Pretty new to partial mash, but have been doing a lot of reading about difference between partial and full mash brewing. I understand most of it, but what I don't understand is if you need to grind up your grains for a partial mash. Initially I didn't think so because you are already getting your needed sugars from the extract...but if I just steep the grain in a muslin what the heck am I getting out of it. Flavor ? I find it hard to believe that I would get the same amount of "flavor" from ungrinded grains compared to grinded . Thanks and happy brewing!

Yes, absolutely crush the grains. With a partial mash, you are getting MOST of the fermentables from the extract, but not ALL of them. You have to crush the grist to get the rest of them.

HTH-

Thanks...wish my local brewing shop had told me to do that. Should I just put the crushed grains back in the muslin bag or in a steeping bag? I would image the larger steeping bag would be easier to sparge with.

You certainly can. As long as you can mash them and rinse them effectively, you'll be ok.
1) Watch your temps.
2) Watch your temps.
3) Did I say watch your temps?

HTH-

If you've already been using steeping grains to add color and flavor to your beer, doing a partial mash differs in only two regards.

1. You will likely be using more grains, some of which contain active enzymes.
2. You will need to pay closer attention to time and temperature to make sure the enzymes can convert the starches into sugars.

Other than that it's essentially the same as steeping.