At its introduction in the summer of 1961, the David Brown 990 was the most powerful wheeled tractor the company had ever made.

Its 52hp was modest by today’s standards, but 62 years ago this was the popular power bracket dominated by the Fordson Major and Massey Ferguson 65.

Enlarged engine for the 990

What set this model apart from earlier David Browns was the engine, an all-new unit which had been under development for several years and was known as the AD4/47.

The four referred to the number of cylinders and the 47 to its nominal power when first mooted as a development of the company’s three-bearing diesel engine, which had, so far, proved rugged and reliable, apart from DB 900 fuel-pump saga.

25D tractor air filter
The original engine from which the AD4/47 was developed provided 32p in the DB 25

The engine in its original form was considered to be at the limit of its development and the sales department wanted a tractor of at least 50hp, so there had to be some major work done on it to bring it up to spec.

Eventually, the extra performance was achieved via two routes: The first was to increase the displacement and the second was to redesign the head.

Factory restrictions

The increase in displacement was limited by the machine tooling available. The blocks were machined on a multi-spindle tool that honed the crankshaft and cam shaft bores, while simultaneously reaming the cylinder bores ready for the wet liners.

It was in this latter operation where the problem occurred, for the cylinder bore spindles were fixed at a specific distance apart and could not be altered, thus limiting the length of cylinder block that could be worked on.

The answer was found by increasing the throw of the crank by half an inch, which was the maximum stroke that it was possible to accommodate on the machine tool.

Various other adjustments were made and the cylinder block itself was redesigned and the crankshaft strengthened.

Switching sides

This new engine now displaced 3,045cc instead of the original 2,705cc and when tested on the bench produced 48.5hp, better than expected, but still not enough.

Attention then turned to the head and the valve arrangement, which up until then had the intake and exhaust valves on the same side, severely restricting the air flow.

The solution was simple and already deployed on some luxury cars such as pre-war Lagondas, and that was to mount the intake and exhaust on opposite sides of the head.

machine tools David Brown
Philip Green at the wheel of a David Brown 990, just one example from his extensive and beautifully kept collection

Believing that the arrangement had never been given a name before, the engineering team coined the term ‘cross flow head’ and this has stuck ever since.

Another advantage of moving the intake to the opposite side was that the air was cooler and so denser, allowing a larger fuel charge to be injected, increasing the output of power.

Mounting the air filer at the front, well away from the heat of the engine, also helped in this respect.

A cloud on the horizon

The upshot of all this work was that the engine now produced 52hp, well above that demanded by the management, and so the highly successful 990 entered production and went a long way to regaining farmers’ trust after the 900 debacle.

However, despite this happy ending there is, in retrospect, the quiet ringing of an alarm bell in the story which later grew to the cacophony which accompanied the demise of the British motor industry as a whole.

Massey Ferguson 65
At 45hp the Massey Ferguson 65 didn’t have the power of the DB 990, but it had a company focussed on agricultural machinery behind i;. Massey Ferguson is still with us

We have seen how the engineers were constrained in their design by the need for the engine to be built with an existing machine tool.

It is not clear quite why this was not replaced with a modern unit at the time, whether it was a lack of funds, or maybe the company’s management was reluctant to invest in the facilities.

There is evidence to suggest it was both. Profits from the agricultural business were rarely consistent and the dated machinery was jealously guarded by factory managers.

Either way, it hamstrung the design of a new product, and new and competitive products are the lifeblood of industry. David Brown had failed to take an essential step in ensuring its tractors could be kept up to date.

Without a full analysis of the company’s manufacturing history it is impossible to tell whether this was the beginning of the end, but the suspicion must remain.