Tuesday, November 25, 2008

Derrick's Xisico/BAM B26-2, Part 2

Continuing with Derrick's teardown...

Once the end cap was removed, the spring guide, mainspring, and piston came right out. I was careful to not cut the piston seal on the slot for the cocking lever. The piston seal is a fairly soft synthetic. Dark brown oil covered everything inside the gun. Well, at least it wasn't going to rust. The seal was stained brown from the oil.
I should mention here that I had only put about 20 pellets through the gun before I took it apart. Assuming again that it's a returned gun, I have no idea how many shots were fired from the previous owner, but as there was virtually no wear on the barrel pivot, it must have been few. The gun was putting lightweight Tournament wadcutters out at approx. 675 fps. A few feet above the 650 fps rating by BAM.

Here's the piston seal looking like a million bucks. I wonder how the seal would have looked 1500 shots later? The piston is sleeved with a piece of sheet metal. Kind of surprising in a (relatively) low-cost gun--as that adds cost and time in production. Typically, this is something a tuner will do to tighten the fit between the ID of the piston to the OD of the mainspring. A snug fit helps mitigate against vibration. As it wasn't a snug fit in this gun, it was probably done so the cocking shoe doesn't rub against the spring coils when cocking.

This is the cheesiest (apologies to Kraft lovers) spring guide I've ever seen. This has to support the mainspring under full compression. Now I see how they could afford to sleeve that piston.

Another view of the seam.

The stock mainspring. No optical illusion. It's not straight. The kinks are likely the result of the wimpy spring guide combined with a low cost spring. The flex and buckling equates to friction and vibration. I anticipated when I bought the gun that I would have to order a spring from Jim Maccari and then make a new spring guide. However, I had a brand new Weihrauch R10 spring and guide that looked close enough to work.
The stock BAM spring was 11.125" long with a 0.122" diameter wire. The OD was 0.743", ID 0.500" +/-.

A silver Weihrauch (often abbreviated as "HW" for Hermann Weihrauch) spring guide next to the mac and cheese model. The BAM spring guide measured 2.937" long and had an OD of 0.473" The HW guide was 3.123" long and the OD 0.522". The single most important thing was the only thing I didn't measure: I made sure that the piston rod would pass through the center of the HW spring guide. And it fit perfectly.

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