Indianapolis-based Allison Transmission Holdings (NYSE:ALSN) is getting bigger -- or at least share offering is.

On Tuesday evening, the world's largest manufacturer of fully automatic transmissions of medium and heavy-duty commercial vehicles, medium and heavy tactical U.S. defense vehicles, and hybrid gas electric transit busesĀ announced that it is increasing the size of its secondary offering of shares by selling insider stockholders. Previously slated for 16.6 million shares in size, Allison's offering will now increase to 20.7 million shares, plus as many as 3.1 million more shares if the offering's underwriters exercise an overallotment option.

Shares will be offered at $22 apiece, making this secondary offering worth up upwards of $520 million before costs, fees, and expenses. However, Allison itself will not receive any of this cash. Rather, all proceeds will go to the insider shareholders selling shares in to the market.

Allison's two largest shareholders, owning 34.5% of the company each, are private equity firms Onex Corporation (TSX:OCX) and the Carlyle Group (NASDAQ:CG).