Manual; repair of Diana double piston systems

[Mr.Fixit-Norm]

Well, after receiving Neil's excellent guide to disassembly / assembly of the original 75 and having another go at mine this weekend all I can report is that I'm still stuck despite having spent over an hour making a spring compressor specifically for the job, and sucessfully getting the giss system back together, the original problem still seems to be present.

Basically, when the action is fully assembled but out of the stock, the gun will cock including setting the trigger. When the trigger is "pulled" there is a very loud click but the gun doesn't fire I can then decock the gun if I pull the cocking lever all the way back ( resetting the trigger to the fire position manually otherwise pulling the cocking lever back will break the tang off the rear hook - tell me how I know this ) then lift the safety lever up whilst slowly returning the cocking lever to the closed position.

I've studied the mechanism over and over again whilst pouring over just about every picture, stripdown and info I can find and I still can't find what's going wrong

The only thing I could do with, and it's a long shot, is to find the side images of the action when the gun is cocked, to see if when mine is cocked, all the parts are in the correct cocked position. This might give me a clue as to if the parts are moving correctly. So.....does anyone have or would be willing to take pictures of the cocked Original 75 action out of the stock, a big ask I know and it may not even help but I'm as you can see, getting a bit desperate

My last option would be to see if anyone on here with more knowledge than me on these guns, would be willing to have a go at it. ( I believe I've seen somewhere on here that Dave Mercer is the man to ask? )

Norm

[Benelli B76]

Have you visually followed the linkage back from the sear as you pull the trigger? There is a part that locks into the base of the piston when it is cocked, think it may be part TR162 on Chambers diagram. This needs to trip out when you pull the trigger so that the piston is released. Inspect the movement of parts when you pull the trigger to see where there is no movement.

Baz

[Mr.Fixit-Norm]

YESsssssss . I've finally got my Original 75 cocking AND firing, I am so chuffed.

After 3 weeks of frustration and loads of help from you guys on here both with suggestions and the excellent Stripdown printout / CD from u.k.neil plus numerous cocking and de-cocking whilst watching what was happening, I finally sussed what was wrong, if you look in the picture below, the small bolt which goes through the "L" shaped slot in the sear and tightens on safety lever, was adjusted wrongly ( and from before I was given the gun as it didn't fire when I was given it ! ) . Once I'd done this it fired first time
It was too far along to the right and not lifting the sear up properly when the trigger was pulled

http://www.thatsmypicture.co.uk/#!album-11-0

I've now tidied the stock, re sprayed the foresight in satin black, but I'm missing one part to finish my project off, it's the but pad end - the bit that goes against your shoulder. Does anyone have one they're willing to part with or know where I can get one?

http://www.thatsmypicture.co.uk/#!album-11-1

I have also chronoed the gun and it's running consistently around 505 - 512fps ( 4.9 - 5 ftlbs ) which, considering the 'O' ring piston seal mod to the gun isn't bad really

I'd just like to say a huge thanks to all you guys on here who offered help and suggestions including u.k.neil, Benelli B76 and Keld Davidson ( aircol ) who emailed pictures and suggestions without which I'd have probably given up and either binned the gun or sold it for parts

Norm

[abellringer]

Norm Hi, now you have really got me intrigued ! what exactly are the mods as in 'O' rings on the pistons. Do you have any photos of these? I ask because when re assembling the gun there is a very important 'clearance' measurement relating to the rear piston washer face and the top hat spacer disc, which is clearly stated in the works manual; if this is wrong the gun will tear bits off the piston and or cogs. If your gun works OK then this measurement must be correct, so how was this achieved with 'O' rings?
abellringer

[Mr.Fixit-Norm]

Hi abellringer,

Unfortunately I didn't take any specific pictures of the Pistons, I do have a picture of the rear one taken whilst I was fitting the springs & Pistons using a homemade spring compressor though and I've done a rough drawing of the compression one.

I believe my late friend Martin did the mods himself, he was a very talented old school engineer who didn't have any computer knowledge but was a genius with a lathe, a coded welded and all round nice guy. Sadly I didn't get round to taking the gun apart until after his death 18months ago. I can only imagine he got the gun with knacked seals and not knowing where to get them from, decided to modify it to work with what he had to hand. He gave it to me about 6 months before he died to see if I could get working, and told me I could keep it if I could. I don't think he realised the reason for it not working.

From what I can see he has machined the reversed dovetail on the piston faces so that it has straight sides then made up two aluminium "collars" which are a tight ( possibly heat fitted ? ) fit. The front piston has a recess where it meets the main piston body to take an 'O' ring. He had a vast knowledge of O ring fitment and how to cut the right shape and size groove to take a specific ring, and it seems to work well. To be honest I'm not sure if the power of the gun @ 5 ftlbs is down to the modified pistons or the fact I used the original springs ( I didn't want to spend out on springs if I couldn't get it to work ! )

The aluminium rear piston doesn't have an O ring but the fit in the cylinder seems perfect to allow it to slide and I've used moly slip to lubricate them both. Only time will tell if it works and is reliable.

As these pistons only seem to be available from Germany at considerable expense, I didn't have much of an option but to use them.

Incidentally, the way I set up the pistons and cogs was to push the compression piston fully forward, then when putting the springs and rear compensation piston in, I lined the two racks up so that the end of the rear piston had the same amount of rack showing in the side cog holes, when I checked the distance of the rear piston face to the end of the cylinder, it appeared to be about 0.5mm ( difficult for me to measure exactly ) , the manual says 0.3mm so I guess it's about right. Whether that's by luck or judgement I wouldn't like to say but it cocks and shoots nicely with no strange noises !



Front piston "O" ring modification. http://www.thatsmypicture.co.uk/#!album-11-2


Rear piston aluminium washer conversion. http://www.thatsmypicture.co.uk/#!album-11-3

Hope this helps.

Norm

[WXYZ]

A general problem is that the Dianas with double piston system (designed by Kurt Giss late 50's), are a little tricky to repair, when the old cork seals are worn out. Sending them to a gun smith will cost a lot. Dianawerk in Rastatt charges about 180 Euros for a repair, which is about the same value as the gun itself (or more).

I have replaced selas and springs in a few (pistols and rifles), and think it is sad to see these fine pieces of air gun history being thrown away just because they are tricky or expensive to repair. The only manual from the factory, for Diana 75, is in German and is not very accurate or detailed.

I live in Sweden and have written manuals (MSWord format) for how to repair these guns.
This manual is in english for Diana 60, but will work to 95% for models 65 and 66 too.

Manual for Diana 75 in swedish. (sorry, I'll translate it to engish some day. There are pictures and stuff you might find useful.)

Manual for Díana 6G air pistol (swedish too....)

If you are interested in sales statistics and prices for old match guns, including the Dianas, I have collected them the last three years from egun.de at this link. This file is updated weekly. File format is XL, and there are some 2500+ prices.

/ Micke

Thanks for simplifying things a little - I am currently sorting a Diana 75. Luckily, I just came across this old thread...

[rojones]

I am looking for an experienced person on diana / original model 75 to fix mines. Any candidate please?

[Garvin]

I am looking for an experienced person on diana / original model 75 to fix mines. Any candidate please?

There's none better than Dave (DM80 on here) if he's still doing them.

[Triple Jack]

Great thread. I have had a similar issue. Sounds like the same problem and hopefully solution. Whilst exploring links I came across this helpful video.

https://youtu.be/4OxDi4vCbl8