I use 13# equipment exclusively and periodically pose this question to each ball company.

I was given the following information from Brian Graham, Hammer Brand Manager,  back in November:

"Hammer does use the Diesel core shape in all 13# performance bowling balls.  The difference in ball motion will all depend on the different coverstocks."

If you check their web site, you will notice that all 13# balls have the same rg and differential -- 2.53 .055.

If you have any further questions, you can call Hammer directly.  I've found them to be very accommodating.

Hope this helps.
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LadyW....

The Hart pertains to the asymmetrical core of the Beat'n found in the 14 to 16 pounders.  If you purchase the ball in 13# it will be a symmetrical ball with a Diesel core.  The Hart might be physically shown on the ball but it doesn't pertain to it's inner core at 13#.

The above is my understanding based upon previous conversations with the Hammer technical staff.

You can call or e-mail them directly to double check this info.

Please let me know if it differs.
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LadyW....

Edited on 8/21/2007 7:17 PM

I'm more than happy to help.  Most of the ball company websites do not provide information on their lighter equipment.  As a result, they give the impression that the cores are the same as the heavier equipment.  In most cases, that is totally untrue.  I have also found that most pro shops are totally misinformed about the lighter equipment.  That is why I've always spoken directly with each company's technical department.  I have found most companies to be very helpful and forthcoming with information.

If you ever need info on cores used by other companies, please don't hesitate to ask.
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LadyW....