How to solve a large scale problem

Q The cooling passages of my Lancia Aprilia project’s engine seem to be blocked with a concretion of rust and limescale, plus a quantity of casting sand. I’d rather not remove the engine, so what can you suggest? Dermot Jones, Edinburgh A It’s often effective to fill the engine with a strong solution of kettle or dishwasher descaler, run it up to temperature, let it sit for a week and flush it with a high-pressure hose. This may have a limited effect on passages that are completely blocked, though, as the descaler can only nibble at the ends of the blockage, rather than softening it along its whole length.

It’s probably a good idea to remove the cylinderhead and undertake as much manual cleaning as possible, both of the water jacket around the cylinders and the various passages in and around the cylinderhead and water pump. Use compressed air or a water jet. Alternatively, find a wet-and-dry vacuum cleaner and reduce the suction nozzle down to a length of flattened copper pipe, which will scrape and suck at the same time as you probe the depths of the block.