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4AGE RWD Conversion: Transmission Options 

This page deals with transmission options for getting the power from your 4AGE to the rear of your car. In simple terms there are only two gearbox options available, one of which is considerably superior to the other.
 

Page Contents 

Gearboxes T-50 Gearbox K-50 Gearbox Power Handling
Clutches Gear Ratios Starter Motor Spigot Bearing
Bolts Speedo Drives Speedo Corrections Differential

Gearboxes 

There are two RWD 5 speed gearboxes that will fit any 4AGE - provided you have the right bell housing to mate the gearbox to the engine. These gearboxes are called the K-50 and T-50. There are also 4 speed versions of these gearboxes, called the K-40 and T-40, that are fitted to other Toyotas, so be careful to ensure you get one with the right number of gears.

T-50 Gearbox 

The T-50 gearbox has a separate bell housing, uses a hydraulic clutch and comprises 4 main alloy casting: bell housing, two diagonally split gear casings and a tail housing. This gearbox is often referred to as the ‘split case’ alloy 5 speed and weighs 29 kg. The ONLY bell housings that will mate a T-50 gearbox to a 4AGE come from a RWD 'A' series engine (3A/4A/4AGE).

The T-50 bell housing has letters cast externally into the top section between the uppermost bell housing to engine block bolt bosses that identify the engine it came from ie 3A or 4AG etc. 3A and 4A bell housings have the clutch fork on the LHS while 4AG bell housings place it on the RHS.

The T-50 gearbox was fitted to other engines as well, such as 1S and 3T etc. The gearbox itself is identical, however, the bell housings differ to suit the parent engine. There are also two types of T-50 gearbox, referred to as 20 and 22 spline. This relates to the number of splines on the gearbox output shaft. Both gearboxes have the same number of splines on the input shaft. There is also a difference in the diameter of the input shaft bearing, which is covered by the bell housing: 62 mm vs 68 mm. The 68 mm bearing is in the 22 spline gearbox. The 22 spline gearbox is more desirable as it is stronger. It is also what all RWD 4AGEs came with.

In general terms gearboxes made 1983 or newer are 22 spline. This means that 3T, 1S, 3A and 4A engines have 22 spline units. 2Ts are 20 spline - so be careful. The AE 85/86 and AA 63 series T-50 gearboxes are 650 mm long from the engine end of the bell housing to the centre of the gearlever mount, however, some tail shaft housings fitted to earlier 20 spline gearboxes are shorter, hence allowing relocation of the gearlever with some mixing and matching of components. 

In reality a 22 spline gearbox is the only one you can use because it is the only one that will mate correctly with an A series bell housing and handle the power.

K-50 Gearbox 

The K-50 gearbox has an integral bell housing, utilises a cable clutch, and comprises two main alloy sections: front casting with integral bell housing and the separate tail housing. There is also a pressed steel plate that bolts to the bottom of the front section.

The K-50 is normally found behind 3A 1500 cc SOHC engines fitted to AE 70 and AE 85 Corollas etc. The ONLY K-50 that will fit comes behind a 3A. Bell housings off any other motor will NOT fit a 4AGE.

In either case ensure you get the corresponding bell housing front cover plate (that bolts to the back of the block just ahead of the flywheel) and the 2 cast stays that bolt between the bottom of the block and the lower portion of the bell housing. Apparently the rear crankshaft bearings can flog out if these are omitted.

The K-50 is ideal for 4AGE conversions into KP 61 Starlets etc, ie cable clutch cars. Although it is not as strong as the T-50 you will probably get away with it in a light car, especially if you get a 22 spline version, although a 20 spline version will fit the starlet drive shaft yoke.

Gear Ratios 

Although there are close ratio gear sets available for the T-50 all factory boxes have one set of ratios, irrespective of the engine. The K-50 also only has one ratio option.
 
Gear Ratios 1st 2nd 3rd 4th 5th Reverse
T-50 3.587 2.022 1.384 1.00 0.861 3.48
T-50 TRD#1 2.63 1.891 1.38 1.00 0.861 3.48
T-50 TRD#2 2.341 1.607 1.195 1.00 0.886 3.48
T-50 TRD#3 2.960 1.607 1.195 1.00 0.886 3.48
T-50 Quaife 2.347 1.733 1.379 1.144 1.0 3.48
K-50 3.79 2.22 1.44 1 0.87 4.32
AE 101 Transaxle 3.166 1.904 1.392 1.031 0.815 3.250
Caterham 6 Speed 2.69 2.01 1.59 1.32 1.13 1.0 (6th)

Some potential sources for T-50 Close ratio gear sets are:

 
 
TRD (Toyota Racing Development) Japan 
Quaife, England 
Housman Auto Sport, Ontario Canada 
Hollinger Engineering, Melbourne Australia 
Note: There appears to be a relationship between Quaife and TRD on T50 parts. It is suspected the TRD gear sets are actually manufactured by Quaife.

