Rebuilding a Diana 75, Questions?

If you are lucky you may be able to get by with just replacing the Breech Seal.

Timing on those GISS springs is very critical. Be sure to mark the cam position prior to taking it apart and reinstall things exactly as it should.

Good luck and someone like Mike Baker would probably be able to do it reasonably too.

That is a fine gun and IMHO worth paying to have it done profesionally unless you have

done a GISS before.

HA

Hotair,

Thanks for the reply.  I should talk to Mike about it.  I'd like to know if he does indeed rebuild GISS type rifles.  I'm not doing it myself to save money.  I want to do it.  I like working on airguns.  The breech seal looks good.  Is there a way to test the breech seal?  The tissue paper test maybe?

Roger

Tissue paper test would work. I would also try and put some chamber lube one drop or two but try one first then shoot and chrony it. Don't over do it though. You want to lightly lube the dried out seals.

A little goes a long way. May do the trick with some fingers crossed then go shoot it.

HA

a little lube might help but if it doesn't honk it more then likely isn't the problem and changing out the breech seal would be the first thing i would do

on the timing thing with gears, i think that has been blown out of proportion over time, now i haven't done a 75 but i own 3 but have done 6 model 6's and they go back together one way, now you do have to set the spacer part number 15 up right and you can only do that if you have all or most of the sizes, extra cost

now the pistol have a .3mm gap needed from the end of the tube to the step spacer and i would have to check the 75's width

so you need the 3 springs, front seal, back dummy seal, breech seal the step timing spacers, you need part no. 16 maybe and any other thing you might need but there's the rub if you have the parts when you take it apart you will not know what you need 

i know your thinking don't set it aside 4 to 6  weeks and then remember how it goes back together 

so how hard are they to work on, the 75's, if you rated a HW 95 a 4 or 5 out of 10 a 75 word be a 8.5 to 10 out of 10, if you had done a half dozen maybe a 7   

so parts Umarex might have them but will they sell you everything you need 

Waffencenter Gotha is your best source for parts Enrico will take good care of you, now is it easy to order from them yes and no but i can help walk you through it if needed and you can be in a hurry it just doesn't work that way and that can be discussed 

https://www.waffencenter-gotha.de/shop/index.php?manufacturers_id=2&filter_id=734

just info pull it down a little 

https://forum.vintageairgunsgallery.com/vintage-diana-resources/diana-milbro-airguns-manuals/#post-2246

so that is a start email me any time you like 

mlwolfram@msn.com

even what i know i'm not sure i would do a 75 myself now, it was something i learned as a kid, if you can't put it back together don't take it apart 

i'm here to help if a can 

mike wolfram

Forgot to mention Joe Wayne Rhea may be able to do it too but I don't see him here at all anymore.

Heard he is retired and may have some time to do such projects. Extremely nice guy!

I think he may have migrated to another forum and you can do a search for Cyclops videos

to find him.

HA

Hi Roger.

I have resealed dozens of the Giss guns, 60, 66, and 75.

Chamber oil will not reserect the rifle. Try the tissue test on the breech seal, but it most likely is the piston seal. You can order the two seals (piston & bumper) from Macarri, airrifleheadquorters.com

It is unlikely that your springs will need replacing, but I have encountered that a few times. Macarri no longer sells the tripple spring set, but jgairguns do.

You won't need any other parts. The spacers mentioned, that go in the rear, behind the bumper seal, can and should be reused. Just measure the exact spacing between spacer and the rear of the compression tube before disassembly. 

Getting the pinion gears alignment is critical.

You also need to partially disassemble the trigger which is a very complicated trigger/anti beartrap mechanism.

I use a light coating of moly grease on all the moving parts.

Take lots of pictures from start to finish of the disassembly, and you should be ok.

Good luck

Scott

Guys,

I just may have lucked out!  I checked the breech seal this morning and it is shot.  Thank you for putting me on this track!  Now I need to dig up the procedure for changing the breech seal and see if this gets the velocity up to where it should be.

