The face of the piston has a noticeable casting sprue, and is hardly flat.
The valve face seems flat.
Putting the piston face against the valve face shows that full contact is not being made.
So I faced off the piston (I took off about .010").
Using some hi-spot blue, I tested the piston face against the valve face. Full contact was still not being made.
So I lapped the valve face against some sandpaper, on a surface plate. The valve hole was filled with a plug of paper towel.
After a few minutes of lapping the face was pretty flat.
As you can see, the faces are making much better contact.
So how much of a velocity gain did this produce?
I found I got between 377 and 380 fps, so a gain of around 5-9 fps. Hardly earth shattering, but nice to know a little performance increase had been realized.
4 comments:
Please buy the Daisy 5501 and tell me how to dissamble it.
All I can say is that with the exception of the 7XX pistols, I find the Daisy designers are insane. Usually it requires 10 hands to work on their guns.
You can get Daisy to send you an exploded diagram and parts pricelist if you contact them through their website. Otherwise I'd suggest getting on the yellow airgun forum and seeing if anyone has disassmbled one.
Good luck!
Since the compression seal isn't at the face of the piston, but at the O-rings, refacing the piston doesn't really have any substantive effect. What difference you measured may well be just sampling error.
The 717 and its kin are really an impressive bit of engineering, squeezing a lot of accuracy into a very inexpensive package. Other than improving the trigger a la Don Nygord's mod, there's not a lot you can do to improve them.
Ensuring that the piston and valve head are flat prevents any potential wobble at contact points in their faces. Any wobble here may pull the O-rings away from the chamber walls - not much, but perhaps enough to allow the last few PSI through.
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