Power Handling 

As mentioned above there are two types of T-50 gearbox - 20 and 22 spline output shaft. The 22 spline gearbox is stronger and is capable of handling 200 horsepower in a Lotus Seven type car (i.e. 600 kg) almost indefinitely, and there are several behind 170+ horsepower 3TGTE 1800 cc Turbo engines in 1000+ kg cars, however, they will wear out eventually.

Apparently one trick is to fit tight steel straps around the split case part of the T-50 to help hold the two halves together. This allegedly helps prevent the two halves separating, dumping the oil and the gearbox then dying. If anyone can vouch for this I would be interested to know.

The K-50 will handle the lesser powered 4AGEs in a normal weight car, however, the T-50 is recommended. The K-50 really comes into its own for easy conversion of Starlets and early Corollas with cable clutches to 4AGE. Although the gearlever may require a new hole in the transmission tunnel it is quite likely the K-50 will bolt to the existing gearbox cross member, requiring only engine mounts to be fabricated. (Note - the 4AGE will bolt into 3A sub-frames - and some KE 70 series Corollas came with 3A engines…)

I would certainly not recommend putting a K-50 behind a 4AGZE…

Bolts to Mate Engine to Gearbox 

T-50
Application Number Of Diameter (mm) Thread Length (mm) Thread Pitch (mm)
Starter Motor
2
10
50
1.25
Bell Housing Bottom
2
12
75
1.25
Bell Housing Top
2
12
55
1.25
Stays to Bell Housing
4
10
40
1.25
Stays to Block
4
10
28
1.25
K-50
Application Number Of Diameter (mm) Thread Length (mm) Thread Pitch (mm)
Starter Motor
2
10
50
1.25
Bell Housing Bottom
2
12
60
1.25
Bell Housing Top
2
12
60
1.25
Stays to Bell Housing
3
10
50
1.25
Stays to Block (1)
2
10
25
1.25
Stays to Block (2)
2
10
35
1.25

The bolts are high tensile and quite expensive so I suggest you buy the bare minimum you require. If the exact bolt sizes are not available, you may have to buy the next longest bolt and cut to size. Don’t forget to use spring washers.

Flywheels and Clutches 

There are 3 basic clutch types, as follows:
 
Clutch Diameter Normal Fitment Notes
7 7/8" (200 mm) Pre 89.5 Normal 4AGE fitment
8 3/8" (212 mm) Post 89.5 May be fitted to some earlier MR2s

Also fitted to 4/7AFE and 20 Valve.

Largest clutch to fit in T-50 bell housing.

8 7/8" (224 mm) All 4AGZE Will not fit in T-50 bell housing.

Clutches and flywheels are interchangeable between all motors, however, they must be changed as a unit - a 212 mm pressure plate will not fit the other flywheels.

The 200 and 212 mm clutches will fit a T-50 gearbox - the input splines are the same, irrespective of the number of output splines, and the clutches and flywheels fit inside the bell housing. The 200 mm clutch fits the K-50, but I am unsure if the 212 mm will. All the clutches  fitted to the various 4AGEs will work with the standard T-50 release bearing etc. Use a 5/8" clutch master cylinder.

The 224 mm 4AGZE clutch and flywheel assembly will not fit inside the T-50 bell housing, and the 224 mm clutch plate has a larger diameter spline than the T-50 input shaft - in fact it fits the W-55 series ‘Supra’ gearboxes. I believe that it may be possible to modify the 4AGZE clutch and flywheel assembly to fit the T-50 bell housing with careful application of a die grinder and substitution of pressure plate bolts for dome headed cap screws. If you decide to go down this path be very careful to ensure sufficient strength remains to prevent the assembly coming apart at 7500+ rpm and amputating your ankles… I personally would not do this.

Custom built paddle/puck/button clutches may be the answer if you suffer from clutch slippage with standard items. By all accounts heavier pressure plates are generally a waste of time provided the correct material is used for the clutch plate. Clutch plates with solid centres transmit a lot of shock to the gears during changes - I have sold a number of Supra gearboxes to people running V8 etc with solid clutches - harsh changes just snap the teeth right off the gears...

Lightened billet flywheels are also available for the engines.

Spigot Bearing 

Any FWD 4AGE will not have a spigot bearing in the back of the crank for the gearbox input shaft to mate with (although the Black Top 20 Valve apparently does have one fitted). It pays to check and replace if necessary. The standard RWD 4AGE bearing fits all 4AGEs. This bearing MUST NOT be omitted otherwise the front gearbox oil seal dies and you get an oil leak and have to pull the gearbox out to change it.

If you need to get an old bearing out the easiest way I know is to squirt/pack grease in through the centre of the bearing to fill the cavity behind it. This area is reasonably large so don’t be surprised when it seems to take forever to fill up. Then take a close fitting bolt, place it in the centre of the bearing and hit it with a hammer. The hydraulic pressure will force the old bearing out. Don’t forget to use safety glasses etc.

