* Extends early participation date * Doubles max acceptance amount for 8.5 pct senior notes * Slashes max acceptance amount for 9 pct senior notes

June 3 (Reuters) - Oilfield services firm Allis-Chalmers Energy Inc (ALY.N: Quote, Profile, Research) said it amended certain terms of the tender offer for its senior notes and extended the early participation date for the offer. The company doubled the maximum acceptance amount for its 8.5 percent senior notes due 2017 to $50 million and decreased the maximum acceptance amount for its 9 percent senior notes due 2014 by $25 million to $75 million.

Allis-Chalmers also extended the early participation date applicable to each of the tender offers to June 18 from June 3.

Last month, the company said it plans to sell 35.7 million shares to its current shareholders for $2.50 a piece to repay outstanding debt.

In April, Allis-Chalmers amended its $90 million revolving-credit agreement with its lenders, relaxing the leverage and interest-coverage covenants.

Shares of the company closed at $2.81 Wednesday on the New York Stock Exchange. They are off about 85 percent from their July 2008 high. (Reporting by Adveith Nair in Bangalore; Editing by Vinu Pilakkott)