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Author Topic: Transfer port diameter  (Read 517 times)

fusion

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Transfer port diameter
« on: July 05, 2017, 09:55:18 AM »
Hello Bob, what diameter would you recommend for the transfer port on a .177 and a .22 PCP air rifle. 
Thanks!
Mike


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rsterne

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Re: Transfer port diameter
« Reply #1 on: July 05, 2017, 11:54:35 AM »
I use 75% of the bore, not larger than 80% or you could have problems loading a pellet without damage.... All the ports (exhaust, transfer and barrel) should be the same size, and the throat area (seat minus stem) should also be the same, or up to 10% larger....

So, for .177 cal the stock Crosman barrel port of 0.134" is perfect.... For .22 cal it can be enlarged to 5/32" or the next number drill up, 0.162".... The exception would be for a high powered bullet shooter, where you want full bore-area on all the ports, but that means oblong barrel ports and a lot more hassle....

Bob
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fusion

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Re: Transfer port diameter
« Reply #2 on: July 05, 2017, 12:59:40 PM »
Thank you Bob!
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Nvreloader

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Re: Transfer port diameter
« Reply #3 on: September 10, 2017, 09:38:39 AM »
Bob

Would the porting size for a 22 cal (.162") diameter remain the same when using the different fuel types using Co2 or Hpa?
If not, what size should be used for Co2?

Tia,
Don
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It is time to restore the American precept, that each individual is accountable for their actions"
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"Speeds fine, but Accuracy is final"

rsterne

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Re: Transfer port diameter
« Reply #4 on: September 10, 2017, 10:17:42 AM »
The transfer port is more related to the bore diameter than the gas used.... It is to prevent damage to the pellet on loading.... You can use up to 80% of the bore, if you carefully debur the port and slightly round the front corner to prevent it working like a cheese grater as the pellet slides past it.... On a .22 cal, that is 11/64" (0.172").... However, I prefer to stay with the 75% width and make the port oblong (longer than it is wide) if I need more power than I can get with a 75% port....

You can, of course, use smaller ports to save gas, but that will also limit the FPE.... For maximum power, all the ports, from the valve throat to the barrel (and inside the chamber) need to be the full area of the bore.... The valve throat can be up to 10% larger in area (don't forget to allow for the valve stem area), and to maximize the flow you will need a retractable bolt probe or nose.... The barrel port will end up being about 80% wide and 120% long.... eg. in a .22 cal I would use about 0.17" x 0.27".... This is really only necessary for a bullet shooter, most pellets can be driven into the 900s without oblong barrel porting....

Bob
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Nvreloader

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Re: Transfer port diameter
« Reply #5 on: September 16, 2017, 11:26:15 AM »
Thanks Bob

I just got a repeater breech for my 79, (to do your ninja 79 build), I was putting it together and found that the bbl has a major defect,
approx 3-1/4" from the chamber end there is a machining/ring defect that covers over half the inside diameter of the 22 cal bbl,
6-7 lands/grooves have this ring,  :(

I measured the bbl transfer port and it is .130" in dia.  So everything is on hold, awaiting the shippers reply.
This kit has the pin type probe, and a OEM stock valve, so I have a backup, if I screw up etc.  ::)

At least I can measure and note all info etc.

Tia,
Don
  • Western NV
NRA Life member

"We must reject the idea that every time a law's broken, society is guilty rather than the lawbreaker.
It is time to restore the American precept, that each individual is accountable for their actions"
Ronald Reagan

"Speeds fine, but Accuracy is final"