Diana Giss advice needed

[NeilnrLincoln]

Hi.
I've recently bought a Diana model 60, it shoots very smoothly but the cylinder end cap has no locking screw & the hole for the locking screw is nearly 180 degrees out (at top instead of the bottom).
I've just taken the cylinder end cap off to have a look & the rear piston is practically flush with the end of the cylinder where I believe it should be set back around 0.3mm?
There is the stepped spacer inside the end cap but no shims behind it and the step was facing the inside of the cap and not (as I would imagine it should be) facing into the cylinder.
I'm guessing the step in the spacer should fit into the end of the cylinder but on mine it tightens up against the end of the rear piston before the cap is screwed all the way home.
Any advice where to go from here?
Thanks.
Neil.

[Garvin]

Hi.
I've recently bought a Diana model 60, it shoots very smoothly but the cylinder end cap has no locking screw & the hole for the locking screw is nearly 180 degrees out (at top instead of the bottom).
I've just taken the cylinder end cap off to have a look & the rear piston is practically flush with the end of the cylinder where I believe it should be set back around 0.3mm?
There is the stepped spacer inside the end cap but no shims behind it and the step was facing the inside of the cap and not (as I would imagine it should be) facing into the cylinder.
I'm guessing the step in the spacer should fit into the end of the cylinder but on mine it tightens up against the end of the rear piston before the cap is screwed all the way home.
Any advice where to go from here?
Thanks.
Neil.

Neil it may be that whoever serviced this rifle found that turning the step towards the seal pushed it in too far when the cap was screwed on and it interfered with the gear timing, because the seal was already nearly flush with the end of the cylinder?

I'm guessing that on some rifles moving the piston rack forward by one whole tooth before putting the ratchet wheel in moved the rear seal quite some way back into the cylinder, making several spacer shims necessary? If you didn't have them handy it must have been tempting to get away without them?

I'm not sure the cap locking screw was really necessary, because the threads are so fine and the cap is very well screwed on anyway. I suppose over time the cap could unscrew itself by enough to affect the smooth running of the piston assembly though. A dab of Loctite necessary perhaps?

[NeilnrLincoln]

Thanks Garvin.
It's clearly been apart & fitted with new seals, the rear seal looks an almost new blue type.
If I'm understanding the Giss system correctly the important thing is that it's timed so the front & rear pistons reach the end of the cylinder at the same time, so based on the fact the front piston must be against the front, if the rear piston is just touching the rear spacer when the end cap is screwed on I guess it shouldn't damage itself?
So it should be safe for the Giss system to reassemble as it was with the stepped spacer turned around so the seal just touches it & as you said, a dab of Loctite on the end cap just to prevent it unscrewing?
Thanks.
Neil.

[JLB]

The spacer should be stepped so the rear bumper of the rear piston just about touches it after firing to prevent over travel of the rear piston and trashing of the gears, the shims if any in the end cap are used to adjust it. I believe the screw is drilled at the factory once set up, so may not always line up on reassembly after a service.

John

[Benelli B76]

When the Giss system is completely assembled you can check if everything is spaced correctly by unscrewing one of the side pinions. They should not be under
stress and unscrew easily. I often see these chewed up because they have been forced to centralise in the thread. Also the little gear wheels should look absolutely central while sitting in the two racks and be easy to lift out.

Baz

[NeilnrLincoln]

Oh ok. If I turn the stepped spacer round so it's facing the right way & screw the cap fully home until it stops the caps over the gears on the sides are free to undo & the gears look central, so is this the correct timing/clearance?
Does this not mean that the rear piston is under a small amount of load when the rifle is unlocked, will it not reach the end of its travel fractionally before the front piston?
Sorry if I'm overcomplicating things.
Thanks.
Neil.

[Benelli B76]

Oh ok. If I turn the stepped spacer round so it's facing the right way & screw the cap fully home until it stops the caps over the gears on the sides are free to undo & the gears look central, so is this the correct timing/clearance?
Does this not mean that the rear piston is under a small amount of load when the rifle is unlocked, will it not reach the end of its travel fractionally before the front piston?
Sorry if I'm overcomplicating things.
Thanks.
Neil.

