What I've found with that basic model on several engines, including cars, is that the 'flat' end that sits on the top of the valve needs an extension making (or a socket or something inserting) between it and the top of the valve.
I made a 'fenestrated' tubular wotsit out of an old box spanner, bunged one end up with a brazed-in steel plug and bolted it onto the open slotted bit, so I could wind the tool well in without hitting bits of casting - and then get the collets out with a little magnet through the windows. Getting the little barstewards back in again was do-able, but a fiddle.
If a compressor with a deeper head (or an adaptor) were available, it would be better I think. But needs must, and if the standard model gets things apart, the issue is postponed until you need to reassemble!