Good evening,
I've enjoyed this site and have a Beeman 850 that I bought new from them. It has not been used lately as chunks of grease are coming out of the barrel and the pellet lodges itself in there as well.
Does this gun need a spring compressor or special disassembly techniques? Also, if any advice on how to do this repair and suggestions for best replacement parts or service would be greatly appreciated as I've missed shooting this gun. Thank you all, mike
PS: I read many pages to search for this answer and thought I'd just post.

This gun uses two pistons operating in opposite directions. It sounds as if the piston seals have disintegrated which is unfortunately inevitable with the original seal material. The current replacement parts are a vastly superior blue synthetic.

The guns are very specialized to work on--the pistons must be "timed" with the controller cogs (that's what's under the little covers on the side of the receiver) and spaced properly. With two pistons, two piston seals, and three springs with a floating guide, it is definitely not recommended if you are not a pretty experienced airgun mechanic.

RWS is the official Diana service center in the US, Randy Bimrose and others do a superb job on these as well. Randy has done two guns for me, including a double-piston Diana rifle.

Here's a link to a parts diagram:

http://www.chambersgunmakers.co.uk/Diana-Original%20plans/Diana%206G%20plan.htm

MDriskill, thank you very much for your input and the diagram also. I'll just have this gun repaired by a specialist then. I bought it new around 1980 I believe. Also, I found on the #28 page of this site, a link to an 4 part discussion of this topic.--- I'll let a pro do it. Thank you again, bulls eye always to you, mike

Although they are very sensitive to gear placement....It should be noted that when the spring is compressed inside the gun, and the endcap properly screwed on (gears and hubcaps are off at this point) The pistons balance themselves out. Just drop in the gears, and hubcaps (make sure they are on the right side) and you're good to go.

I'm making a rebuild tutorial for these right now

James, Thank you also. I'll look forward to your tutorial! Some more knowledge would be helpful. Tho I'm leaning to having a pro repair this pistol. Bulls eyes for you, mike

Actually the factory specifies a .3 mm gap between the timing disk and the end of the main tube. This is achieved by using the timing disk of the correct thickness. They range from .2 mm to 1.4 mm. The one in the gun may or may not fit correctly with the new seals. I've seen it turn out both ways. Putting the pistons and springs in, then dropping the gears in works, but its not right (at least w/o checking for the correct gap at some point). They aren't much more difficult to work on than a normal gun, but you need the right info (which I just gave you) to do it correctly. Please incorporate this info into your guide, or hold off. I'm tired of seeing these guns ruined by people who are capable, but don't know better.

Earl VanBeek

Earl,
Thank you for your advice as well. I've decided to have a 'pro' do the repairs and I've packaged it now for shipping. I agree as to the uniqueness of some machines and without understanding their principles? It can possibly and often enough leads to misery later- (I'd rather not disclose how this insight came to me).
I'll post when I receive this pistol back again.

Thanks again, mike