Thanks again,

Roger

Here is a little known tip. You can use Moen tap washers for the breech seal. Work even better than factory breech seals and cost next to nothing. I forget the exact size but you can get a variety pack for a few bucks that contain several of each size. Perfect fit, both in diameter and height. Also check the length of the cocking arm rod. When set to factory specs it should cam over slightly when closing the cocking arm. This ensures a good seal.

RG

Talldog,

Thanks!  I'll check them out.

Roger

Posted by: Talldog

Here is a little known tip. You can use Moen tap washers for the breech seal. Work even better than factory breech seals and cost next to nothing. I forget the exact size but you can get a variety pack for a few bucks that contain several of each size. Perfect fit, both in diameter and height. Also check the length of the cocking arm rod. When set to factory specs it should cam over slightly when closing the cocking arm. This ensures a good seal.

 

RG

That is very nice to know! On lesser valued guns such as ones of Chinese origin, in case of emergency, I had first flipped the seal or O-rings around or soaked them in automotive Gumout carb cleaner or Acetone to force them to swell back up then they worked fine as a temporary quick fix until I got the proper seal or O-rings.

https://www.bobistheoilguy.com/forums/ubbthreads.php?ubb=showflat&Number=1854960

HA

Scott on the gears, when i did the model 6 pistols you load the guts and compress and add the gears and caps 

now the gear and cap don't have a choice but to go in one way in my view, now after that you would check the rear step washer for tube to washer clearance and put in the right washer 

half of the pistols i have done for myself they needed changing and the others didn't 

i would think the rifles and the model 10 would have the same rules of assembly

now on the ARH seals, i have used one set on a pistol and front one is to big and once you install on the front piston, they are bitch to resize,  the factory seals are easy to install 

so would i put in new springs if needed or not, well you don't know what a new length might be unless you had a pair so buy them and use them or not 

so my bottom line is have more than you need and proceed with caution 

so the bottom line is we might have different views of the same job but that OK 

just an idea on the pistols , the rear cap can be a real problem to get on RIGHT and get the screw in

so the urethane spacer that goes in the back of the cap comes in 3 sizes   2 - 2.5 - 3mm and that can be a problem if you don't have them

what are they for,  remember the step disc, washer, well the cap spacer pusher that up tight to the tube and in doing so unloads the gears with that .3mm of clearance 

so too little spacing and the cap will go on but not do its job, too much and you can not get the cap to rotate far enough to get the screw in and if you look at a gun for sale you will find the screw in the wrong area of the tube on not screwed in 

so i was think of a cheat to make that spacer or spacing and have the ability to fill the space at any thickness and compress the step disc and get that screw in

so i came up with teflon sheets, on e in .3mm and the other in .5mm and with those two thicknesses you can come up with endless thicknesses 

i use some .6mm plastic test pieces and with 5 i get to 3mm, so i test with the .6mm pieces and get the screw hole lined up and then replace them with the teflon with and add .3 ro .5mm extra 

is it factory no but it solves a problem and should last 

be aware that once the gears and caps are in, the back cap can be remove and the spacing tested as you like

the teflon was bought on Ebay and came from China 

so just idea on the pistols 

model 75 breech seal size  3.05 thick     13.93mm diameter     4.25 mm ID  and they are  hard in nature you can't really compress with your nail hard 

oh  just a suggestion, if you buy from Germany or over seas the shipping is a killer so buy way more then you need, those parts are worth every dollar you paid and could be sold to offset your costs 

all i got for now  take care

miike

Hotair  Joe tries hard to get his 75 rebuilt but he ended up sending it to Lobowhitefang, even after all of the FWB 300's he has done the 75 got the best of him, so there is a high learning curve to these 

"model 75 breech seal size  3.05 thick     13.93mm diameter     4.25 mm ID  and they are  hard in nature you can't really compress with your nail hard .. oh  just a suggestion, if you buy from Germany or over seas the shipping is a killer so buy way more then you need, those parts are worth every dollar you paid and could be sold to offset your costs" 

Germany .... yeah really ... Maccari sells them for $6.00 a pop. Or just buy Moen washers .. 10 for .98 cents (sorry Jim). I've used factory, Maccari and the Moens. The Moens actually work best. The dimensions are not overly critical as guns vary, but that being said the tap washers spec up almost perfect. Regardless of what one uses, the cocking rod must be set properly.

