"Which Oil?" is an interesting and useful book, written by a Richard Michell, an Australian oil industry specialist and classic car owner. The one big Q he didn't write about was the Trunnion one. So I wrote to him, and he was kind enough to reply, at length, and to say I might copy his response to Triumph owners.

When reading, please note that Richard has never owned a Triumph. Don't mark him down for not being familiar with details of the Triumph trunnion and where the oil/grease goes in.

"First I must confess to having never owned a Triumph vehicle although I did once seriously consider a Dolomite Sprint. Plenty of other British cars - MG TC and TD, Morgan 4/4 Competition, Austin A30, A40 and Seven amongst them. In fact my first car was a one-fifth share of a Standard Flying Twelve, a name that would fall foul of Trades Practices legislation today. Perhaps as a consequence of my Triumph abstention, I had not heard of the trunnion lubricant controversy until you raised it. Also, I have never seen the tunnion concerned so I may not fully understand the design.

At first sight, this would seem to be an application where a grease would be the preferred means of lubrication. The nipple would preferentially be located at the bottom of the trunnion and the seal at the top. The trunnion would be regreased at appropriate intervals and the fresh grease would displace the old. Grease would be preferred over oil for the same reason it is elsewhere on chassis, it does not leak out. It also does a better job where surfaces settle onto each other when not in use, as is presumably the case in these trunnions. Oil is squeezed out whereas grease soap is not.

So why did Triumph change from grease to oil? From a quick Google search, apparently it was because grease was drying out and a hard white powder was being formed.

All greases will bleed some of their oil when put under pressure, particularly if there is a large ratio of surface contact area to grease volume, as would be the case in the trunnion. However, in the layout that I have suggested, the outcome should not be catastrophic. The oil would remain in situ and would not evaporate. It would require gross neglect to cause failure of the type observed and, as such failure requires loss of the oil, switching to oil as the lubricant would not seem an immediately-logical corrective step.

The following is pure speculation. Back when these trunnion designs came into being at Triumph, the lubricating grease specified for them was presumably chassis grease. Such greases would have been based on Calcium chemistry at that time. A 'feature' of calcium greases is that they require some water to be present in them if they are to retain their structure. Without water they separate into oil and a solid soap phase. This soap is a 'gritty' solid, white or pale amber in colour. This behaviour is part of the reason that the upper operating temperature of calcium greases is about 80C.

If Calcium greases are held at elevated temperatures ( say over 65C ) for extended periods they can slowly lose their water content and separate in the manner described. As Triumph vehicles became faster - and particularly with the introduction of disc brakes - the increased heat generated under braking may possibly have regularly brought the trunnions up into this temperature danger zone. Once the grease destructured, regreasing would not necessarily fix the problem. The new grease would tend to displace only the separated oil, leaving the gritty soap behind. Over time, soap would build up and the vertical link could bind in the trunnion.

Turning to Triumph's solution - moving from grease to oil - it appears that the objective was to have the trunions sitting in a bath of oil. The oil addition 'nipple' is at the top, as is the seal. This design will control the friction when the vehicle is under way but will not be as good at controlling wear or initial friction (for the 'squeeze out at rest' reason I mentioned earlier).

To overcome this, Triumph specified use of an Extreme Pressure (EP) gear oil. It is here that they got a little confused, in my opinion. If you have digested the relevant bits of 'Which Oil?' you will be aware that EP additives are used to control wear and friction in boundary lubrication conditions - the conditions that apply in a starting-from-rest Triumph trunnion or when it suffers an impact when motoring, such as a pothole. However, Triumph seem to have missed two critical points. The first is the one that current Triumph owners have cottoned on to - EP additives can be chemically aggressive to 'yellow metals' such as bronze. However, this chemical attack does not become significant until temperatures of over 100C and so it is not a significant concern in the bronze trunnion application. Also, modern EP oils are far less aggressive than those of 40 years ago.

The big howler, at least to me, is that EP additives work only on ferrous metals. They have no impact on non-ferrous, other than the potential for corrosion. For the same reason, moving to the GL-4 specification for the gear oil, to reduce the possibility of corrosion, does not necessarily give any wear protection to the bronze trunnion. The GL-4 wear tests are also on ferrous metals. So we appear to have the situation where, if anything is being protected from boundary layer wear, it is the harder steel vertical link rather than the softer brass trunnion. The protection of the bronze wear surfaces is solely by the oil portion of the gear oil, not the EP portion.

To my knowledge there are no chemically-active EP additives designed for bronze. An oil film is an excellent lubricant for bronze where full hydrodynamic lubrication conditions can be maintained but designers usually avoid the use of bronze where boundary, non-film conditions will exist regularly. Bronze will wear and may even distort. I presume that Triumph used bronze in this location to avoid binding between the two threads, e.g. from corrosion. Also, the softer bronze would sacrificially shear relatively easily if it should bind at any localised spot.

So what alternatives are there for the lubrication of a Triumph trunnion? Solid lubricants could be helpful, molybdenum disulfide being the obvious first candidate. Using a gear oil with some truly-colloidal moly in it - at a few percent - could be beneficial. Some moly will stay between the surfaces when the trunnion/link are at rest and give a soft, easily-sheared protective layer on both the bronze and the steel surfaces. It will also protect the surfaces against impact loads. However, the moly would have to be truly-colloidal - completely stable in the oil - or it could settle out and ultimately lead to binding problems of its own.

I don't know whether it is feasible but the design I described earlier - a grease nipple in the bottom of the trunnion - would be the preferred way for me. The top oil addition plug could then act as the vent while the trunnion is being greased. The grease I would suggest is a CV joint grease with 2- 3% moly. Modern greases will not separate out into a solid soap phase and an oil phase in the way a calcium grease could historically. However, regular regreasing should be carried out at the standard chassis maintenance intervals.

Richard Michell, Jan 2012"