I'm not sure if all the 24Vs have this or just the 3.2s, but mine has this heat exchanger:



Obviously this has to come off so that an engine mount can go on. After removing it, I found that the 'center' hole for the oil filter is an 18mm unthreaded hole:



What has everyone else done to facilitate connecting up an oil filter or relocator plate here? I am hesitant to tap a thread there if I don't have to as I don't want stray metal filings in the oil system...

The risk is not the steel filings as you can put grease in there. The Risk is tapping square as if your slightly skew the filter will not sit properly and wham on start up - all your oil is on the floor. The thing is some filters the O ring projects more which is neccesary as the steel of the filter will catch the block first on that set up and wham all the oil on the floor on start up.
Check thread
.viewtopic.php?f=5&t=2648

Also there are two types of threads that Alfa use . 164 type and 166 type. A 166 filter will screw on to the 164 thread and feel ok to the average mechanic when fitting (no space above steering rack in 164) and it will come off after a few minutes. Ironically the 164 has the 166 thread on the inner of the exchanger.
You must decide which filter you gonna use and check your threads if you are going to make a ''stud'' 16 3/8 is one type.

My advice is to have one uniform thread like the one on the left in the pic. The top one prevents the filter screwing on properly. Once tapped add loctite to new insert. Also check protrusion of insert - stud as if it protrudes to far into the filter there is no gap for the oil to go into it and once more you will have an oil filter explosion on the floor.
Basically three checks. 1) filter you gonna use 2) protrusion of o ring 3) depth of insert to allow oil flow.

Thanks Kevin, very helpful as usual.

My engine is out of a GT, and the heat exchanger has a 3/4 -16 thread for the oil filter. I am happy to keep this thread as it is the same as many Nissans (which i also like to play with) so it is easy to get a relocator block for.

I stole a fitting out of a Nissan block hoping to be able to use it, it is similar to the one you recomend but it switches from 3/4 -16 to M18x1.5 part way along (the nissans have an M18x1.5 hole in the block it screws into). I could probably cut the M18 section off and use it.

You are correct it will be very important to make sure the thread is square.

After you do that you can put on the 164 'S' heat exchanger it fits with the mount.
I have one when I put the 'S' in my 75 I think they are a must. as they heat up the oil. and I think cold oil does more damage then too hot oil ever did. And with the large amount of oil that Alfa have it needs some help to come up to temp.
and it also cools too as the temp it locked to the coolant temp..