benjamin 397 accuracy issues tips and tricks

benjamin 397 accuracy issues tips and tricks

Joined: March 12th, 2010, 4:53 am

March 10th, 2012, 10:07 pm #1

Ok so I brought a benjamin 397 back from the dead....thank the airgun gods for brass !!!! this thing would have been toast. Any way.
After cleaning and resealing It came back to life(bolt seal was to big) so I stuck with the old one. Upon removing bolt to replace I noticed bolt cam missing..In the mean time I was holding the bolt in place as I tried shooting this 397. 8 pumps was weak I thought @ 500ish fps. I cleaned the barrel with nothing more than patches..now in the wait for the cam and the bolt seal I removed all the paint from inside muzzle end of barrel and recrowned per DIY instructions found here.

No I still think its lacking a bit on the FPS. 8 pumps gives me just under 700fps sometimes going over ?
Now as far as accuracy: 8 pumps @ 25 yards with CPs was so so touch under 1". Predator poly mag was consistent at just +/- .5 inches......then I brought out my favorite pellet that tends to do well in my other .177 cal. H&N baracuda 10.34 grain. Now before the bolt seal and cam replacement I did not notice anything unusual about the cudas except they grouped well pulled off several 5 shot groups touch under .5

Now with everything fixed and the trigger a bit lighter the Cudas need a nudge to go in all the way. pushing the pellet back out it seems to have rifling marks on the head. Accuracy now stinks with the cudas and horrible with the CPs...remains acceptable with the polymags ?

I also noticed this valve had what appeared to be a release or check valve in it ? does this regulate it from going higher than 700fps.
I been trying to post pics but it says I have to resize, I just have not been able to figure that out yet.
PS this is my first rehab of a 397

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this is one of the first test I did before finishing the rehab. its the only pic I got to work also.
Last edited by mac1911 on March 10th, 2012, 10:36 pm, edited 1 time in total.
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Joined: April 1st, 2009, 3:18 am

March 11th, 2012, 6:28 am #2

Well, if you view the schematic on Crosman's web site, that may help you out to see if all the internal vlave parts are the same.

There should only be three parts inside the valve itself. A white piece, spring, and the exhaust stem. Does the valve dump all the air after 8 pumps? If not, a new vlave spring may be in order for it. Also, you may want to inspect the valve to see if it is a Canadian market valve/gun. The exhaust stem, if a Canadian model, will have flats on it (to allow air to pass by) where the U.S. model has a fully round stem.


"The majority of things in our lives are created by folks no smarter than the rest. Afterall, the world is comprised, and operated by C average people intellctually, academically, and morally. These people are often the great pioneers that set the precedent for what excellence should be."
Last edited by Duane30 on March 11th, 2012, 6:32 am, edited 1 time in total.
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Joined: March 12th, 2010, 4:53 am

March 11th, 2012, 4:04 pm #3

I did not think to take pictures of the parts when disassembled. Funny thing is I did get some 775+ fps readings when I first replaced the bolt seal and cam. But now the FPS has seemed to settle down.
As for discharging of the air over 8 pumps it does not. 10 pumps only shows a very small increase in velocity. Its consistant with the velocities just not up to what seems to be normal. reading around the forums should be closer to 800fps with 8 pumps.

What i did notice with this valve was a ball bearing looking thing inside a hole on the valve body ?
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Joined: April 1st, 2009, 3:18 am

March 11th, 2012, 4:38 pm #4

http://www.crosman.com/pdf/manuals/B397 ... %20PL1.pdf

Also, is your Benji second hand or did you buy it new? Crosman, for the Canadian market guns, put flats on the valve stem. Unless, for the Benji line of guns, they decided to add a preloaded ball bearing seal in the exhaust valve half (the rear portion)? Much easier machining task to to put flats on the stem.

"The majority of things in our lives are created by folks no smarter than the rest. Afterall, the world is comprised, and operated by C average people intellctually, academically, and morally. These people are often the great pioneers that set the precedent for what excellence should be."
Last edited by Duane30 on March 11th, 2012, 5:21 pm, edited 1 time in total.
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Joined: March 12th, 2010, 4:53 am

March 11th, 2012, 8:39 pm #5

yes the valve looks like that, but does have a ball bearing looking thing in the rear part of the valve. No flat sides on stem. I was tempted to solder that hole up before assembling. Im not really into taking pictures or I think of it after. Other than the lake of FPS it shoot great. I have found 2 pellets it really likes of the rest with the williams peep sight it shoots 8 grain poly mags in the 1/2 to 7/8 groups size along with some beeman wad cutters. It will shoot consistant 1/2 groups of the rest @ 25 yards with 5 to 8 pumps if I take the time to weigh out the pellets and inspect the skerts.

