Dieseling Air Rifles

by: Glenn and Danny

Editor’s Note: This articles comes directly from gunsmiths who work at Umarex USA.

The mainspring of a rifle that broke due to heavy dieseling.

The mainspring of a rifle that broke due to heavy dieseling.

Have you noticed smoke coming out of the barrel or breech of an air rifle or an unusually loud shot? Dieseling is the ignition of a fuel resulting from the heat generated by the compression of air in a cylinder. When the air within a spring-piston airgun’s receiver tube is compressed, the temperature of the air rises to a very high level, igniting the tiny oil droplets that lubricate the piston seal and walls of the compression chamber. Thus, a combustion and sometimes detonation can result.

Dieseling in an air rifle is natural due to the lubrication required for smooth operation, but too much lubrication can be damaging to your airgun, and may even result in injury to you or others. This is why we stress you follow the RWS recommended air rifle 1000 shot maintenance schedule (also see 100 shot schedule) and specified amount of lubrication for all of your air rifles.

A carbon mainspring with heavy buildup and damage caused by dieseling.

A carbon mainspring with heavy buildup and damage caused by dieseling.

The accompanying picture of a broken spring is an example of what may happen if you over-lubricate your air rifle. The excessive amounts of oil can detonate, creating a powerful force within the receiver tube that violently slams the piston into the spring. When conditions are right, or rather wrong in this case, the force can cause the internal spring to break, rendering the rifle unusable.

The same thing can happen if you use the wrong type of lubricant. You should avoid using petroleum-based lubricants in your air rifle. These lubes are more prone to combustion and some formulas are known to deteriorate the piston’s seal.

While a broken spring due to over-lubricating is one of the worst things you can do to your spring-piston rifle, excessive dieseling can also lead to tears in the piston seal, which will cause a decrease in power and consequently, the velocity.

Many of the air rifles we receive for repair have been over-lubricated. We’ve seen this so often that it’s immediately recognizable, be it the black build-up of carbon on the piston seal, a torn seal, or a broken mainspring. We’ve even seen situations where excess oil has seeped out of the chamber and onto the stock.

Dieseling can cause heavy damage to the inside of an airgun if it is not controlled for.

Dieseling can cause heavy damage to the inside of an airgun if it is not controlled.

Dieseling can cause heavy damage to the inside of an airgun if it is not controlled.

To avoid dieseling, be sure to use non-petroleum based lubricants, like RWS Chamber Lube and Spring Cylinder Oil. Then, only apply the amount as specified in the gun’s manual or in the RWS recommended air rifle 1000 shot maintenance schedule (also see 100 shot schedule).

If you have an air rifle that has been over-lubed, or lubed with petroleum-based oil, the best and safest thing you can do is to contact us at Umarex USA. Ship the gun to us un-cocked and unloaded with the Return Authorization number we’ve provided. We have the specific decompression equipment necessary to disassemble and clean your air rifle and we’ll send it back to you shooting like new.