MartinL, I have run into the same issue more than once. Thing is, most aftermarket grips were apparently made to fit the traditional forged frame Hi Powers. Most quality aftermarket grips I've encountered fit my forged frame Hi Powers almost perfectly without any trimming or bedding, etc. Your Hi Power is the later cast frame version. The cast frame dimensions are slightly different. Just enough to make one crazy trying to get new grips to fit properly.

Several years ago, a grip outfit by the name of Raasco posted a picture of the area that might need to be trimmed in order for some grips to better fit a cast frame hi Power. Perhaps your grip fit would improve by trimming the area shown in the attached photo...

 

Not sure this will help. But, it does contain factory specs. L/R

 

When I bought my Israeli surplus FN Hi Power it came with some thin wood grips. Having worked at my LGS I knew these grips weren't OEM. I decided to by EOM grips from Numrich so that my FN would have the correct grips. The are wider than the aftermarket which I like.

The aftermarket grips on my FN didn't fit as well as they should. Who knows who did such a sloppy job of manufacturing

 

Take off the other side and see where it's hanging up. Minor scraping with an exacto knife will remove the offending material. The hipower grip isn't just a flat slab smacked on the side of the frame. Some fitting is needed.

 

My aftermarket grips were loose enough that I could feel them moving, and the tongue that fits under the edge of the front strap cracked on one of them.
I bedded the grips with epoxy putty, applied some blue loctite to the screw, and no issues since.

 

Craig Spegel sends instructions with his grips that sometimes you need to lightly file the front of the screw hole. Didn’t seem right but I had a set that didn’t seem to fit anywhere. A couple of passes with a round file, and they dropped right in perfectly.

 

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Fitting grips is always a challenge on Hi Powers. Your grips have one of the more difficult problems I have encountered, unfortunately. If you notice in the first pick above, the back edge of the grip does not have enough material to sit flush with the back of the frame. When you install the grip screw and crank it down, it bends the back of the grip down and that causes the front of the grip to lift up from flush against the side of the frame. The overhang on the bottom of the grip is likely a different issue.

In this scenario, I have tried to build up the back of the grip with layers of black tape or have used epoxy to build it up and then sand it down to fit. These options will work mechanically, but they look horrible.

I have found only 4 grip brands that I can consistently get to work well on cast frames:

1) OEM wood or plastic - Kind of obvious but they do fit well. You must source the specific wood version made by FN for cast frames. Older FN wood grips can look similar to the naked eye but they will not fit. OEM plastic grips with the thumb rest always fit.
2) Craig Spegel wood grips or the Uncle Mike's version of his grips in rubber
3) Navidrex micarta checkered grips - These need fitting but it is only to keep them from twisting. They bed against the frame well.
4) Checkered wood grips from KSD Grips - They are simple and cheap and often require no fitting like Spegels. Not a lot of options (plain walnut, black painted walnut, or rosewood)

Here are some pics of my cast Silver Chrome with black painted walnut KSD Grips for comparison. These required ZERO fitting.

Finally, I have also tried the new Hi Power grips from Lok. They bed well on a cast Mark III and feel OK in the hand but they have a goofy look on the bottom of the frame when installed. So I guess you can say they work but I don't like the cosmetics.

Scott

 

I have one type of grip that fit a cast frame Hi Power well without any fitting, Nill. I used them on my 1993 transitional cast frame Hi Power. Mine was an early cast frame, without the serrations on the bottom of the grip frame, but definitely a cast frame gun. Don't know how Nill accomplished it, but their grips fit both my cast frame and forged frame Hi Powers quite well..

 

Those are quite nice looking. I am not familiar with Nill grips but I am looking them up now. Thanks for this.

Scott