Is there any reason not to use a bottomless rancilio 58 mm portafilter in a brewtus iv, or any other 58 mm group?

I would imagine they are interchangeable but I figured it can't hurt to ask, since the search feature didn't turn anything up for me.

Just because a group is stated "58mm commercial group" does not necessarily mean a portafilter will interchange. One of the key areas is the "lock up" lugs on the portafilter into the grouphead, and the
depth/spacing for proper lock-up.

That being said, having previously owned a Brewtus III, I am fairly certain the Rancilio portafilter fits correctly to it, although I never purchased one and tried it. The basic standard here is the original Faema E-61 group, and certainly fit the widest variety of machines. On Chris Coffee site it shows the brands of machines the Rancilio portafilters will typically fit, and it lists the Expobar Brewtus.

My Rancilio Silvia portafilter fit my Brewtus 2 fine but not all 58 mm portafilters are the same.

I found that out the hard way when I ordered the wrong extra heavy 58 mm portafilter for the Silvia I used to own. After spending a lot of time filing both the ears and the outer rim I got it to work fine. My final conclusions after the ordeal was it was not worth the amount of work I put into it and I should have sent it back.

Some brands of portafilters interchange with others do some research before buying because 58mm portafilters are not standardized.

What Pat said -- 58mm baskets are interchangeable, but many PFs are not. Rancilio and E61 PFs are interchangeable, and the bottomless Rancilio is sold by a few on line vendors as an E61 bottomless, so you got lucky.

There's a second issue with PFs and groups that comes up even whe everything is of the same brand -- soemtimes it takes excessive force to get the PF in far enough. (The PF should always get to around 6 c'clock - the handle pointing straight out toward you). If the PF requires to much force to get to 6 o'lock, you should fle the bottom of the ears to make them a little thinner. On ver old groups, the PF can go to far and never get tight. In that vase, you will need a spacer or thicker group gasket -- these are standard parts at suppliers. You always want the PF to insert easily and securely.

another_jim wrote:you should file the bottom of the ears to make them a little thinner.

A better method that I read about here on HB, is to use a flat surface and a sheet of sand paper to lap the top ridge of the portafilter that the filter basket mounts against. Filing the ears & maintaining the proper angle and an accurate thickness on both ears is very difficult.