(Reuters) - Honda Motor Co (7267.T) was so concerned by mounting recalls for Takata (7312.T) air bags, and frustrated by its supplier's failure to explain why some have exploded, killing five people with shrapnel, that it bought used and scrapped cars in Japan to conduct its own tests.

Around 21 million vehicles have been recalled globally by automakers since 2008 for defective Takata inflators. Honda alone has recalled about 14 million cars, mostly in the United States, where four of the deaths occurred. The fifth was in Malaysia.

Two senior Honda insiders, who asked not to be named, said the tests on 100-150 Takata air bags at Honda's quality center near Utsunomiya, north of Tokyo, in the first half of this year indicated shortcomings in Takata's manufacturing quality and cast doubt on the competence of a company Honda considered part of its core group of suppliers, or 'keiretsu'.

"We doubted if Takata was producing air bags to the specifications we had mutually agreed on," one of the insiders said. "When we did not receive a clear analysis of what was happening, we decided to conduct our own tests ... and we found the quality of those inflators to be all over the map in term of key quality metrics."

Takata denied that Honda tested used air bags because it was dissatisfied with Takata's explanations and said it had not been told of any quality problems found.

A Takata spokesman said: "If Honda did the kind of quality tests on Takata inflators you're describing, wouldn't you assume Honda would communicate with us, to ask us about the quality lapses they supposedly found? As far as we know, and we looked into it extensively, there has been no such communication between us."

The Honda insiders said the tests did not identify the cause of the defect but led the company to believe manufacturing quality issues could have played a role.

Honda's chief spokesman Kaoru Tanaka confirmed that it conducts component quality tests at times when defects are suspected, but doesn't usually release the results or comment on specific tests.

"What's most important to us is our customer, and to take necessary action as quickly as possible for their safety ... and remove fears and worries felt by the customer as part of the air bag recalls," Tanaka said.

'EXTREMELY SLOPPY'

One of the Honda insiders said inflators from the used cars examined by Honda showed Takata was "extremely sloppy" in making the propellants that ignite to inflate the air bag in a fraction of a second in a collision.

He said the inflators contained varying amounts of the prescribed mix of ammonium nitrate and secondary ingredients, with many going beyond predetermined margins of error, and varying amounts of chemical compounds that strayed from Honda-approved recipes for inflators.

Some propellants that one of the individuals said he saw at the test center showed colorings he attributed to damage from exposure to moisture, a leading theory for the fatal defect.

Honda CEO Takanobu Ito's view on Takata, which supplies roughly a fifth of the world's air bags and whose ties to Honda, its biggest customer, go back more than a quarter of a century, soured around mid-year, after the tests. He felt he wasn't getting enough information from Takata on the cause of the defect, one of the insiders said.

The person said Ito felt "angry" and "betrayed" by Takata, and considered Honda a "victim" in the crisis.

It was not clear whether Ito ordered the tests.

Since the tests, the insiders said he had told people close to him that he had no sympathy for Takata and would not step in to rescue the company if it slipped into financial trouble.

They added that Ito's personal view did not necessarily reflect how Honda as a company might respond.

As the number of recalls has risen steadily, Honda has said it is looking to rival air bag inflator makers Autoliv Inc (ALV.N) and Daicel Corp (4202.T) for replacement parts to carry out the Takata-related fixes.

It wasn't immediately clear if Honda carried out similar random testing in the United States, but 10 automakers involved in the U.S. recall, led by Toyota Motor (7203.T), are expected soon to hire an independent engineering firm to test Takata air bags.