This gun was bought new buy the privious owner. He claims it just stopped pumping up one day. Claimed it was never apart ????? He had left it the corner of his garage since about 2002ish It was covered in a dusty crudy film, not so much of a whisp of pump...I had never seen a 397 or 392 but when I seen the brass barrel I had a feeling it could be saved....
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Joined: May 31st, 2005, 5:24 am

March 12th, 2012, 3:03 am #6

Ok so I brought a benjamin 397 back from the dead....thank the airgun gods for brass !!!! this thing would have been toast. Any way.
After cleaning and resealing It came back to life(bolt seal was to big) so I stuck with the old one. Upon removing bolt to replace I noticed bolt cam missing..In the mean time I was holding the bolt in place as I tried shooting this 397. 8 pumps was weak I thought @ 500ish fps. I cleaned the barrel with nothing more than patches..now in the wait for the cam and the bolt seal I removed all the paint from inside muzzle end of barrel and recrowned per DIY instructions found here.

No I still think its lacking a bit on the FPS. 8 pumps gives me just under 700fps sometimes going over ?
Now as far as accuracy: 8 pumps @ 25 yards with CPs was so so touch under 1". Predator poly mag was consistent at just +/- .5 inches......then I brought out my favorite pellet that tends to do well in my other .177 cal. H&N baracuda 10.34 grain. Now before the bolt seal and cam replacement I did not notice anything unusual about the cudas except they grouped well pulled off several 5 shot groups touch under .5

Now with everything fixed and the trigger a bit lighter the Cudas need a nudge to go in all the way. pushing the pellet back out it seems to have rifling marks on the head. Accuracy now stinks with the cudas and horrible with the CPs...remains acceptable with the polymags ?

I also noticed this valve had what appeared to be a release or check valve in it ? does this regulate it from going higher than 700fps.
I been trying to post pics but it says I have to resize, I just have not been able to figure that out yet.
PS this is my first rehab of a 397

[/IMG]

this is one of the first test I did before finishing the rehab. its the only pic I got to work also.
700 fps on a later model benji with a non adjustable piston rod is pretty good on 8 pumps. the older benjis in .177 that had a different valve and adjustable flat top pistons would do maybe 750 on 10 pumps and that was pushing it with everything adjusted correctly and a light pellet. i don't have a clue why your accuracy changed, but i shoot nothing in my old and new benjis and dans but jsb pellets. all the different pellets i tried for 50 years, none shoot as good as the jsb's. if you want to try a couple of things i can tell you how to bring it up some, but you will need to remove the valve again. first, get the right size 0-ring for the bolt and adjust it to where it just bottoms out on fully closing the bolt. second, replace your hammer spring. third, get a valve that doesn't have the checkvalve or solder that one closed. fourth, the little white piece is usually about 1/4 inch long with a piece of round metal about 1/4 long behind it, or it won't have the piece of metal and the white check valve will be about 1/2 inch long. if it is the shorter one, leave the piece of metal out and get a slightly longer and weaker spring to replace the stock one. if the check vavle is the long one, cut it in half from the side so it will measure about 1/4 inch and use the longer and weaker spring. fifth, put new orings on the front, middle, and rear on the outside of the valve when yoy put it back together. with your thumb you should be able to tell if the spring is long enough to close the exhaust valve. i set mine so close that they usually have to be cocked before pumping or they will belch air out the barrel. when you screw the valve together with the new oring in the center and tighten it down scribe it and then back the front half off 1/2 to 3/4 turn. this will still seal and will take almost all if not all of the headspace out of the non adjustable rod. either that or get someone to make you an adjustable rod which is what i do on mine. simple and inexpensive. the reason for removing the piece of round metal or shortening the white checkvalve is this will give you slightly higher volume and you'll probably need to use 10 pumps to get maximux velocity. like i say the older 317, 347 would definitely shoot faster than the 397 but they had adjustable piston rod for zero headspace and different type valves, which by the way, were a lot more trouble to reseal than the later ones and weren't nearly as moddable since they were pretty much maximum to begin with. i have old benjis in 310, 317, and 347 that are .177 and old 312, 3120, 342 that are .22 cal. i also have 392 and 397 and c9 dans that are .20 and you'dbe surprised how much more efficient the .20 and .22 cals are vs the .177 and what this means on the same number of pumps from these three guns STOCK FORM, they are going to shoot pretty close to the same velocity with the .20 and .22 being 675-700 and the .177 being 700-725. i would be surprised if you EVER get a 10 grain pellet to shoot anywhere near 700 fps on a stock gun. these mods i have given you are proven workers.
Last edited by johnny366 on March 12th, 2012, 3:06 am, edited 1 time in total.
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