Talldog,

Do you have a part number or some other reference for the "Moen tap washer".  I tried googling Moen tap washer without success.

Thanks,

Roger

Guys,

Just wanted to let you know that replacing the breech seal resolved the problem.  Before replacing the seal the velocity was 310 fps and gun failed the tissue paper over breech test.  After replacing the seal gun passes the tissue test and velocity is 620 fps with 7.7 grain R10 pellets.  I used a DANCO Inc. flat washer, stock # 35063B for the seal with a tiny film of silicone grease.  I do have JM seal on the way here, but I couldn't wait. (:-)

Thanks for all your help!

Roger

good to here, so how did it fit, tight enough to stay put and what was the thickness 

Marflow,

Seemed good to me.  I had to push it into the pocket.  The washer is 1/8" thick with an OD of 17/32" per the manufacturer.  I don't expect it to move any.  I'm thinking the tiny bit of diver's grease helps, too.

Roger

Glad to see you were lucky enough to get it resolved with just the breech seal and a cheap fix at that!

HA

While you may want to disassemble a Giss airgun, unless you have the correct tools and special vice, you could damage the gears.    It's a tough AG to disassemble/reassemble without them.    And damaging parts like the gears (which may no longer be available) can add to the frustration.

Just saying (I've done about 5 and will not do mine anymore).

Chuck

It's been a while since working on a Giss but if I remember correctly you should not continue to shoot these guns if the velocity drops off. Please correct if I'm wrong but I believe the gear rack can be damaged. 

so if a Giss system rifle, pistol stops shooting up tp spec and the breech seal is fine do what you would do with any spring gun stop shooting to 

on the giss system you will just pound the front piston into the front of the tube  and could damage the piston itself 

now on the gears, they could be damaged if the piston seal was damaged but i doubt it because when the piston seal and i'm talking old original factory front seal, they just fall apart and pack themselves in the end of the tube and are a pain to clean up, never seen a back seal that was damaged, the cap spacer, no that's a different story

the gears are always talked about, like they are some sort of nightmare, they are not and here is way 

the gears are taken out and replaced when you compress the spring, a little compression and a magnet and out they come and that compression should be done before the caps are removed, the compression take any pressure off the pin in the cap to c=keep the gear centered and the gear itself 

putting them in is straight forward but the trick on the rifles is a small rod to keep the springs straight when compressing, the pistols that is not need 

so you compress the spring 1mm pass the end of the tube install the gears and the caps and that part is done 

oh sure you have to get the right step spacer and this and that but the pistons, spring, gears and side caps are straight forward 

and as stated have more tools then you need helps 

now on the model 75's everyone could put in a set off seals and springs but the problem is all of the parts needed to be removed to get there and put back in the right order 

so is a model 6 pistol hard yes and no but a good place to start on your Giss System practice, is a model 10 easy, never done one and the trigger assembly is way different then the 6 so your challenge is increased 

a model 60 thru 66 rifle would be a good place to start on the rifles, all the piston and seals are the same as the 75 but the trigger assemblies are not as complex  and i have never done one 

so are the giss system rifle and pistols easy to work on, NO but in reality it is not all about the gears, they are straight forward 

one note i alway put the gears and caps back in and on the sides they came from, mark them with a little scribe 

Good tips.   I may need to venture into my 75.    The breech seal is definitely starting to  fail . 

So, I have a model 75 in need of servicing. Who would recommend for repair? I do not want to do it myself and I want quality work.

Fred

Contact Scott Schneider "Motorhead" 916-201-7237. He is an outstanding airgunsmith and the proud owner of a beautiful Diana 75.

Jim in Sacramento

Worth mentioning is that if you break something parts usually come from doner rifles. I have 3 and David Slade of Airgunwerks rebuilds and tunes all of mine. A professional rebuild and tune is hard to beat

can you tell me how you would tune a Diana model 75 besides putting in new parts and  lubing in the right amount and with the right type 

you can't do anything with the trigger group or much of anything else

I see the---Tune---word used very often on things that can't be tuned, they can be rebuilt and is that a tune ?????

I to own 3 model 75's and have you looked at Waffencenter Gotha for parts, more then you think