Fremantle coach Mark Harvey has started the mind games ahead of Friday night's AFL blockbuster against Collingwood in Perth, questioning the legality of the Pies' off-the-ball tactics.

Harvey claimed Collingwood block "a fair way off the ball" and said he would seek clarification from the AFL umpiring department about whether their tactics were legal.

"I'm just going to ring up the umpiring department today to just make sure how far you can block off the mark from a kick-out," Harvey said on Wednesday.

"(And) if you go back off your mark whether you can get someone blocking you from behind.

"I just need some clarification on the blocking and how far off the ball (you can do it) and when you can.

"They do (it a lot), and it appears maybe a fair way off the ball."

As the build-up intensifies ahead of the top-of-the-table clash, Harvey also said the Dockers were on red alert for Collingwood spies.

"We've had a couple of sniffer dogs at the airport and they tell me that two assistant coaches for Collingwood are here trying to get some research on us," Harvey said.

"They're doing their intelligence, just as we are."

Harvey said he was still considering making four or five changes to the side that defeated Brisbane by 13 points last Saturday.

Nick Suban, Byron Schammer, Jay van Berlo, Antoni Grover and Alex Silvagni are all in line for recalls, while back-up ruckman Kepler Bradley appears the most likely to be dropped as Fremantle attempt to match Collingwood's running power.

"We've got a difficult selection process ahead of us," Harvey said.

"It (the selection decision) is a bit about the right mix of playing a different side.

"How they structure up is different

"Do we need more running players or do we need more key players?"

Fremantle, perennial underachievers since entering the AFL in 1995, will surge to the top of the table for the first time in their history if they defeat Collingwood.

"It's a different type of pressure," Harvey said.

"The pressure is we're playing the best team in the competition at this point of the season.

"It will be a great judgement for us of where we're at.

"Our young players are looking forward to it, our older players are showing the way.

"They're more experienced than us Collingwood.

"What that counts for in a big game like this we're about to find out.

"Internally we have to be measured about the game and obviously realise the enormity of where the two teams are."

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