Update on the Diana piston (would like some input )

[Barryg]

For those who don't know what I am talking about
http://www.airgunbbs.com/showthread....-(A-bit-scary)



Over on the DWC a member might be cutting a piston in half so we will have more idea what is going on and also one of the owners of a piston that has separated stem seems to be saying that the the groove in the stem is not lined up with the crimp on the piston body, so until we can look at how the stem is held in it seem to be a guess.
Does anyone on here want to have a guess or does anyone actually know how the stem is fixed in the piston head because I don't yet

The few that have failed seem to be recent pistons, just think how many Diana pistons out there (and they are all fixed the same way) in three parts that have not failed.

A close up pic of the piston parts, HOW IS THE STEM HELD IN ?

[robs5230]

My guess is that only the solid piston head is held with the crimps.

The rod is most likely held in the head by the splines and a cross pin.

Thats the way the Diana 52 barrel is held in (splines and cross pin).

Those crimps are nowhere near deep enough to extend to the piston rod.



That groove in the piston rod can ONLY be there to receive a cross pin.

Either the pin has broken or not been installed at factory allowing the rod to come loose.


If you fancy "crowd funding" a new piston for me Barry, I'll cut one apart Saturday morning
In fact I'd be very surprised if Tony Leach doesn't know the answer to this as he's done the 22mm piston and skirtless kit on enough of them.

[robs5230]

Just another comment, the guy in the video on the link has an american accent. The transfer port restrictor found on UK rifles is not present.
I'd suggest that particular rifle was running at FAC power.

[robs5230]

Link to a thread I commented on last year where I explained the attachment method of the 48/52 barrel.
http://www.airgunbbs.com/showthread....WS-Diana-48-52

[bozzer]

I'm 100% with Rob.

The head is held on by the crimping.

That groove has to be machined in the piston rod nose for some reason. The most obvious one is that there is something that fits into that groove to retain the piston rod to the head. That must be a pin through the solid piston head and into that groove. The splines line the rod up ( reasonably ), central to the piston body.

So, like I say, 100% agree with what Rob says.

Those that have failed ...

Error at assembly and no pin put through piston head to locate groove.

Pin not knocked in far enough to locate groove. This should be a difficult error to achieve if the pins are a set length and go in flush to o/d of head. So maybe a batch of under tolerance sized pins that have been knocked in but haven't located groove deep enough to retain rod under high pressure.

Pin failed/sheared.

So again, agreeing with Rob.

PS ... I've looked at the video a few times and kept pausing and I can't see any damage to the nose of the rod or any damage on or around the turned groove. So it doesn't look like the rod has been forced free of a slightly undersized hardened ( presume must be hardened ) pin. So I'd have to go with no pin fitted error.

He's just pulling that rod out with his finger and thumb. There's no way that that will hold when cocked with USA FAC pressures on that spring. So surely that gun can't be useable like that?

Plenty talk on the net about rods with 'wobble' and people saying that they have 'pinned' theirs to tighten up that play. Surely Diana can't have thought about Pinning through the head into that groove and thought the splines forced in under pressure will hold and so not bothered with the pin? It must be pins that have failed or been well undersize and not gone through into the groove enough.

[Bruno27]

Just got a 1968 Diana 35 yesterday and piston rod came loose with seal screw broken
I haven't had a proper look yet

[robs5230]

On the barrel, the cross pins were a F.T. fit and needed some getting out and then the barrel needed pressing out with considerable effort.
The barrel however is under no real pressure. Certainly not the amount of pressure that the piston head sees.

I'd hazard a guess that as it looks as the rifle in the vid could be possibly running FAC, the amount of pressure / heat created may have allowed the solid head to enlarge and loosen the pin. Repeated occasions would eventually result in a sheared pin. Once the pin shears, its only a matter of time till the fit of the splines would shake loose and the rod drop out.
The piston head is comparitively small so would be affected by heat to a fair degree.
Any wear to the piston seal could also be a contributory factor. The end of the seal is open faced. Any wear to the seal could see that piston head go crashing into the end of the cylinder. Shock alone could break a pin over time.

[robs5230]

Just got a 1968 Diana 35 yesterday and piston rod came loose with seal screw broken
I haven't had a proper look yet

Shame but its a rather different situation. The age of that alone would possibly cause that screw to shear

[Barryg]

Yes it looks clear that the piston body is held by the crimps but perhaps instead of a pin locking the stem what about something like Woodruff keys that go around the groove and held in place by the crimps, I am itching to find out

[robs5230]

Yes it looks clear that the piston body is held by the crimps but perhaps instead of a pin locking the stem what about something like Woodruff keys that go around the groove and held in place by the crimps, I am itching to find out

Far too complicated and it would involve machining a slot / square hole for the key.

