Author Topic: Three-Screw Maximus  (Read 224 times)

CraigH

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Three-Screw Maximus
« on: July 20, 2018, 03:33:49 PM »
Why make a 3-screw Maximus?   Well, this project was still-born about 18 months ago thinking of a high shot count, so it is not a response to the Crosman/Benjamin Fortitude that is coming to market at Gauntlet speed.   The idea was add air capacity to support a regulator, not to make a 3000 psi operating Maximus.   The project parts, Magnum Airport high-strength valve retention screws, were in hand back then but other projects, airgun and otherwise, took priority.

This three screw project has been done this morning, but not without reconsideration of the components.   The reason for that is mostly the fact that the Magnum Airpower screws have a lower head than Crosman 8-32 low-head valve retention screws.   Three different screws are shown in image number one.   Dimensions are as follows:

                      Maximus    Magnum Airpower     Crosman #1700-010 (Prod,1701,1720)
Head Diamete    0.266”          0.2695”                       0.266”
Head Height       0.083”          0.064”                         0.085”

Also note the Maximus screw has two tiny notches in the head – these notches on both left and right side screws were precisely pointed toward the rear of the air tube.   The notches may be a light peening to avoid loosening, but if so are barely if at all able to be discerned.   My Marauder pistol screws have no such notches.

This Maximus tube had a hole where a third valve retention screw would be placed, but was 0.2475” diameter instead of 0.266” diameter.   The tube also had both forward and rearward 4-48 breech screw locations drilled.   Seems like something of a semi-generic tube.

So in the end I used Prod valve retention screws in place of the Maximus screws because of their head height, and carefully fitted a Magnum Airpower screw in the third location.    I milled the seat depth with a 17/64” end mill to 0.076” to center the screw, depth-wise, in both air tube and valve.   The screw diameter was spun and sanded to fit within .001”.   And I must say it is fitted infinitely better than the factory screws.

Image number two shows the 0.247” third air tube hole; image number three is the completed project.   Thanks for reading.


  • Lone Tree, Colorado
Craig

Beeman QB78D - Beeman P17 (2) - Beeman Chief
Benjamin Marauder Pistol
Benjamin Maximus Hunter
Benjamin Wildfire
CCS 2400KT; .22 14.6" LW barrel "Verminator"
Crosman 2240; 3 Carbines, 1 Stock Pistol
Crosman 1322C; 18" Carbine, FTP, Steel Breech
Crosman 1377; Phase 2
Crosman 1077 - Crosman 2100B
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SPA PP700S-A .22
Umarex 712 - Umarex P.08

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Alan

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Re: Three-Screw Maximus
« Reply #1 on: July 21, 2018, 03:49:54 AM »
Very interesting, and valuable information.
  • Roswell, New Mexico
Alan

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rsterne

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Re: Three-Screw Maximus
« Reply #2 on: July 21, 2018, 08:41:48 AM »
Nice job!....

My only comment would be that by fitting the 3rd screw perfectly to the hole in the tube, you want to make sure the other two screws are still tight against the back of the holes.... If they are centered when the 3rd screw is installed, and a poor fit in the holes in the tube.... then the new 3rd screw would be doing most of the work.... You want all three screws to press against the rear of the holes at the same time, to equally share the load....

Bob
  • Coalmont, BC

CraigH

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Re: Three-Screw Maximus
« Reply #3 on: July 21, 2018, 10:06:53 AM »
Thanks for the comment, Bob.   Yes, I took that into account.    Since this was an "add-on" vs a new install, I had the original screws locating the valve at their rearmost position against the air tube.   This in itself was a bit of a concern as they were not centered but had seen some small amount of movement in a gun that was never over 2000 psi.    And as an "add-on" I fitted the third screw with the valve in situ as that seemed the best arrangement due to the aforementioned appearance of the two original fittings.

Obviously this makes for a custom arrangement, but the screws/valve should move together if such movement were to occur.   Any future issues could or would be addressed with #10 screws.

Thanks for your concern!
  • Lone Tree, Colorado
Craig

Beeman QB78D - Beeman P17 (2) - Beeman Chief
Benjamin Marauder Pistol
Benjamin Maximus Hunter
Benjamin Wildfire
CCS 2400KT; .22 14.6" LW barrel "Verminator"
Crosman 2240; 3 Carbines, 1 Stock Pistol
Crosman 1322C; 18" Carbine, FTP, Steel Breech
Crosman 1377; Phase 2
Crosman 1077 - Crosman 2100B
Gamo Urban
SPA PP700S-A .22
Umarex 712 - Umarex P.08

Vintage
Crosman 130 - 150 - 180 - 600 - Mark I - 38C; .22's
Hahn Super BB Repeater