Sunday, June 15, 2008

The 1377 Project: A New Bolt

Well, I keep chipping away at my 1377 carbine conversion. This time I decided to make a new bolt. There were two reasons: One, the original bolt supplied with the Crosman steel breech is a bit loose. Two, I wanted to improve the airflow from the transfer port to the end of the pellet.

The bore of the Crosman steel breech is .257"

The supplied bolt is .248" diameter. That's a lot of slop.

I turned down a piece of 3/8" free machining steel to .256"

As always, protect the ways of the lathe when polishing or sanding...

Turning an extended nose/probe and the 60 degree cone that seats in the barrel.

Carving away the o-ring groove. This was a case of measure, cut, measure, cut...

The bolt cut off to length.

Facing and chamfering the end of the bolt.

Using a transfer punch to determine where the bolt handle will mount.

Drilling for a #8-32 tapped hole.

Tapping #8-32

I mounted the bolt and found that the bolt was hanging up a bit, which can be seen as a slight raised ridge. So I turned down that end for about .1" to the original .248" diameter.

The finished bolt, all mounted. Velocity, after shooting a bit, averaged at 575 fps. for 5 pumps, and 726 fps. for 10 pumps with Crosman Premier Light pellets. That's a negligible increase over the previous improvement (568 fps. at 5, and 724 fps. at 10) gained when I made the adjustable piston.

The most interesting development, and something I hadn't noticed before, was that the valve was not dumping all the air I was pumping in. I could take a second shot after pumping ten times and get 254 fps. with the CPL's. I made sure that I exhausted all air in the valve by dry firing between each shot for the new averages. This makes the original velocities measured after the piston mod a bit suspect, as they likely had a boost of remaining air in the valve, and may have been slower had all the air been exhausted.

So the next installment will cover a bunch of modifications to the valve and porting...

1 comment:

Riggsy said...

Are you going to do any internal valve mods.