Is it necessary to add more or stronger screws to a 1377 when increasing power o

[Gerald K]

If I were to increase power with a ft valve and piston on a 1377/1322 do I have to use extra screws or a stronger one for the valve?

[Motorhead]

unless changing the swept stroke and actually producing higher pressure ... you are fine.
Simply reducing the dead space between piston and valve only raises the compression potential a small amount.

[subscriber]

That begs the question; how much does a "flat top piston" reduce the compressed volume?  If by 50%, then the peak compression pressure can be double.  This will be noticeable as double the peak pumping force, after an equal number of pumping strokes.

Then the question is, are the pivot pins and screw closures strong enough to stand that?   I would consult with the source for the modified parts, or the entity that provided the recipe for the modification.   If aftermarket commercial, or a common modification, a bunch of people would have experience to share; including about reduced part life; such as increased pivot wear - if any.

If the question is, "should I use all the parts supplied in a "booster kit", the answer is probably yes.  My answer would be that any modifications should not be made in a vacuum.  The person performing them needs to understand what is being done, and why.  What each part does.

The question about part strength cannot be answered without explaining exactly what is being done; or pointing to a commercial site that sells parts, or explains the recipe for the modification.  Else, it is like asking, "if I remove the rev limiter on my engine, will it blow up?"  In like manner, the answer to that is "probably not" if you exceed redline by 250 RPM.  Highly likely to be "yes" that something will break, if you exceed redline by 1000 RPM.


Rather than speculate, finding what people report with similar mods might be useful:

https://www.crosman-air-pistol-owners-forum.com/board/index.php?topic=6448.0

https://alliancehobby.com/flattop.htm

Prior posts about 1377 flat top pistons on GTA Forum:  www.google.com/search?q=site%3Agatewaytoairguns.org+flat+top+piston+1377

[Rick67]

Wyoman/Mr. Gary is the expert when it comes to these stuff.

His builds are top notch 

[subscriber]

Thanks Richard.  I tried to send Gary a PM, but his mailbox is full.

I will have to try another strategy to get his attention.

EDIT:  My attempt at getting Gary's attention - I replied to his last post on forum:  https://www.gatewaytoairguns.org/GTA/index.php?topic=192876.msg156238853#msg156238853

[CraigH]

Everything you know - Look around Gary's work:

https://www.crosman-air-pistol-owners-forum.com/board/index.php/topic,11146.msg139111.html#msg139111

https://www.crosman-air-pistol-owners-forum.com/board/index.php/topic,11147.msg139113.html#msg139113

https://www.crosman-air-pistol-owners-forum.com/board/index.php/topic,11149.msg139116.html#msg139116

and about pressure possible with FTP, Bob Sterne's work;

https://www.gatewaytoairguns.org/GTA/index.php?topic=77463.msg737664#msg737664

[subscriber]

Thanks, Craig

Bob's measured pressures on his testbeds show that an easy doubling of pumped pressure is possible, using modified pistons to approach "zero headspace".  Thus the loads on all the parts are double.  Whether the stock parts are up to that or not, long term may be moot, as Bob then went on to create his Millennium Pumper.

This is where the Crosman forum links are sure to be useful.  What is odd is that I can see some on that forum, and not others, without being a member.  I strongly encourage Gerald to register at that forum.  My interest is more in pointing Gerald to those who have the best answers, than for myself.

[Rick67]

I remember Mr. Gary saying that valve pinning increases the FPS a bit.

He actually showed a graph when Tack Driver said that doing such does not increase the FPS.

I have done the valve pinning method on my two 2400KTs, and I like it—-makes the valve very solid even without the belly OEM screw.

[CraigH]

Thanks, Craig

Bob's measured pressures on his testbeds show that an easy doubling of pumped pressure is possible, using modified pistons to approach "zero headspace".  Thus the loads on all the parts are double.  Whether the stock parts are up to that or not, long term may be moot

There is a stop stamped into the 13xx air tube.   Some years ago when building a 1322 with an FTP, I followed Gary's work and installed the set screw under the steel breech and into the valve.

When later reading Bob's work on pressures possible, I installed two machine screws, one on either side of the air tube and into the valve.   I did not do a countersink into the air tube as would generally be done with PCP's, the thought being three 8-32 threads and the factory design would be sufficient.   I did note that it appeared that Bob did not add any additional valve retention.