Trigger Spring Specifications QB78

[nocalories]

Hello, Bob and Lloyd. I'm pursuing a 2-stage trigger mod to my QB78D. I'd like to know the specifications of the OEM trigger spring (wire size, outside diameter, etc.) so I might change it as necessary. Can you help? A B.C. native, I speak metric, but I use imperial measure, too. I am aware that at least one 2-stage mod kit is offered for sale, and I've read of the DIY threaded spring-plunger mod (I have the part in-hand). Any advice or links to wise writings about 2-stage mod appreciated. Thanks!

[nervoustrigger]

Here’s my take on it
https://www.gatewaytoairguns.org/GTA/index.php?topic=125063

The location of the spring plunger relative to the pivot point gives a lot of leverage so it needs to be a fairly firm plunger.  I initially used one that had roughly 1lb of initial force and even with a weak spring on the trigger, the wall of the 2nd stage was barely detectable.  It was much better with a 2lb plunger.

All that is in the thread linked above, just trying to summarize the part most relevant to your question.

[rsterne]

Yep, you will need the strongest spring plunger you can get in the tiny size there is room for.... Mine was a 6-32, and I had to re-tap the hole for it....



Part #2 in the photo above.... Screw #1 adjusts the static sear engagement.... Areas circled in yellow show where I polished the trigger and sear....



The gap to the plunger sets the length of the first stage.... Make sure to maintain enough sear engagement for safety.... The trigger blade spring S must be replaced by a lighter one as well, and adjusted to suit....

Bob

[nocalories]

Thanks for the replies. Variations on the spring plunger mod seem a great idea. I do have an idea for a simple, world-beating 2-stage kit, with a two minute install time. My wife and I live and travel full-time in an RV, so I've no workshop. I'll post a drawing tomorrow. If the community agrees it's practical, perhaps someone with a shop (and a QB family rifle) could produce one for testing. Thanks again for the replies.

[Gertrude]

Thanks for the replies. Variations on the spring plunger mod seem a great idea. I do have an idea for a simple, world-beating 2-stage kit, with a two minute install time. My wife and I live and travel full-time in an RV, so I've no workshop. I'll post a drawing tomorrow. If the community agrees it's practical, perhaps someone with a shop (and a QB family rifle) could produce one for testing. Thanks again for the replies.

Ooohhh, I think there might be one or two of us here, who might be willing to test out your design. Bring it on   
 

[Nvreloader]

Nocalories

Like Gertude said, "Bring it on",
as there is several us QB air rifle junkies around here, LOL

I have several of the spring plungers sitting here,
But have been testing my plastic washers trick in the trigger, seems I cured a problem I found etc.

Tia,
Don

[nocalories]

As promised, here is my drawing. Wouldn't this work on the old Crosmans (without altering a vintage gun), too? My children aren't giving me grandchildren, so this will be my legacy. Please name this the "MUNRO MOD" if you like the idea. I'm certain there's a market if someone wished to produce them for sale. Mine's free, correct? BTW: I have three of these guns!

This is a replacement for the original grub screw. Brass would work for all except springs. Let's call the parts the grub screw and the stem, and the small and large springs. This would be an easy machining job in brass.

SPRINGS: Springs of any dimension (wire size, outside diameter, length) are available for pennies by post from China. The big spring would be lighter wire than OEM. No pressing reason to change the outside diameter of the big spring, I think. The small spring would be tiny, but stout wire for it's size - 2 mm or thereabouts INSIDE diameter, four to six mm long.

STEM: It can be pretty spindly, because their is no significant load thereon. The original overtravel screw will arrest everything before strain results to the stem or small spring. A pinch point for the small screw, and a guide for the large spring is incorporated on the stem. Despite my bad drawing, the small spring need NOT be under tension until just before the trigger breaks, and can slide freely on the stem otherwise.

