Idiots guide to classic airgun strips (pt 1)

[Phil Russell]

DIANA 26 underlever

!933-1940. Note that this model, sometimes referred to as the 26U is not related to other model 26, break barrel rifles.
This model, .177, stamped 2 38 on the serrated butt (February 1938) is in overall very good condition although most screw heads show signs of removal due to some screw head slots being a bit mangled. Cocking was very smooth and quite easy. A chrono test with JSB Exact 8.4, Superdome 8.3 and Marksman all gave performance hovering on 400fps for around 3 ft lb. I believe this is about on spec for this ‘youth’ rifle and is on a par with other junior rifles. A tap test using damp tissue showed the tap to be good.
I did wonder if performance could be improved a little though so embarked on a strip to see what the mechanics were like. If you plan a strip of this rifle I recommend a spring compressor … see later.
Stock removal is 2 screws on the forend and the front trigger guard screw. There is no need to remove the trigger guard rear screw as it only fastens the guard to the stock.
Unscrew the end cap and look at the trigger. Note the trigger pivot pin also holds the trigger cage into the cylinder and with it, the mainspring. The mainspring etc cannot be removed until this pin is pushed out and the trigger removed. With action in a compressor, use a suitable drift to take up spring pressure on the cage to relieve pressure on the cross pin. I used a suitable ½” square drive socket. Push out the pin. The trigger is now free to remove complete with the trigger return spring. With trigger removed, release tension on the trigger cage and let the mainspring push the cage out of the cylinder. There was very little preload on this rifle. With spring tension gone, remove the cage, mainspring and spring guide.
Spring spec: 30 coils, 2.65mm wire, 18.45 od. Spring guide was a good fit.
Tap inspection: The tap is held in position by 3 screws on the lever side. The central screw needs to be removed before the 2 outer screws as it acts as a lock screw. As you remove the side screws, keep pressure on the cover plate as it is lightly sprung loaded. Remove carefully as there is a small ball bearing under it which sits on top of a small spring in a hole in the tap body. The tap can be removed for cleaning but note there may be shim(s) under it. My tap had a single, home made looking, shim fitted. With all cleaned, I applied moly grease to the tap and surfaces before reassembling. I tested tap alignment by firing a pellet into a rag and examined the head and skirt. I could see no deformation meaning that the tap alignment was OK.
To remove the piston, the cocking lever needs removing: remove the pivot pin lock screw and then the pivot pin. It helps to have the lever released from its locked position. Remove cocking lever. Piston can be removed. The leather piston head looked fair but inspection showed it was rotten with the sides separated from the central disc. It is held in place by a screw; this was tight so application of releasing fluid and light taps on the end were needed. Remove screw and the dished steel washer followed by a leather spacer washer, the remnants of the piston seal and a further backing washer. The piston seal is 25mm diameter. I made a new seal from 3mm thick leather.
Rebuild: All sharp edges on the piston body were removed and the piston cleaned. With the new, supple leather piston seal in place on the piston I compressed the seal slightly using a jubilee clip such that it would enter the cylinder easily. A smear of moly grease was put on the piston and it was pushed into the cylinder; pre-compressing the seal made this easy but do not waste time as the seal can quickly expand a bit to make fitting more difficult.
To see if performance could be improved I decided to fit a new mainspring. I did not have an exact replacement of 2.65mm wire x 30 coils so decided to try an alternative of 3mm wire for which the id was a good fit on the spring guide and the od was fine for the piston. I calculated that 26 coils would suffice to prevent the spring being coil-bound. After fitting this I noted, unsurprisingly, that cocking effort was higher but performance was hardly improved even after allowing for the new piston washer to form itself. Given this lack of improvement I removed the new mainspring and refitted the old one to give the easy cocking cycle.
Rebuild continued: Refit the cocking lever. With action in a compressor, add spring and spring guide, suitably lubed with moly, and the cage. With the drift in place, compress the spring until the holes in the cage and cylinder line up such that the cross pin can be replaced … but not yet as the trigger needs to be added. On some rifles that use this arrangement it is possible to add the trigger and return spring at the same time but I found this difficult: the spring is quite strong and the trigger is difficult to hold in place while the cross pin is inserted. So I installed the trigger first. The 3 sear parts on the trigger make it a bit fiddly but I used a thin punch to capture the sears as I inserted the trigger. Holding this in place I inserted a punch about the same diameter as the cross pin to get the sears aligned before pushing the cross pin through as it pushed the punch out. The trigger spring can now be replaced. This is best done with the action still in the spring compressor as you get more stability. I found it easiest to locate the spring on the dimple in the trigger blade before using a flat blade screwdriver to push the other end of the spring up and over the tang in the cage. Take care not to let the spring ping away. Remove action from the compressor and replace the end cap.
Replace stock and test. I had not increased performance from the initial test but at least I knew the piston washer was now OK.
Cheers, Phil