A pin for sure. I'd bet my left nad on it (I lost my right one in a bet a few years ago )
There'll be an off centre round hole bored through the solid piston head.

[robs5230]

Plenty talk on the net about rods with 'wobble' and people saying that they have 'pinned' theirs to tighten up that play. Surely Diana can't have thought about Pinning through the head into that groove and thought the splines forced in under pressure will hold and so not bothered with the pin? It must be pins that have failed or been well undersize and not gone through into the groove enough.

Just read this bit again and had another think. Its possible though unlikely that no pin is fitted in any.
The fitting of a pin has to be real precise or it will send the rod off centre. Maybe the original intention was for a pin and now they dont use em at all. My theory is that the cross pin retaining the barrel on the 48/52 causes the barrel droop.
Its possible that the rod is just bonded in on the splines. As I said my barrel didn't half take some effort to press out with the pins removed.
Regarding wobble of the rods, if they do start to move at all, the fit on the splines will worsen, pin or not. More movement = more wear = more movement etc etc. If the pin isnt a precise fit this could still happen and eventually shear the pin due to movement.

So the likely causes in my mind..


Heat causing enlarging / shrinking. This will affect with pin or not (more likely on FAC)
Head crashing into cylinder end. Again whether pin fitted or not (worn seal but again more likely on FAC)

[bozzer]

Okay ... a few more thoughts ...

Woodruff keys ... Just can't see it. As Rob says it will be a far too complicated job trying to machine slots for the woodruff keys to sit in. That compared to a drill/mill through head and tap a pin in ... It's the pin option every time.

Do they just not bother with the pins and rely on heavy pressure fit of the splines. I can't see that either. There are many reports on the USA sites of wobbly rods. Wobbly rods means those splines have become loose. So the pressure fitting on the splines holding the rods in theory doesn't hold water. Wobble equals loose splines equals rods coming out. The reports are of loads of wobbly rods, but most continue to work indefinitely with no issues. So that must mean despite the loose splines there is a pin in there.

So either the odd one ( rare ) hasn't had a pin fitted and the splines loosen and the rod comes out ... or the odd one has the pins failing. Re the pins failing I'll go with Rob's repeated slamming of piston head maybe causing a fail.

... and yes ... the old rifle with the piston head screw fail is a different story.

Me and Rob should get a room.

PS ... Maybe more than one pin ( both sides ) to prevent off central pressure and a safety measure if one pins fails. So that would mean all pins fail for rod to come out. Maybe if one goes first then the other follows soon after.

[gc93]

my take , its nothing airgun but it was a piston of sorts "hydraulic and air rams " the piston head has a groove to allow for crimping the groove in the rod ideally would line up with crimp and head groove but close would work ( belt and braces a cross pin) the splines in the rod to help fix and stop rotation of rod , if the gun piston and piston head was soft the crimping would travel down the piston just catching the rod groove (talking thous here doesn't need much to hold it fast ) then complete piston would be heat hardened after. all items would need to be fairly tight if not press tight . as said only true way would be get the grinder out of if posh mill the piston. maybe cost cutting has crept into the D piston making (no cross pin ?) from what you read its raised its head elsewhere . atb gc..

[Barryg]

I wonder if the T01 pistons were fixed the same way

[bozzer]

Now I'm starting to doubt myself.

Looking at the video again there is absolutely no evidence that there has been any pins in that groove on that nose of that rod. I'd expect to see something where a failed pin had started to go and some wear around that groove. There's nothing.

I know little about pressure bonding. Could those two large pressure 'crimp' dents put enough pressure on the steel head, that part of the inside diameter of the head where the rod fits in is forced into that turned groove on the piston rod? Just enough so that it holds the piston rod in place. On the wobbly pistons there's been enough wear to give some movement but not enough for that ' pressed in ' areas of the inside of the head to let go of the piston and allow it to come out.

It's an easy option re manufacture. You don't have to accurately drill/mill holes in the head for pins or make/buy pins and fit them. You just allow one high pressure crimping action hold the piston body to the head and the head to the rod ( the latter via that turned groove ).

PS ... I think gc93 beat me to it whilst I was typing. Well done gc93 ... that's now my favourite option.