You are a bit. The goal is that the gear teeth are not under load when the pistons stop. If they are assembled incorrectly the gear teeth received a shock loading and one or two can break off. The proof is as I already said above, the caps will be hard to remove because of the side pressure, and the little gear wheels will be slightly off centre when the caps are removed.The large back cap must always be in place when you do this kind of check.

Baz

[NeilnrLincoln]

Excellent, thanks for your help. In that case I'll turn the stepped spacer the correct way round so when the rear cap is tightened up the load is taken off the gears & put a blob of Loctite on the end cap to prevent it unscrewing.

[Garvin]

Excellent, thanks for your help. In that case I'll turn the stepped spacer the correct way round so when the rear cap is tightened up the load is taken off the gears & put a blob of Loctite on the end cap to prevent it unscrewing.

I'm sure you're planning to do this anyway, Neil, but as Baz says, make sure you unscrew one of the little gear caps and check the toothed wheel inside is free when the end cap is screwed on (before you Loctite it, obviously!).

[NeilnrLincoln]

Will do Garvin, thankyou all for your excellent advice. I'm reassured now that it's (hopefully) not going to destroy itself .

[dvd]

Will do Garvin, thankyou all for your excellent advice. I'm reassured now that it's (hopefully) not going to destroy itself .

Hi,
it looks like you have a firm grasp of what is happening in the action and you are nearly there.
Three other things I would like to include;

1) Before you do the very important gear cap cover test to see that the gears are not loaded, make sure that the caps turn freely on the threads without any binding, before you insert the gear wheels. Other wise you can get an incorrect feel and think it needs further adjustment at the rear shim.

2) There are about 5 or 6 stepped washers available in 0.2 mm incremental sizes going all the way up to 1.2mm if I remember correctly. The reason is that in the eventual replacement of the buffer and piston-seal these may be slightly thicker or thinner than what was in the gun when it left the factory. So it could be that the new seal and buffer might require a slightly thicker or thinner stepped washer to achieve the correct timing, or you could be lucky and be able to use the original stepped washer because its thickness coincides exactly with the dimensions of the replaced washer and buffer.

3)Last thing is to make absolutely sure that the cylinder's bottom is clean of any left-over pieces of old seal. Otherwise you get a false reading when it comes to getting the right thickness of stepped washer.

Regarding the rear lock screw on the threaded end-cap, the threads on the cylinder are 0.5mm pitch. So if yours is approximately 180 degrees out, that would be half a turn which equates to half of 0.5=0.25mm. So you can insert a 0.25mm shim between the cap's inner face and the stepped washer to align the lock screw's hole.
The lock screw is essential because if the end cap unscrew even by a small amount,, then the timing of the two pistons will be lost.

Good luck with your rebuild........Fantastic guns these Giss type Dianas

[thisisdonald]

All good info... I've got a 75 here that I've bought a service kit for. I bought one spacer just in case. I figured I could make up others from shim material easier than ordering from Germany.
Lots of good info on this video as well:
https://youtu.be/8rEIZas_v-M

[Benelli B76]

One of the 6 M I have done, gear just drops in loose and central. You are able to screw in the pinion caps quite a way with fingers, no mole grips !

Baz

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[NeilnrLincoln]

Some brilliant information on here, thankyou. I'm a lot more confident in the workings of the Giss system now.

[abellringer]

Have to agree with the comments and good advice given by members, there should always be a second thin shim in the end cap on rifles, and the small retaining screw is indeed fitted after the end cap is tightened up, as is evident when you first strip an Original and find swarf in the rear piston head [if it's still in enough pieces to see]. The smaller diameter is definitely fitted into the cylinder, the timing is needed to be:- when the springs, pistons, cogs and covers are fitted; and the pistons are at rest under spring pressure, you offer the end cap up to the rear piston washer and look for a tiny gap between the cyl. end and the front face of the spacer, there should be a gap of 0.3mm [ factory instruction]; this ensures when the end cap is fitted tightly the rear piston is pushed forwards slightly, this takes the pressure of the mechanism and gives the cog ''freedom'' described in several posts. This freedom is needed, as has been said to avoid stripping cog and piston teeth.
Hope this helps.