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kiralinas · New Member

first:excuse my english
what diameter she has to be at the exit of the compresion chamber??
i have a hunter440 .22 and a chinese.177 and a romanian .177
at the last 2 is a little smaler then .177 at hunter 440 is much smaler then .22 she is smaler then it is at the chinese and at the romanian air gun
if someone knows somthing.........

AirGunEric · Site Admin

There is no "magic size" for the transfer hole- Depending on how much air volume is being moved (which is entirely dependent on the specific gun) and the overall length of the barrel, this will dictate the 'optimum' size of the transfer port passage.

Some people have enlarged them with some success, others have found enlarging them (especially on .177 cal rifles) they actually lose power.

I don't have any specifics on the rifles you mention, unfortunately.

Slavia · Moderator

I have read that opening up the transfer port reduces piston braking, so you're forever more stuck with using heavy pellets to compensate.

Here is a source where the guy maintains that it's not the size, but the smoothness of the port that matters. He advocates leaving the exit (breech end) of the port alone - square and original diameter. He talks about chamfering and reaming the entrance (piston end) to make it smoother, but not so much that it creates "lost volume." (Adding extra volume to the cylinder head that is not swept by the piston.)
http://www.filestube.com/a/airgun+tuning

I'll try modifying nearly anything and everything, but I leave the transfer ports alone.

kiralinas · New Member

thanks Slavia,very useful!
i love information like this,if u have more i want to have.

rsterne · Moderator

It would make sense that enlarging the transfer port would have a similar effect too using too light a pellet.... a very harsh firing cycle because the piston hits the end of the compression chamber.... Likely to cause damage as well....

There obviously has to be an optimum for any given gun/pellet combination.... but I'm with Slavia.... leave it up to the manufacturer....

Bob

Amigo · Veteran Member

Kiralinas:

Welcome to the forum!

I also agree with the chamfer (piston side only) suggestion given by Slavia, which is an excellent starting point.
Closer to an ideal shape after initial chamfer is the type of shape seen at the output end of musical instruments such as tube, trombone and trumpet.

The idea is to create a "funneled" air flow undisturbed by any sharp corners or edges. Sharp edges and corners create air turbulence which impedes both volume and speed of the air flow.

Finish off with as high a polish as is practical with what you may have available.

Unless there is an obvious manufacturing defect, opening up a transfer port is usually a "fools errand". The exception in this case might be the .01 or .02 millimeter port diameter increase due to very slight metal removal during the polishing process.

Best regards,

Amigo