MASCOT May 1994

ROADSTER NOTES - Trouble With Trunnions

Bill Haverly

Though the earlier beam axle Roadsters gave a more bumpy ride than the later cars with IFS, at least they didn't suffer the wear points associated with the 4AB-4AD Series. The points in question are the pivotal ones at the top and bottom of the swivel pins and link the wishbone arms to the swivel axle units. They're usually called the top and bottom trunnions. Although some critics of the top trunnion assembly say that it should have been made stronger by having the pins on the forged trunnion block larger than 9/16" diameter, it seems to me that the main wear problem at top and bottom is caused more by inadequate greasing arrangements than a basic design fault.

The top greaser serves both the block bore/swivel pin bearing and the two pin/bush bearings. The grease makes it to the first point, passes through the centre of the pin , then relies on a blanking plug at the end of the top suspension arm to redirect it back into the pin bearings. But even with a tight plug and new bushes fitted it's doubtful whether much grease will be able to force its way back to lubricate the whole bearing. When the pressure builds up, the grease usually oozes out around the top and bottom of the trunnion block - the line of least resistance. A more direct feed to the trunnion pins was an omission.

The 3/4" bottom trunnion pin copes with its heavier load, but like the top bearings suffers from pivotal wear due to limited movement. Here the greasing provision would be adequate if only the angled nipple were accessible! Tucked in behind the back-plate on the bottom wishbone it's difficult to get a grease gun in unless the nipple happens to be pointing in the right direction. So caught between ill-fitting blanking plugs and inadequate pressure at the top and inaccessible greasers at the bottom, it's easy to understand why satisfactory lubrication is difficult to achieve, and excessive wear so common.

As a postscript to the gloomy scenario above, the good news is that parts are available when wear has developed to MoT failure proportions. John Lamb arranged the production of bottom trunnion kits a while ago, comprising pins/nuts/thrust washers and bushes and I still have these in Club stock at £40 pair (£36 to Spares Fund Members).

Recent problems with my own top trunnions on the 4AB have prompted the manufacture of undersize bronze bushes. These are made to 1/2 " inside diameter, the absolute minimum safe limit advised by 'Engineer Brains' (alias Ashley Crossland) who adds that a good radius between pin and trunnion block must be maintained when pins are machined down to fit undersize bushing. Otherwise, in his words, the pins will 'snap like a carrot'.

These remade bushes can be reamed to any size between 9/16" and 1/2", to suit the minimum amount needing to be skimmed off the pins. Blanking plugs are. also available to keep the pressure up and the grease in. The last batch was the closest fit obtainable from a standard size - these new ones have been made to the exact diameter. Top trunnion bushes 1/2" ID £2.50; std top trunnion bushes £1; blanking plugs 50p, all each.