Well known in the US as the TurboHydro 180 (fitted to countless US postal vans), Down Under we call it the Trimatic. It’s the M40 according to Holden’s transmission code. Whatever you call it, it’s a three-speed automatic introduced with the HG in 1970. Until they were phased out in 1988 — with the last of the carburetted 304ci V8 VL Commodores — these humble transmissions served behind literally hundreds of thousands of V8 and six cylinder Holdens.

First published in the November 2007 issue of Street Machine

Although never considered ideal for high-performance applications, the unpopular Trimatic (often referred to as the Traumatic) can be turned into a competent performer, capable of handling up to 450hp reliably. Some users have stretched this to 600hp but reliability quickly becomes an issue at that level.

The Trimatic’s best features are being smaller, lighter and more efficient than the bigger TH350 and TH400, robbing less power from the engine. One of the biggest issues with rebuilding or beefing up a Trimatic is the years of wear and tear. Other than basic rebuild kits (below) — which are of quite good quality — very little exists in the way of replacement or upgrade parts due to the low demand. That means it’s not uncommon to have to sift through three or four gearboxes to build one good one.

Precise Automatics kindly offered to take us through a typical rebuild to show what’s involved in building a tough, reliable and nice-shifting Trimatic.

STEP-BY-STEP

1. Patient and methodical checking is the key to building a good Trimatic. Wear and poor original machining tolerances cause many problems. Spend time to verify all clearances and surface flatness and the ’box will work very nicely.

If you don’t get everything right, it doesn’t matter what quality frictions you use or how well you set it up, it’s not going to give good longevity.

2. This piston compression tool (see arrow) prevents tearing of the O-ring seals during assembly. While it is possible to do without, there are about half a dozen specialised tools needed to reliably assemble a Trimatic — even with a good workshop manual, rebuilding a Trimatic at home in the garage is not recommended.

3. Factory pump gears are adequate for most performance applications. However, worn gears cause low hydraulic pressure, which is one of the main reasons older Trimatics don’t perform properly.

When fitting the ’box, always install the converter into the ’box then bolt the whole assembly into place, otherwise the converter can misalign with the drive tangs (centre) and chew them off.

4. Trimatics are highly sensitive to engine vacuum. Different vacuum signals from different engines will make the ’box shift very differently. Changing the line pressure regulator spring allows you to dial in the ’box’s line pressure to suit your particular car and engine combo.

Precise prefers Teflon second clutch sealing rings, as they work better than the original cast iron versions.

5. Factory V8 ’boxes come with five frictions in the second clutch (it’s engaged both in second and top) and four in the six-cylinder versions.

Precise increases this to six in its high-performance builds. To accommodate the deeper clutch pack, it removes the wave washer and use a shorter second gear spacer, as there’s a range of different heights available.

6. Stock Trimatics can slip from first straight to third if you back off the throttle just as you’re approaching the 1st-to-2nd shift point — tall diff gears exacerbate the problem. The fix is to increase the 2nd-to-3rd shift pressure (via an aftermarket replacement spring) to twice that of the 1st-to-2nd shift.

This doesn’t always work and must be fine-tuned in service by dropping the pan and changing or modifying the spring.

7. When bolting the pump to the bellhousing, a centring jig must be used otherwise front bush and pump gear damage will occur — the centre area on the pump side of the bellhousing gets worked, which swages it upwards. It must be machined flat if this occurs.

Due to ample pump thickness, warping is generally not an issue. However, it should be thoroughly checked for wear and straightness.

8. It’s necessary to increase hydraulic pressure throughout the ’box to give the frictions more bite, allowing them to transfer more torque.

However, these higher pressures tend to snap the original alloy reverse-gear pressure plate (left). On all its Trimatic rebuilds, Precise replaces the alloy version with a steel one (right).

9. While output shaft failures are rare, the same cannot be said for the planetaries. Although they’re well balanced and have four pinions, torn teeth are a common failure in high horsepower applications, usually caused by rapid downshifting when entering a corner.

Although it’s easy to repair, there are no upgraded or aftermarket versions available to give the ’box greater reliability in severe applications.

10. The input shaft (it runs up out of the picture) is welded to the clutch drum. It’s a busy component as it houses the third frictions and the segmented one-way clutch (also known as the sprag), while the second clutch frictions run on its outer groves.

In high-horsepower applications, the weld joining the two can fail, allowing the input shaft to tear itself clean out of the centre.

11. Trimatics commonly strip clutch material off the reverse clutch frictions. The debris goes right through the ’box, jamming the valves up and eventually burning out the ’box.

Soaking new frictions overnight ensures the oil gets deep into the material rather than just sitting on the surface. This helps prevent the frictions’ top layers from being ripped away the first couple of times they bite up.

12. The valvebody is the brains of any automatic transmission. To change the way the ’box shifts, Precise recalibrates the valvebody by performing a range of mods, including selecting the correct combination of springs and valve diameters, along with enlarging strategic apply-holes in the separator plate.

The Trimatic’s factory hold-on servo is quite sufficient, with no need to go to a stronger aftermarket unit.

13. The factory accumulator pin is press-fitted. This is converted to a screw-in fitment to better handle the increased hydraulic pressure and improve longevity. Once you get into modifying ’boxes it becomes quite technical as there are a lot of specific mods to suit particular vehicle combinations.

Final calibration can only be done in service and it can take a lot of time and effort to get the valvebody just right.

14. Where the bellhousing bolts to the main case is a weak area in Trimatics — they have a tendency to crack around the mounting bolts. In non-Holden fitments, the bellhousing is often cut in half, with a new front welded on.

Unfortunately, there’s not much meat in the pump area and if you end up with more than 20thou run-out, you’re going to run out of meat before being able to true it up.

15. The hydraulic pressure that runs the gearbox is determined by engine vacuum, via the vacuum modulator. Once vacuum becomes unmanageable, Precise recommends going to a full-manual set-up.

Drilling out the hole in the vacuum source up in the manifold (no bigger that 60thou) will help stabilise vacuum. Any bigger and you risk slipping from first to third (when backing off) and burning out the ’box.

16. An aftermarket cast alloy pan isn’t necessary as the original unit is quite generous. The main casing is relatively strong in this area — more flex comes from the bellhousing.

Trimatics are especially sensitive to oil levels — even slightly low oil will hurt them — so fix any oil leaks ASAP. Precise recommends changing the oil and adjusting the bands every 20,000km or 12 months.

WRAP UP

The longevity of any Trimatic largely relies on how fussy you are. Due to the lack of new parts to replace high-wear components, it’s tempting to take the attitude that ‘it’s not perfect but it’ll have to do’. However, unless you’re ultra-diligent with every aspect, the ’box’s life expectancy can be dramatically reduced, especially the steels — if they’re scored, burnt or distorted you’re going to need to source another set. If the ’box has been built properly, yet you’re still tearing them up, you need to look at a different ’box.

As for fitting a transbrake to a Trimatic, Precise says this is a no-no due to the change in operation. In a stock Trimatic, the third clutch and the rear band are both engaged when first (low) is manually selected. In transbrake-equipped ’boxes, the third clutch is disengaged when the transbrake is disengaged. That means all the torque must be transmitted by just the rear band and the sprag (one-way clutch) and they will not hold in high-horsepower applications.

Precise Automatics has a wealth of experience with a wide range of gearboxes, including finicky electronically controlled four-speed autos. Many top competitors and tough streeters swear by the strength and reliability of its gearboxes, so call them at (07) 3287 5270 or visit their website.