GRUB SCREW: Available in brass in different lengths for pennies. It should be machined (or otherwise sourced) with the profile on my drawing. This would incorporate an anvil pinch-point for the small screw and a guide base for the large screw. The grub screw must be drilled end to end to allow the stem to protrude. I'd suggest that the grub screw drilling can be oversize, and the edges chamfered to ensure no binding. Grease, too.

Before installation: 1. ADJUST OVERTRAVEL SCREW. 2. REMOVE TRIGGER SPRING GRUB SCREW.

1. Lift out the trigger.
2. Insert new grub screw (a sheet metal screwdriver relieved to go over the stem for adjustment would be nice).
3. From the trigger side, insert the stem (with both springs in place thereon) into the grub screw.
4. Replace the trigger.
5. Turn the grub screw in a little at a time. The trigger will function as a single stage trigger at first. Operate the trigger mechanism as you turn the grub screw in by degrees. At some time the small spring will begin to be pinched between the two anvil surfaces in the diagram. You want that small spring to come into play sometime just before the trigger breaks. Turn further until the heavier 2nd stage travel is sufficient for safety.

Notes: Determining the spacing between the small spring anvils to fit the chosen small spring can be figured by prototyping this and then using right-sized washers on the stem to adjust for testing. Too, washers or other spacers might be used to fine tune the large spring performance.

By reason of the grub screw now adjusting the second stage, large spring choice(s) and washers or other fixes would adjust the first stage trigger weight.

As stated earlier, a couple of different weight small springs, and three different large springs should please anybody. What do you think?

[rsterne]

The idea will certainly work, but to change the relationship between the 1st and 2nd stage pull weight would require a spring or spacer change.... As you screw in the collar to get some 2nd stage effect, you are also increasing the 1st stage spring tension....

I prefer the separate adjustment of using the plunger instead of the overtravel screw.... although I understand that it requires drilling out the hole and running a larger tap through it.... What would be VERY helpful is if somebody sold a kit consisting of a lighter trigger spring, the proper spring plunger, a drill and a tap, and instructions on installation and adjustment.... including what parts of the sear and trigger to polish (and maybe even the sandpaper necessary)....

Bob

[nocalories]

I understand your concern about effecting both springs while adjusting the 2-stage function. However, unlike my poor diagram, the large spring would be three or four times longer than the small spring. I imagine that the maximum safe and useful adjustment range for the second stage (small spring) is about 1.5mm. So, the grub screw is only moving in or out 1.5mm (2mm to allow for variations amongst guns). A maximum 2mm of large spring compression with a spring already carefully chosen for the job would have little effect. I do suggest that three large spring choices (light, med, heavy) provided with a kit would satisfy most. In any case, clever spacers/washers for the large spring could be supplied, and folks could delete or add these to fine tune that large spring.

My wife and I live and travel full-time in our RV, so I left my backyard shop behind. I know that someone with a metal lathe could get the stem part turned quickly. It would be necessary to then experiment (using tiny washers will speed the process of measuring before turning more test stems). Only the spacing of the anvils (with the small spring between) would take trial and error time. That testing doesn't require the large spring to be in place. The large spring could be chosen later, after the grub screw position is known. Just about any light large spring would work for testing purposes. If you folks don't know, mail order spring offerings are endless. The best of the brick and mortar home improvement stores can't begin to match the selection online.

[Manning]

Did the spring plunger mod a few days ago.  Used a 5mm plunger with short nose, and IIRC, a 4 lb inital force. {thank you Ribbonstone and/or nervoustrigger' for sharing your experience with the 1 lb force plunger}

There's plenty of room for a 5mm plunger.  not sure a 6mm would fit. 

With the stock trigger return spring, it's still a bit tough to tell when the second stage wall comes in.  I've settled on using two lighter rate and longer springs wound together.  Now it works like a two stage trigger should as far as pull, but the light springs won't quite return the trigger if you release the trigger after pulling to the wall when cocked.