* Honda says won't use Takata front air bag inflators
* U.S. imposes fine of $70 mln, could be more
* Takata shares close down 13.4 pct, briefly down 20 pct
* Takata CEO says to phase out ammonium nitrate (Recasts, adding more Honda comment, Toyota, Nissan, analyst. Changes slug, pvs USA-TAKATA/)
By Naomi Tajitsu and David Morgan
TOKYO/WASHINGTON, Nov 4 (Reuters) - Honda Motor Co said it will no longer use front air bag inflators made by Japan's Takata Corp, raising questions about the future of the embattled parts supplier.
Takata, which counts Honda as its biggest air bag customer, was fined $70 million by the leading U.S. auto safety regulator on Tuesday. Shares in the Japanese auto parts firm slumped by as much as a fifth in Tokyo on Wednesday.
Regulators have linked eight deaths - all in Honda cars - to the Takata inflators, which use ammonium nitrate and can explode with too much force, spraying metal fragments inside vehicles.
The fine adds to a growing list of potential bills Takata could face. To date, automakers have primarily borne the cost of 'investigative' or voluntary recalls as the root cause of the inflator defect hasn't yet been found. If recalls become official, the cost could switch to Takata.
Having to pay around $100 to fix each of the many millions of cars would cost Takata more than its current market value of around $820 million. Around 40 million cars have been unofficially recalled worldwide since 2008 over Takata air bag inflators.
Along with the fine, the U.S. National Highway Traffic Safety Administration (NHTSA) ordered Takata to stop using the potentially dangerous propellant in its products.
In a harshly worded statement, Honda said it would no longer use Takata's inflators in front-seat air bags, saying it was "deeply troubled" by evidence suggesting Takata "misrepresented and manipulated test data for certain air bag inflators".
Honda makes up around 10 percent of Takata's global sales, and is its biggest customer for air bags, which account for 38 percent of the supplier's revenue.
The air bag scandal dominated Honda's quarterly results presentation in Tokyo on Wednesday, with the company saying that quality-related issues, including air bag recalls, accounted for 1.9 percent of second-quarter revenue, almost double the amount since 2013.
Honda's move highlights its impatience with Takata, and raises fresh uncertainty over the supplier's prospects. Takata already faces criminal investigations and lawsuits in the United States.
Honda Executive Vice President Tetsuo Iwamura declined to comment on possible legal action against Takata, or the future of the firms' relationship. "Once we determine an official cause of the defect, we plan to discuss issues of responsibility (with Takata)," he said.
Honda did not specify what alternatives it would use, but it has been buying more air bag inflators from Takata rivals including Autoliv, TRW Automotive Inc and Daicel Corp.
"Honda has taken a significant hit from this," said Christopher Richter, senior analyst at consultants CLSA. "If the maker can't figure out what the root cause is, can they afford the risk of continuing to use them as a supplier? For Honda, that's a no," he added.
Among other leading Japanese automakers, Toyota Motor Corp said it would take further action on air bag inflator issues as necessary. It has previously said it was testing rival air bag inflators. Nissan Motor Co said it would defer to NHTSA on actions related to Takata, but noted it was "surprised and disappointed" with the company's conduct.
"Because they make their own inflators, Takata earns high margins on its air bags. If more automakers use (rival) inflators, Takata's profit could suffer even more than sales," said Koji Endo, auto analyst at Advanced Research Japan.
"Other companies see the risk of using Takata inflators so they will likely follow Honda and avoid using them."
In its statement, NHTSA accused Takata of providing "selective, incomplete or inaccurate data" from 2009. The regulator also said it could demand an additional $130 million fine if Takata does not comply, or new violations are found.
Takata said it would phase out ammonium nitrate in all its inflators by end-2018, even though it had not yet determined the root of the problem.
"We still think our product is safe, but we realise there are many concerns from consumer, automakers and U.S. regulators," Takata chief Shigehisa Takada told reporters.
Takata will pay the $70 million fine in six instalments through October 2020.
Experts say the ammonium nitrate used as a propellant in the inflators could become unstable after being exposed to high humidity over a period of time. Some have suggested that auto design, including how well sealed passenger compartments are against humidity, could also be a factor.
Takata shares closed down 13.4 percent at 1,189 yen, their lowest close in nearly a year. (Reporting by Naomi Tajitsu and David Morgan, with additional reporting by Maki Shiraki and Chang-Ran Kim in TOKYO, and Bernie Woodall and Paul Lienert in DETROIT; Editing by Ian Geoghegan)