Ok so I was looking at aluminium blocks today..when most have steel blocks they use aluminium mains ..why does the aluminium block come with steel mains..i understand they are cross bolt but why not aluminium..just a hummm thought

All aftermarket blocks both iron and aluminum use a billet steel cap. The aluminum cap deal came along to try to keep production blocks alive in big power applications. The thinking is that they help absorb the "shock" that tends to crack mainwebs and caps. The real problem is usually from detonation but that is the thinking. Does it work? Well if you look at all aftermarket blocks (not just mopars) they nearly all have a steel cap.
Todd

Thanks Todd. I see your point and I have always heard detonation was the killer.

My BAE block has alum main caps.

Both style caps work, but the change in clearance with aluminum caps suggest a race-only application.

What brand of block are you looking at?

I was looking over the Indy maxx which states billet mains ..

If its for a street car, I would ante up for one of the new KB units. I like the valley in them better than the Indy for oil drain back....just my 2-cents.

The aluminum main caps are to absorb shock loads, that's the thinking. They do this because aluminum is 1/3 as stiff as steel or nodular iron. Of course it would also be possible to triple the section thickness to make the aluminum part as stiff.

In the case of an aftermarket block with sturdy main webs, stiffer caps are a benefit, because they retain their shape and thus the shape and clamping of the bearing shell. I believe there's a lot more bending and flexing than most of us think possible.

R.

100% false statement to say ALL aftermarket blocks come with one certain set of caps.
I have a shortdeck r3 Pro stock block here at home that's Iron and has aluminum caps.

OK OK OK. In the one sentence I said all and should have used MOST. HAPPY now! YES there are SOME aftermarket blocks that use aluminum. Clearly I should get out more often. Clearly I have much to learn. THE LAST sentence I typed CLEARLY says MOST. I know that there are blocks that use them. I was pretty sure the original poster was looking at the aftermarket Big blocks that to my knowledge all use a steel cap. I will go crawl back in my hole now. What was I thinking...Todd