The Aluminum rotor housings will save you 15 lbs each.
I'm not sure how much (in weight) the Aluminum side housings will save you but they are available as well.

The 20B Center Housing can be had (in aluminum) as well but it is a bit pricey at $5K +.

All available through www.rotaryengine.com

Bruce T. put me on to them.

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From: wrjjrs@aol.com
Sent: Tuesday, August 03, 2010 4:36 PM
To: "Rotary motors in aircraft" <flyrotary@lancaironline.net>
Subject: [FlyRotary] Re: Bad rotary week

Thomas George was speaking of the side plates/intermed housings.

Bill Jepson

-----Original Message-----
From: Thomas Mann <tmann@n200lz.com>
To: Rotary motors in aircraft <flyrotary@lancaironline.net>
Sent: Tue, Aug 3, 2010 1:33 pm
Subject: [FlyRotary] Re: Bad rotary week

 ..... or go with the aluminum housings already P-Ported. It saves 15 lbs per rotor.

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From: Dwayne Parkinson <dwayneparkinson@yahoo.com>
Sent: Tuesday, August 03, 2010 3:20 PM
To: "Rotary motors in aircraft" <flyrotary@lancaironline.net>
Subject: [FlyRotary] Re: Bad rotary week

Lightweight carbon steel housings?  Where exactly do I find those?