David Padfield inspected truck and trailer towing components in 2014, following a crash at Staghorn Flat on August 7 of that year.

The BP trailer decoupled on the Wodonga-Yackandandah Road, hitting two vehicles, with the prosecution in the trial of Heavy Mechanics Pty Ltd alleging the business failed to properly service the truck and trailer.

The prosecution argues the tow system needed to be completely rigid, or else wear would occur.

Mr Padfield on Wednesday detailed to the Wodonga County Court the wear to various components in the towing system.

He took a tow eye pin, which connected between the truck and trailer, and inserted it into a block which had been connected to a trailer component.

He demonstrated how it could move back and forth and up and down.

"I'm able to move the tow eye in the block quite easily," Mr Padfield said.

"So it's not a good fit or a snug fit."

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He told the jury on Tuesday "there shouldn't be any wiggle room between the tow components".

Mr Padfield also showed jurors photographs of the worn components and said such wear must have occurred over time.

"The threads are basically flat is what we're seeing," he said of one component.

Prosecutor Duncan Chisholm asked on several occasions if the wear had occurred quickly or over time, and Mr Padfield repeatedly said such wear had occurred over time.

The prosecution alleged that unless the wear was detected during the service, the tow components would eventually fail.

Jurors were shown videos testing new tow eyes in blocks, which showed movement even when tight.

Heavy Mechanics last serviced the truck and trailer six days before the crash.

The court earlier heard the tow eye pin and a nut, which holds the system together, had last been replaced on June 17, 2011, more than three years before the crash.

Mr Padfield said a previously installed towing pin could have caused some of the wear on the block attached to the trailer.

Judge George Georgiou told jurors they could hear the final evidence in the trial Friday and possibly start deliberations on Tuesday afternoon or Wednesday.

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