If it was relatively easy to increase the sludge trap capacity on a Square Four, the means to do so would be common knowledge by now - some 80 plus years after the pushrod Square first appeared. One problem is that Square Four crankshafts flex to a surprisingly large extent - as related by Val Page in his recorded interview by Jim Lee back in the early 1970s. The transcript of this recording exists in the Club archives. Given that the cranks flex, any reduction in cross - sectional area will reduce their life before fracture. Fracture of Square Four cranks, particularly the front crank, is not uncommon. This might be why the factory did not increase the sludge trap size over the production run of about 33 years, or about 29 years if you delete the period during WW2 when Squares were not produced. And even if you could safely enlarge the trap size, removing all 4 sludge trap plugs without dismantling the engine would remain on the somewhat impossible side of extremely difficult.
So we go back to the question of how often should you clean out the sludge traps? The Healey brothers reckoned that an engine run on non-detergent oil would throw a rod in 500 miles or less once detergent oil was used. They should have known, since they were the Square Four experts when detergent oils came into use. My own first Healey had an engine (Healey engine no 2) which had suffered a broken rod and the Healey engine I sold last year (Healey engine no 5) was the same. So we have evidence that the Healey brothers were well familiar with the sludge trap problem. One of the features of the Healey 1000/4 was the provision of an oil filter to extend the intervals between sludge trap cleaning.
Former AOMCC Square Four spares organiser David Jones threw a rod on his Mk II because of "waiting till the winter" before he cleaned out the sludge traps. Former AOMCC Chairman Lester Grant did likewise with his Mk II.
So I have never run a pushrod Square engine without cleaning out the sludge traps first and fitting an oil filter. I have subsequently stripped both my 1948 4G and my (first) Healey 1000/4 and checked the sludge traps again. Like David Anderson, I was pretty horrified to find how much sludge they had collected. The 4G had maybe done 6000 or 7000 miles when I stripped it but the sludge traps were around 1/2 full. The Healey had done about 15000 miles and the traps were around 3/4 full. Hence my recommendation that, even with a filter, cleaning the traps every 20,000 miles is necessary.
What I don't know is how quickly the traps fill over time and miles. I suspect that the first few hundred miles puts quite a deposit into the traps from the inevitable debris left in the engine when it is built, plus the particles which wear off during the running in period. I believe that my current Healey engine had done no more than 200 miles before I stripped it but there was appreciable debris already in the traps. They were perhaps 5 to 10% full. I would expect an engine in good condition to deposit relatively little once it has run in but as wear takes place the rate of deposition will increase.
Few people ride their Squares enough to exceed the 20,000 mile threshold for sludge trap cleaning. For those who do, it would be of great benefit to the other Square Four owners if they could asses how full the sludge traps were after whatever mileage they have covered - and let us have the information. Otherwise, you are all working on the data David Anderson and I have assembled - and I think that amounts to only 4 Square Fours.
36 4G, 37 VH, 53 ex ISDT KHA (project), 54 KH(A), Healey 1000/4 (project)
Former Machine Registrar & Archivist, General Secretary and Single Spares Organiser (over a 25 year period).
Now Archivist once more - but not Machine Registrar.