Three-Screw Maximus

[CraigH]

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.

[Charlie51]

Nice write-up, Craig.  I'm glad to see the project getting a completion check-off.  I guess I need to get with the program!

[oldpro]

 Needs Prod SS valve!

[CraigH]

Needs Prod SS valve!

Instead of regulator for shot count and tight ES?

[Ribbonstone]

Not sure where you are going with this...high power or low power/long shot count...but better valve retention is NEVER a bad idea. Just aren't many times folks complain that things are put together too srongly.

[CraigH]

Not sure where you are going with this...high power or low power/long shot count...but better valve retention is NEVER a bad idea. Just aren't many times folks complain that things are put together too srongly.

For sure - I was surprised the original two-screw retention has had a small amount of movement - nothing serious, but still - and the gun has never had more that 2000 psi in the tube.   I ensured that all three screws are in contact with the air tube.

I will play with power increases for a short time, but the goal, perhaps not clearly stated, is shot count and minimal ES.

Thanks for your reply!

[moorepower]

Needs Prod SS valve!

Instead of regulator for shot count and tight ES?

.                                                                                                                                                    Could do both 🤔 high shot count, low cocking force and tight es. The one thing I dislike about the Max is the hard cocking.

[CraigH]

Needs Prod SS valve!

Instead of regulator for shot count and tight ES?

.                                                                                                                                                    Could do both 🤔 high shot count, low cocking force and tight es. The one thing I dislike about the Max is the hard cocking.

Thanks for the SS feedback.   That may not be able to be fitted on this tube as I carefully centered the machine work on the factory pseudo third screw hole.   I likely will be limited to hammer stroke and hammer spring tuning.

Edit: And of course port size.

[Ribbonstone]

Not a bad place to be, if you are careful and test a whole lot.


Non regulated, just some adjustment to strikere weight, stiker spring tension, and transfer port, there3 are 40-45 good non-regulated  shots in a non-regulated Disco/Maxi...it's just not all that easy to bring them out.