Starter Motor 

Easiest option is to use the starter motor that is correct for the gearbox, so make sure you buy it when you get the gearbox. Don’t forget the cover plate that goes between the back of the engine and the front of the bell housing too - you will have to remove the flywheel to get it. The 4AGZE starter motor is incompatible with RWD gearboxes. 20 valve or red top flywheels (they are the same) work just fine with AE 86 starter motors.

Speedo Drives 

The standard speedo drive comes directly out of the gearbox tail shaft housing. If you have a really tight transmission tunnel then a VDO etc angle drive can be used to allow the speedo cable to lie parallel to the gearbox. Many automatic RWD Celicas etc had a factory angle drive that will mate with a T50 etc - and it costs a lot less from a wrecker than a VDO stockist...

Speedometer Drive Corrections 

If you change your overall tyre size or diff ratio, or mate the gearbox to a completely different speedo then you will need to make a speedo error correction. My thanks to Neil Fraser for the following information:

"The gearbox speedo worm drive ratios I know of include 5 x 19, 6 x 20, 6 x 22 etc. (ie 5 starts on the worm gear and 19 teeth on the driven gear). Most speedos are 637 rpk, 675 or 1000 rpk (revs per km). So lets say that to cover 1 km of distance your output shaft at the gearbox will turn how many revs?

Say a tyre diameter of 590mm: Circumference = 22/7 x .59 = 1.854m
Axle revs: 1000/1.854 = 539.37
Output shaft revs: 539.37 x 4.11 = 2216.8284
Say your speedo is 675rpk (should be written on back somewhere) - required speedo cable revs: 2216.83/675 = 3.284, so therefore best ratio is about 6x20 (3.33)

The small gear will have written on it the number of starts on the worm it would be compatible with (ie 5 or 6). Using a 5 start gear on a 6 start worm is always terminal. The worm slides off easy enough but requires practically stripping the whole gearbox…"

Differential Ratios 

The 4AGE loves to rev. The factory RWD cars come with either a 4.1 or 4.3 differential ratio. 4.3 is recommended. For any car running a Ford Escort type diff 4.44 is the recommended ratio. If you take an AE 86 with a 4AGE for a drive and you find it is uninspiring check the axle code on the chassis plate - if it is T-292 it is 4.1 ratio. A car with a T-282 (4.3) ratio goes much better. If the last digit is a 3 or 5 then you have just found a limited slip diff. As a comparison the AE 111 20 Valve uses a 4.312 diff ratio.

Remember that a 0.86 5th gear combined with a 4.44 ratio diff is equivalent to a four speed gearbox and a 3.8 diff. 

Any good tyre shop should be able to supply rolling diameter specs for the tyres it sells. If not then  rolling diameter (excluding tyre squash due to car weight - which you can assume to be consistent) can be figured thus: tread width *  aspect ratio gives sidewall height. Multiply this by 2 (two sidewalls contribute to one diameter overall) and add to it the rim diameter.

Thus 195/60 x 14 equates to (195 x 0.6 * 2) + (14 * 25.4) = 589.6mm = 23.2 in. Multiply this by pi to give circumference = 3.1412 x 0.5896 = 1.85 metres, so for every revolution of the wheel the car moves by 1.85 metres.

My car does 6500 rpm in top gear at max speed - that is 6500/0.86 (fifth gear ratio) = 7558 drive shaft rpm. This in turn passes thru a 4.44 diff ratio, so divide 7558 by 4.44 to give 1702 axle rpm. This means that a wheel is doing 1702 rpm, which is 1702 * 1.85 = 3149 metres/minute.

Multiply by 60 (minutes in hour) then divide by 1000 (metres in kilometre) and you get 188.95 kilometres/hour speed, which if you divide by 1.62 gives 116.6 mph. My car indicates 115 mph at this speed - not bad speedo calibration huh?

To compare a 185/70 x 13 tyre (what I used to have): (185 * 0.7 x 2)+ (13 * 25.4)= 589.2 mm diameter vs 589.6 mm for a 14 inch 60 profile - pretty close to the same, especially once tread wear occurs.

A friend runs the same box and diff as me, but 185/60 x 13 tyres: (185 * 0.6 x 2) + (13 x 25.4) = 552.2 mm diameter vs my 589.6: now divide one by the other: 552.2/589.6 = 0.936 ie his diameter is 0.936 or 93.6% of mine. Since this directly relates to engine rpm, and our gear ratios are the same (otherwise you have to account for these in the same way) his engine will do 1/0.936 more rpm at the same speed in the same gear = 6940 rpm.

Alternatively if I put his wheels on my car, my speed would drop to 116.6 *  0.936 = 109.1 mph at the same revs (6500).

Next: Modifications 

 
 


Copyright © 2000 SpeedTECH Last modified: January 23, 2000