Is the Lewis Gas Chamber a Friction Fit?

[1gewehr]

The Form 1 was finally approved and I got the welded-up barrel out. I was looking to remove the gas chamber from the barrel and it slid right off! It is not a very tight fit on the replacement barrel and wondered if there was any method to fitting the gas chamber/gas block on the barrel? Currently, it is snug, but easily moved by hand with a little effort.

 

I was thinking of putting a small set screw down through the top to hold it in place on the barrel. But it looks to me like there would be some gas leakage because of the loose fit.

 

The gun is a US Model 1917 .30-06 ground gun which had the radiator and shroud removed for use by the British Home Guard in WWII. The gas chamber/gas block is an aircraft model with a dovetail for the front sight mount.

 

Also, if anyone has the correct 4-port gas regulator cup or some extra .30-06 pans for sale I would appreciate hearing from you. I know IMA-USA has pans for absurd prices. I have yet to win the lottery so they are out of my price range.

[DougStump]

First, this is based on a .303 BSA ground gun, but I should think it is the same. The barrel band slides on the barrel without much effort. When you screw the gas cylinder into the band, the tapered end of the cylinder should bottom out in the recess on the barrel. Don't torque it down hard, just until it bottoms out then back off so it will match up to the gas tube.

 

Doug

[Black River Militaria CII]

The gas port ring has probably been installed on a barrel backwards at some point thus expanding the smaller ID of the inside taper to the same size as the other side which is slightly larger ID so it won't fit either way. A decent interference fit is important here so as not to lose gas pressure since the gas block tapered joint is never gas tight either. A set screw will pull one side off the surface of the barrel and just stretch the ring. I have shimmed the ring with a carefully constructed .001-.002" stainless shim glued to the inside of the ring with Loctite. The gas block hole is carefully punched through once the ring is in place.This has held up well and seems to be pretty durable and prevents loss of gas that escapes from the gas block joint. It doesn't allow removing the barrel from the shroud to clean since then the shim has to be replaced, but I can clean the barrel sufficiently without removal from the shroud. Just one idea.

I have tried with some success shrinking a ring slightly to restore a tight fit, but the method is tricky and rings were easy to find at the time, but are not now.

The gas block screws into the ring and can also be quite loose from over tightening and a source of leakage. The threaded shank on the gas block can be shorted very carefully to get an extra revolution that allows the block to start tightening up before correct alignment with the axis of the barrel. The tip of the threaded port shank must be flat and perfectly 90 degrees to the port hole. Cleaning up the tip from pitting from corrosion and also cleaning the bottom of the barrel port will help to reduce leakage but it must be done carefully. Also, have had one Lewis gun with the gas block brazed to the ring!! Had to unbraze it to get it all apart.

The threads of the gas tube at the gas block connection are also usually damaged and a source of gas leakage. The tubes are often over tightened so have to be backed off to correctly fit the receiver joint for the tube, leaving it loose at the gas block. Careful trimming of the threaded end of the gas tube will allow the tube to be tightened one more revolution for a tight fit to the gas block, but that is all that can be removed without shortening the tube too much to fit the receiver joint correctly. does depend on the tube though and with some it is possible to remove the equivalent of two revolutions, but that is not necessary.

Anyways, just some ideas…..

Edited September 7, 2016 by Black River Militaria CII

[1gewehr]

As usual Bob and Doug have answers! Thanks!

 

For now, I'll see if the fit is tight enough to cycle reliably. If not, I'll take Bob's suggestion and make a shim from some stainless foil.

I'm really excited about making this Lewis run again!

[DougStump]

The specs I have show the threads to be .394" 32 tpi, not exactly a common size anymore. And of course tapering the hole to match the barrel would be fun. I good machinist should be able to make a new one, but it would be very time consuming. However with modern CNC machining or metal 3d printing it's possible. The only question is how much?

[1gewehr]

IT'S ALIVE!!!

 

Finally got to a range yesterday that allows full auto rifle caliber. I first used a single live cartridge and a dummy to make sure that the spring tension was set properly. I purposely set the spring tension high and the gas on low to ensure that I didn't have the bolt hitting the buttstock mount. I reduced spring pressure until the dummy fed properly. Taking the spring down two more notches, I loaded two live rounds. Both fired fine. I then loaded five rounds to see if releasing the trigger caused the sear to engage properly. Again, perfect two round bursts with the final round fired singly.

Lastly, I loaded ten rounds. Again perfect functioning. At this point, I checked the gun over carefully to make sure everything looked OK.

Then I fired two drums in a variety of bursts. Again perfect function!

 

I am very pleased with this gun! It is very controllable, accurate, and smooth.

 

The one peculiarity that I don't like is the way the feed holds one round in the slot if you remove a drum partway through. I just removed the drum and then fired the last round singly. But it is something that could be a potential hazard.

 

Thanks for all of your help. This is my first rifle-caliber machine gun and I feel privileged to get it working again.

 

Now, I just need to locate a correct 14-fin radiator, front sight, and shroud front section! Plus more .30US drums!! (I know IMA has some for ridiculous prices. I can't afford those.)