Benjamin Hand Pump - Sun Optics Repair Instructions

[Bruce Jarrett]

Zae333
You said in another post the #60 o rings at Lowe's was too small. I noticed Lowe's has a ticker #78 o ring did you try them ? Since you are repairing every 6 weeks or so where are you getting the o rings ? Is Sun Optics sending these to you or are you buying them . O rings don't cost much it's just knowing the correct size. Are you replacing all the o rings every time or just P30 ? Did the 10w 40 motor oil seem to work better then the silicon grease that was in it originally ? Do you think full synthetic motor oil would be better ? I would like to stay with the Benjamin pump if practical. Sorry for all the questions but with my gun shooting great I'm trying to learn about the weak link , the pump. Thanks

[zae333]

I noticed Lowe's has a ticker #78 o ring did you try them ?
- Yes, they are too big. I also tried a #36, which is very, very close to the right size but still, too big.

Since you are repairing every 6 weeks or so where are you getting the o rings ?
- The repair kit Sun Optics sent me came with 6 o-rings. I am down to only two left. I have not found a source for good replacements yet.

Are you replacing all the o rings every time or just P30 ?
- No, the other o-rings are holding up just fine. It is only P30. I've replaced it so much that I can do it in about 15 minutes now.

Did the 10w 40 motor oil seem to work better then the silicon grease that was in it originally ?
- I have not tried using this on the whole pump, just the P30 O-ring and unfortunately, it doesn't make much of a difference.

Do you think full synthetic motor oil would be better ?
- It actually might, at my next replacement I'm going to try that.

I'm actually in the same boat with you Bruce. My gun shoots amazingly, 1/2 inch groups at 45 yards and sub 1/4 at 20 yards. The pump is definitely the weak link but it can be repaired very quickly once you master disassembling it. And my 6 week repair schedule is a rough average, I shoot a lot, sometimes filling my Marauder from 2100 psi back to 3000 psi 3 - 6 times a day on weekends and at least 3 more times throughout the week. So my pump may get quite a bit more use than say your average shooter's. I'm looking into buying higher quality Viton orings or better, I'll post back with the exact sizes for the P30 oring. Got to get new batteries in my calipers.

[Bruce Jarrett]

ZAE333
I called Sun Optics and he said P30 was size -007 That is an international sizing . The numbers in Lowe's and Home Depot on their o ring packages are their own numbers. Sun Optics is sending me some but he sent them last week and I have still have not recieved them. Since I have had the pump apart in my living room for 4 days I went looking for one. I found a -007 at O Rally auto parts behind the counter ,sold by the piece. The sales person didn't know the material makeup and it didn't say on the box. They were 36 cents each. The link is to a sizing chart and if you click on materials they say urethane is the best Wear/Abrasion but I don't know if that would be the best for the pump because I don't know enough about o rings and applications. Sun Optics did say that Full Syn motor oil may be better and weight didn't make much of a difference. I polished the inside of P29 with JB bore paste and Flitz and the outside of P14 since that is the only places an o ring is sliding on. I put pump back together and it is working fine so far. Instead of time I will keep track of the number of pellets before failure. The original was about 700 pellets before it failed. This run may be short because I'm using an o ring of unknown material. If you or anyone knows enough about material types then maybe we could get a better o ring for p30. The thread size on the inlet screen is 1/8 npt and since it is water/moisture that is causing the pumps to fail I'm looking for an inline filter that I could put in the inlet.

http://www.marcorubber.com/sizingchart.htm

[Bruce Jarrett]

I added a Desiccant filter to the Benjamin pump. The filter is 1/4 npt on both ends and the pump is 1/8 npt so I had to add reducers. The Desiccant is blue but turns pink when it is time to replace. Pump is working fine with it on there and should keep the moisture out of the pump and rifle.

[hawkeye]

I added a Desiccant filter to the Benjamin pump. The filter is 1/4 npt on both ends and the pump is 1/8 npt so I had to add reducers. The Desiccant is blue but turns pink when it is time to replace. Pump is working fine with it on there and should keep the moisture out of the pump and rifle.

Hey Bruce,

Great post. Just wondering where you obtained the parts to do the mod? I like the idea.

Thanks,

Tom

[Bruce Jarrett]

Northern Tool & Equipment has a store near me so I bought the filter at the local store but here is a link for online. Two come in a package http://www.northerntool.com/shop/tools/ ... _200319833
If you do a search online you can find a lot of these filters and ranging in prices. I just bought this one because it was local and I could return it if it didn't work but it does seem like a nice filter. The reducers came from Lowe's part numbers A-734, A-715, A-738 all in the pluming section. One end took one fitting to reduce it the other end took two parts.If you search online you can find reducers that would only require one part for each end but again I went to Lowe's because it was local. Total cost for filters and fittings including tax $32.29 and that is two filters. That's a lot cheaper than a Hill pump filter.

[jlands]

Bruce,
thx very much for sharing.

jared

[zae333]

Bruce,

Outstanding idea man. I never even considered this setup. I will definitely be trying it out before next weekends tree rat hunt. Drying the air as it enters the pump could almost completely solve the p-30 deterioration problem. Thanks.

[Bruce Jarrett]

ZAE333
The reason sun optics went to motor oil instead of the silicone grease is because the moisture was creating a black goo in the pump. I didn't like the idea of motor oil because every other manufacture highly advertises against petroleum products around high pressure. That's why I suggested full syn motor oil before. Anything in the pump will eventually go to your rifle air tank and then the barrel. It may be better to go back with the silicone grease now with the moisture gone but I haven't decided on that yet ? With more use maybe we can figure out what works best. I looked at the diagrams for all these pumps including the Hill pump and they all look like the same parts. The Hill and the Axor pump had the filter that the Benjamin pump lacked. I would like to find out the o ring material type that the Hill pump uses at the P30 so we could buy the same type. The Benjamin pump will still have to be taken apart every once in a while so you can lube the seals but at least that's very easy with this pump.

[Roger]

Bruce,

With one end of the filter screwed in the pump and the other end caped how is air get through the filter? It appears to be a inline filter. AIR------> into filter Air out of filter------->

shoot straight

Roger