearance to the guards and track width requirements. Most of the axle shaft fitted to the RN10-25 models have sufficient spline length for the axle to be shortened a small amount without the need for re-splining, but work out what you need and measure the axles before purchase.
The Escort drum brake backing plates will need the centre hole size and bolt pattern changed to fit over the Hilux bearing carrier. The backing plate will need a spacer behind it so that the drum properly covers the brake shoes.
Upgrading the brakes to discs has been done and reuse of the Hilux drum system may be possible.
The ratio and type of the centre that is factory fitted to RN10-25 Hilux may not be suitable or serviceable considering the age. Options are to fit a diff centre from another later model Hilux or Hiace that has a suitable ratio or Limited Slip centre for example.
Other options are to fit aftermarket ratios or Limited Slip Differential centres and new bearing and seals.
If the chosen ratio is different to standard and an accurate speedo is required, the speedo accuracy will need checking and correcting.
If Escort drum brakes are retained the handbrake linkage brackets and hydraulic line bracket will need fitting to the Hilux housing.
If the factory style anti tramp rods are to be fitted, then these will need fitting to the Hilux housing at the correct angle relative to the spring platforms. For originality sway brackets would also need fitting to the Hilux housing at the correct angle relative to the spring platforms.
If discs, tyre clearance etc. dictate shorter axles then the housing will also have to be shortened by an equal amount on each side.
With the exception of tramp rods, the brackets can normally be cut from a donor Escort housing.
Shackles and Shackle Plates
The RN10 - 25 Hilux housing axle tubes are Ø65mm and 5.5mm thick compared to an Escort axle tube at Ø63mm and 3.2mm thick. The Escort shackles fit over this housing, alternatively the slightly larger diameter Hilux shackles will fit through the Escort shackle plate. The MkII Escort shackle plates will not require modification to use this Hilux diff housing.
To prevent rapid universal joint wear and tailshaft vibration, the operating angles of front and rear universal joints need to be similar, e.g.: less that 1° difference in operating angle. The angle required will vary depending on whether anti tramp rods are fitted and the curvature/rating/ride height of the leaf springs. Anti tramp rods and stiffer leaf springs reduce the amount the diff nose lifts under power where as leaf spring curvature and ride height alter the static position of the diff nose.
The angle of the spring platform to the pinion needs to be checked and altered, if necessary, by relocating the spring platforms around the axle.
If this diff is fitted to MkI leaf springs, the 50mm wide MkI Escort spring platforms will need to be fitted to the Hilux Diff.
The 1 piece Escort tailshaft that has replaceable universals has the same universal as most of the Hilux/Hiace units, but the tailshaft will need shortening and a Hilux flange fitted to the universal.
Other Hilux & Hiace Diffs
Later Model Hilux 2WD
The later model, particularly 80's on, 2WD Hilux diffs are to wide to be useful in an Escort fitted with wider tyres. The axles do not suit re-splining as they are machined down behind the existing spline for too great a distance.
They may provide the ratio you are looking for though.
The Hilux 4WD diffs are generally 20mm narrower than an equivalent year 2WD diff. Some of the axles have an extra length spline that may be shortened without the need for re-splining, in particular check out the RN46 manufactured around 4/83. All 4WD axles have 6 wheel studs and can be re-drilled for an Escort stud pattern without the need for welding up the old stud holes.
The 4WD housings are the least suitable as they are step up in diameter toward the centre and will cause mounting hassles for brackets and shackles.
The 4WD centres will in some cases be Limited slip units and may contain suitable ratios.
The Hiace diffs are all too wide for an Escort. The housings are the same style as the 2WD Hilux but the axle tube dimension is 68mm x 62mm oval and 4.5mm wall thickness. Use of this housing represents a small weight saving which may be cost effective if the 2WD housing for your application requires shortening and other alterations.
The Hiace centres may also have the ratio you are looking for.
The mid 90s Hiace is fitted with a wheel stud that has extra spline length. These may prove useful for a disc brake setup.