Canadian Airgun Forum

custom HW98 scope stop pin
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Author:  cbf123 [ Sun Jul 03, 2016 11:49 pm ]
Post subject:  custom HW98 scope stop pin

I recently rebuilt my HW98 with a new spring to bring it back to factory power (it had been running around 6 ft-lbs). Previously I hadn't been using the scope stop pin, but with the increased power I thought it might be necessary, and I didn't want to risk damaging the holes the way I'd seen online.

I decided to replace the little pointy set-screw in my Hawke mounts with something a bit more substantial. I started with an M5 cap screw, wrapped some tape around the threads to protect them, and chucked it into a drill. I then used a belt grinder to reduce the thickness and the diameter of the cap to match the diameter of the hole and the thickness of the compression tube. There was a bit of trial and error here, finished diameter was a hair bigger than 6mm. I then cut a slot in the other end to allow the use of a screwdriver to adjust the projection once mounted.
stop_pin.jpg [ 42.58 KiB | Viewed 823 times ]

I then needed to make a recess in the base of the mount to allow for actually fitting the mount to the dovetail ways on the gun. Lacking a milling machine, I chucked up a carbide-tipped router bit in my drill press and ran it at 2500rpm.
milling.jpg [ 52.04 KiB | Viewed 823 times ]

It worked nicely, thought I was slightly off-center due to eyeballing everything. Good enough to do the job though.
mount.jpg [ 57.94 KiB | Viewed 823 times ]

Once the dovetail was engaged lightly I then screwed the stop pin down into the receiver hole and tightened everything up, making sure to align it all as near to exact as could.

It seems to have worked, we'll see how it holds up long-term.

Author:  Chevota [ Mon Jul 04, 2016 4:16 pm ]
Post subject:  Re: custom HW98 scope stop pin

I like it but I worry about a couple things, one being the new hole in the mount isn't supporting the screw as well since it's recessed, ie the force at the screws head has more leverage to bend or break the screw, or possibly tear up the alum threads. But a non-issue if the screws head is still inside the mount when deployed, but your head looks short. So I'd check it often at first because "if" the screw bends you know what that'll do to the gun, and we'd both hate that.
Also, the hole in the gun is centered as is the hole in the mount, but the mount sits off center on the gun. Not a big deal but I was thinking if you decided to go to a bigger screw size you could strengthen everything and could make the new hole offset to match the gun. For example I often replace the oem M5 with an M6 for strength, but with the much larger hole in your gun I suppose an M8 could be used.
Btw, I think the mounts might hold without the pin on that gun, but only if the clamp screws are tightened well enough which apparently not everyone does. Some people read online about some teeny tq value like 14 inches, but that's not right, plus they vary a lot from mount to mount. Or maybe it got lost in translation and was meant to be 12Nm.
I used the one-piece version of that mount on my gun, then added two extra clamp screws for both looks and clamping force, and no stop pin so it wouldn't damage the hole. I doesn't budge.
Just some thoughts...

Author:  cbf123 [ Mon Jul 04, 2016 11:58 pm ]
Post subject:  Re: custom HW98 scope stop pin

All valid concerns. :)

The Hawke installation instructions are pretty useless. "Using the enclosed hex key, tighten the screws on the side of the clamp." Gee, that's helpful.

Author:  Chevota [ Tue Jul 05, 2016 8:10 pm ]
Post subject:  Re: custom HW98 scope stop pin

I know, it sux. Even if they did give real tq specs, who has a tool that can do it? The mounts like yours have been made with M4 and M5 clamp screws. Your appear to be M5, but if not I suggest you drill/tap to M5. Then since the screws are cheap asian they'll likely be very rough which will tear up the alum threads, so I wire wheel the threads to smooth that up, and often retap the holes with a new sharp US tap. Some have kinda short screws so I'll buy some longer ones for more contact on the alum. Then I dry lube and grease the threads. From there I tighten by feel based on experience so I can't really say what but it's pushing near the limits of the threads. If all fails you can use helicoil which is probably ideal but $pendy. Or just get the one-piece mount and add screws like I do. Or however you want, just suggesting based on what I've had great luck with.
Hopefully your Allen key and screws are a good match, I've had lots of cheap sets where between being cheap and a loose fit the Allen and/or screw will round out and you're screwed. This is another reason I smooth the threads, and the main reason I use the special lube because you get more tightening per torque applied, ie less tq on that risky allen. I often use US Allen wrenches for this kinda work, and measure to find the thickest one, and if possible US screws but they're hard to find. Torx would be ideal, like a 35, but I've never seen M5 in Torx.
I was thinking about your use of the screws head vs the shaft, which is a great idea except for the heads length. So how about like a 1/4" or so steel shaft (1/4" shoulder of a Gr8 bolt) that simply slides into a smooth 1/4" hole in the mount? Make the shaft length so it's in the gun all the way and just barely under the scope, which I guess would be ~18mm? Optionally, to keep the shaft from rattling etc, you could add a set screw at the fwd top side. Or maybe just a piece of rubber on top or the pin that's squished down by the scope.
Just sharing thoughts passing thru my brain...

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