BSA Improved Model D rebuild

Strangely enough nobody seems to have made a dismantling/reassembling a BSA underlever rifle video yet, as far as I know. But it's incredibly simple and you can't go wrong if you follow the advice of experts such as Lakey in this thread:

https://forum.vintageairgunsgallery.com/vintage-bsa-resources/vintage-bsa-lincoln-jeffries-technical-tips-faqs-2/#post-481

Perhaps you could film the process from the point of view of a novice and I could post it in the gallery for reference?

Without a video, first make sure the gun is not cocked. Make sure you have good fitting screw drivers very important also good screwdriver techniques to be able to tackle worn screw slots and very over tightened screws, skills to avoid slips broken and burred screws You also need a good table or bench to place the gun on. Best to use a bit of carpet or cloth to hold the gun on. Before you start make sure you keep an eye on the front sight as these can be delicate and had to find or repair. Find a small pot to put screws in or make small holes in cardboard to push the screws into and write down what they are from ie trigger screws. Make notes and take pictures if unsure where they go back.

First undo the two trigger guard screws and the trigger guard should wriggle out forward.  The next step is to remove the trigger guard housing and wood stock as one piece by unscrewing anti clockwise from the cylinder barrel,section, as you are doing this you may have to pull the trigger back, as this interferes with the slot on the cylinder.

Now comes the tricky part as you are unscrewing, both parts are under the piston spring tension. So you need to push down against both parts as you undo. You can place the end of the barrel on a soft cloth or bit of carpet so the end of the barrel is not damaged, watch out for the foresight. As the last thread is turned the pressure of the spring is released, but its not that much.
you then have the two parts, the trigger housing and stock. The other part is the cylinder and barrel section with loading tap.

To remove the piston, remove the spring or springs as there can be two. it just slides out. Then cock the gun so the piston moves to the trigger end of the cylinder.

Removing the cocking lever assembly, Remember there is a small keeper screw holding the main screw in place, this is under the loading tap. Make sure that the main screw is not holding tight the small keeper screw. Loosen up the main screw left and right slightly. Then remove keeper screw but be careful as its a small thread about 1/8 x 40 tpi. after this remove main screw and remove cocking lever.

With screw driver or bit of wood slide out the piston and all you have left is the cylinder and barrel and loading tap.

Before removing the loading tap this is the time to check the tap alignment to the barrel. 

To remove the loading tap undo the two screws holding the plate in. Watch out the two screws may be tight and the slots worn. Also remember that these two screws are similar to the trigger guard screws and can be mistakenly mixed. When the two screws are removed the plate holding the tap in can be removed and the tap removed after.

Next step, would be the other cocking link lever which is two parts, the screw assembly is the same as the main cocking lever screws with a keeper screw holding the other screw in place. The cocking latch on the barrel catch is held in with a pin. To remove pin these pins are tapered. So you need to find the smaller and large side of the pin. With the correct punch knock the smaller side, but be careful not to miss as you can damage and mark the lever.

with the piston all you need is to remove the washer, check for damaged screw. If replacing the screw and washer make sure the screw fits down below the inner washer. So the screw is not protruding so it hits the end of the cylinder. if the piston is going to hit the end of the cylinder its best it hits the inner washer first or the cup shape of the main outer washer cushions the impact.

Id you need to replace the cocking rod which engages the trigger, this is held in place with a keeper screw which is the same keeper screw as the main cocking lever keeper screw.. The piston body needs to be held in a soft grip vice cusioned with card or wood and the rod un done with a spanner. This is screwed in place very tight so be careful.

To remove the stock from the trigger housing the but plate needs to be remove by undoing the two butt screws. Then you need a large and long screw driver to engage the stock screw and turn anti clockwise. After the stock screw is removed just pull apart the stock from the trigger housing.

Trigger housing, trigger and sear and sear spring. Just undo the both trigger screws making note of screws and the assembly of the position of sear and trigger in housing.

Assembly is the reverse. Things to watch out for. Don't go mad and over tighten screws, The main screw with keeper screws need screwing up but backing of to fit the keeper screw. 

The piston washer if new should be soaked over night in neat-foots oil. Or the old one cleaned and re soaked with neats foot oil. To fit the piston and washer back into the cylinder you have to get it padt the threaded section of the cylinder. First cup in the cylinder as much as you can and push and twist into the cylinder, take a bit of time. Once past the next part is to get the washer past the cocking slot in the cylinder. With a bit of wood or metal push the washer down to get past the slot as not to damage the washer in the process. Just before you do grease up the rear part of the piston. and then push home. Once in line up the piston slot with the cylinder slot and then re fit cocking lever assembly.

Re fit spring or springs with a little covering of grease and grease up the assembled trigger housing spring guide rod tube attached to the trigger housing. Line up the two parts and push and screw together at the same time. Screw up both parts then back off until the two trigger guard screws fit.

Loading tap assembly is the reverse just add some new grease to fit. 

If the need to remove foresight rear sight or barrel cocking catch. These are dovetailed in place and can be drifted out from left to right or the other way round. The process to replace is the same as drifting out. If lose fit replacing, just carefully peen the top of the female dovetail part down to tighten. An ideal punch is a small plastic punch made of derilin.

Hope this helps your video.

@bc312 Thanks very much for your help. Most useful