Can a SSP airgun really diesel?

[mr-rat]

I must be joking, right? Only springers are known for dieseling, right?

Well, there's a reasone why I ask...

Some time ago I got a Feinwerkbau 103 SSP pistol that was sold as defective and leaking.
I figured it would just need new seals.

Wrong.

To my surprise air was leaking out from an almost invisible hairline crack in the cylinder wall,
rearmost where compression would peak.
But still it held enough pressure to fire the pellet.

I see two possible explanations:

1. Overpressure from dieseling.
2. Metal fatigue from repeated compression decompression cycling or cocking.

Observations:

I found no evidence of dieseling inside the gun, all looks nice and clean.
The gun is in very good condition with no signs of abuse.
The compression cylinder is made of aluminum and has a hexagonal shape on the outside.
Thinnest parts of the cylinder walls are no more than a millimetre thick.

Normally I wouldn't think of dieseling in an SSP but the manual is very clear about it:

"
When cocking your air-pistol, a very high
pressure will be produced in the
compression chamber, which may
considerably increase the temperature of
the gas resp. air for a short time.
If there are still residues of weapon oil or
oil mist in the compression chamber,
the may become inflamed and result
in a recoil of the cocking lever, which can
then cause injuries to the shooter and
damage to the weapon.

For all moveable part and also for the
outside conservation you must
therefore exclusively use our FEIN-
WERKBAU special grease, which is
suitable for high pressures.
"

Anyway, I swapped out the cylinder for a new one and it's working perfectly.

 

[Dhc8guru]

I am going to say no on the dieseling. I have several SSP pistols and when I installed new seals in them, I gave them a healthy dose of oil for reassembly and never saw any dieseling. The reason is, an SSP is pumped and the compression is held in the piston cylinder and when the trigger is pulled the air and what ever else is in there just escapes into the barrel. If you filled the air cylinder with oil when it was un cocked and then tried to close it. you probably wouldn't be able to compress the cylinder or you would break a mechanical item trying to.
A springer on the other hand is cocked and when you pull the trigger, it releases the spring, rapidly sending the piston through the tube compressing the air, then out the barrel. When you dump a bunch of oil into the piston tube, the fast moving piston rapidly compresses the oil and air which causes it to diesel. Put enough flammable oil in there and it will explode.

So in the case of your cracked cylinder, I would say it was from fatigue or poor design. Its hard to believe but the cylinder could have worn thin from pumping?

 

[scrane]

Good question.
I think that the relatively slow pressurization and low discharge impulse from a SSP would be insufficient to generate much heat.

 

[Alan]

Maybe, maybe not.

I have an old, under-lever, .177 caliber Chinese airgun, that I bought (on a fluke) at a tool show some 15 years ago. With the 6 grain pellets it came with, on a good day it'll achieve 450 FPS. But is does diesel on plain motor oil.

 

[scrane]

"Alan"Maybe, maybe not.

I have an old, under-lever, .177 caliber Chinese airgun, that I bought (on a fluke) at a tool show some 15 years ago. With the 6 grain pellets it came with, on a good day it'll achieve 450 FPS. But is does diesel on plain motor oil.

And it was a single stroke pneumatic? Do you remember what model it was?