The market leading Thwaites 1 Tonne Hi-Tip dumper is the most popular skip loading dumper in the UK both amongst hire companies and with end-users and is powered by a Yanmar 3TNV76 engine. As well as the Thwaites being the UK’s most popular dumper the Yanmar 3TNV76 is one of PES’s biggest selling engines and predominantly is sold to go into the Thwaites dumper. ‘Why is this’ you might ask, is there a problem with the Yanmar engine or the Thwaites dumper? The answer to this is an emphatic no, the reason lies in the inherent fault of the narrow access skip loading dumper inasmuch as when the skip is raised the centre of gravity goes up and the machine becomes theoretically unstable. If you are moving a narrow wheelbase dumper and raising a skip with potentially 1000kg in it and you drive over a brick or are on a slope or even turn the wheel too aggressively the skip-loading dumper becomes unstable and will tip over.

So why does that damage the Yanmar 3TNV76 engine you might ask? What happens is that when the engine is on its side the oil in the Yanmar engine sump can get ingested into the intake system of the dumper so instead of air on top of the piston you have oil as well. In the normal combustion process the piston moves up the Yanmar engine block, both inlet and exhaust valves are closed and the air becomes compressed and superheated and then the diesel injector sends a spray of diesel into the superheated air which ignites and causes a mini-explosion driving the piston back down the Yanmar cylinder block. When there is oil on top of this Yanmar piston this oil cannot be compressed like the air and therefore that force has to be released somewhere. What normally happens is it finds the weakest link and the Yanmar con-rod which connects the piston to the crankshaft bends and, in the worse-case scenario, snaps resulting in the conrod coming out through the side of the Yanmar block, the infamous ‘leg out of bed’.

How do I avoid this happening to the Yanmar 3TNV76 in my Thwaites dumper then?

  1. Make sure your operators are adequately trained, just because its small doesn’t mean its any less dangerous than a normal site dumper.
  2. As soon as it tips over turn the Yanmar engine off. Speed is essential, as soon as there is sufficient oil on top of that piston it will bend and then snap that con-rod. Don’t just stand there looking at it wondering what to do. Bent con-rods means the engine is cost-effectively repairable, a leg out of bed means its scrap!!!


It’s not just the Thwaites with the Yanmar that this happens to. The Benford/Terex/Mecalac fitted with the Kubota, the JCB fitted with the Perkins and the Wacker Neuson and the Belle also with the Yanmar 3TNV76 all have the inherent instability problem caused by operator error.

If you are unfortunate enough to find yourself in this position, as the largest stockists of Yanmar engines in the UK PES can supply you a new complete engine from stock for next day delivery and in 9 times out of ten a new engine is the most cost-effective way of getting your dumper back on the road. However, it’s better to train your operators properly in the first place than pay for a new engine!!!