At 09:44 PM 11/18/04 -0500, Jonathan Stein from mgatwincam group wrote:
"Quite a few of us have had highish compression pistons made up by Arias to a design conceived by Ralph Steinberg at Cloverleaf Auto near Philadelphia."

At 01:20 PM 11/19/04 +0000, Don Haynam wrote:
"I too have the Arias pistons and I believe the design is such that the pistons are all made to the original 9.9/1 and they then machine off the top to whatever ratio is requested. I ordered mine at 8.3/1 not knowing that you could get higher. When they arrived they were 9.9/1. I called Arias and they gave me the dims. to remove material to make them 9.3/1, which I did."

On 8/3/2010, Mick Anderson wrote:
I have attached a couple of drawings for interest. [see links below]
I have arranged the drawing of a Twin Cam 9.9 CR standard bore piston.
I am rebuilding a Twin Cam engine at the moment. If I cannot get a set of originals I will get a set made up by Arias.
I would specify the BMC design from the top of the cylinder bore to the top of the crown and Arias design for the lower section.
The diameters would of course be increased as required for oversize bores.
This should give a much lighter forged piston.
There are very slight differences from the measurements in the Technical Data Book, but I think that measuring a production original is a better idea as BMC were messing about with diameters, particularly ovality, and the latest details may not have been recorded in the book.

MGA-P-01 - MGA Twin Cam 9.9 CR Piston Drawing (401-KB pdf)
MGA-P-02 - MGA Twin Cam 9.9 CR Piston Skirt (121-KB pdf)

Addendum May 12, 2020:
Notes: The drawing MGA-P-01 from Chris Halliday in 2010 is incomplete and has at least one error: -- Crown angle 52 deg is wrong, should be 50 deg.

Erroneous Drawing Detail Erroneous Part Correct Original Part
This drawing documents existing geometry/dimensions of the piston for the MGA Twin Cam 9.9 CR. It does not detail any manufacturing tolerances of either dimensions or geometric features of the piston (except gudgeon pin bore).

The original pistons used by the factory had the Hepolite name inside the piston.

At 11:54 AM 5/12/2020, Bill Spohn wrote:
"I sent a stock Twin Cam piston to Ross Racing so they could scan it to make me a set of forged pistons. I went conservative on the race engine. It turns out that a stock high compression dome results in 12:1 in a 1950 cc engine, so I wasn't really worried about detonation because I had changed the shape. If you scale the stock low compression crown shape up to the same size, you wind up very close to 10:1, making a street 1950 cc engine fairly easy, at least from that aspect". --BILL

More piston notes on the following pages.