Correcting the danger of air rifle "Auto Fire"

[Basu]

Hmm.....Briha I knew you could say so.
I wish for beginner it is better to start with
90° at sear, take the feel and then
reduce it to 1-2° more, if need be.
Mine is less than 90° for certain.
benne- my experience with SDB ,I supect that piston
sear has lost its required angle due usage .
Trigger material is of hardened steel .
You may be needed to set right the piston sear first
to get proper engagement.

Basu

[bennedose]

Thanks for the suggestion basu, and thanks for the information Brihaji

[bennedose]

Today I opened up the old SDB again. Just before my repair to the trigger sear failed cocking had become noisy with a scraping sound. On remivong the spring I found that the scraping was because the spring had broken in 2 places and was lying in 3 pieces. The piston sear, as Basu had predicted was worn out and the recurence of auto fire was because the piston sear had shifted by a fraction of a millimeter due to the broken spring. The repaired trigger sear was OK.

I went back to the same workshop and got metal welded on to the piston sear. it is brass and not mild steel as I had thought. Filing into shape was easy. I replaced the spring by taking a new locally made 40 coil "35 size" , 11.5 inch spring and cutting off 6 rings because I anticipated that the new spring would have too much tension and make my SDB misbehave as it had done earlier.

Now the trigger is fine but I am left with an unfamiliar new-springed rifle whose behaviour is totally different. Let me see what happens over the next few days. I know that it is a fundamentally good rifle but it may need a lot of tweaking

[mercury]

Bennedose....notice the light angle given to the trigger sear. the piston sear is kept at 90 degrees.

the reason I put up this picture is for you to understand that when a rifle is cocked these two sear points are under the extreme high load of a compressed spring. notice the shape / thickness of the metal towards the rear edges of both sears. there is enough metal there, rounded off to ensure that they withstand the load / pressure they are under.

now switch to the before and after pictures of the trigger sear that you have put up in your first post. notice the difference in the shape of the original sear ( shape and thickness ) compared to the modification you have done. the modified sear , besides being angled in the wrong direction and elongated.... it is not thick enough to withstand the pressure it will be under.

quote... "I went back to the same workshop and got metal welded on to the piston sear. it is brass and not mild steel as I had thought". can you elaborate ??

I would go with what Pratik suggests...drill bits. hard steel would need special tools but is worth the trouble. mods on triggers are tricky and can be dangerous if not done right.

[bennedose]

[ Image ]

Bennedose....notice the light angle given to the trigger sear. the piston sear is kept at 90 degrees.

the reason I put up this picture is for you to understand that when a rifle is cocked these two sear points are under the extreme high load of a compressed spring. notice the shape / thickness of the metal towards the rear edges of both sears. there is enough metal there, rounded off to ensure that they withstand the load / pressure they are under.

now switch to the before and after pictures of the trigger sear that you have put up in your first post. notice the difference in the shape of the original sear ( shape and thickness ) compared to the modification you have done. the modified sear , besides being angled in the wrong direction and elongated.... it is not thick enough to withstand the pressure it will be under.

quote... "I went back to the same workshop and got metal welded on to the piston sear. it is brass and not mild steel as I had thought". can you elaborate ??

I would go with what Pratik suggests...drill bits. hard steel would need special tools but is worth the trouble. mods on triggers are tricky and can be dangerous if not done right.

Thanks for the useful tips. Will use them next time round. The angle on the trigger sear I am sure will make the trigger lighter - but I was ovrdoing the safety (correction of auto-fire) bit while attempting a half job by just looking at the trigger sear without loking at the piston sear. That failed very soon so I extracted the piston.

I the simply "jugaad"-ed the new brass coated piston sear to be at 90 degrees and left the slightly worn new trigger sear alone. I need to see how long this brass-brass combinatiion lasts. I deliberately left the tip of the piston sear "extra thick" hoping that the soft brass would take the strain better. It is about about 75 shots old but the spring has become the immediate concern now. Getting that broken in and tested will test the jugaad sear repair and I will post my experiences.

I must give credit where it is due. The old SDB sear has lasted at least 6 to 7000 shots. Apart from that even the other screws holding the stock have not got worn out. I can't say that for my new IHP 35 in which one of the side screws and the hole in the receiver have got worn out after perhaps 1000 shots.

[brihacharan]

Mercury wrote.....
Bennedose....notice the light angle given to the trigger sear. the piston sear is kept at 90 degrees.

The reason I put up this picture is for you to understand that when a rifle is cocked these two sear points are under the extreme high load of a compressed spring. notice the shape / thickness of the metal towards the rear edges of both sears. there is enough metal there, rounded off to ensure that they withstand the load / pressure they are under.

Now switch to the before and after pictures of the trigger sear that you have put up in your first post. notice the difference in the shape of the original sear ( shape and thickness ) compared to the modification you have done. the modified sear , besides being angled in the wrong direction and elongated.... it is not thick enough to withstand the pressure it will be under.

> Great observation there by mercury
(Angled in the wrong direction and elongated.... it is not thick enough to withstand the pressure it will be under)
> Perhaps the main cause for "Auto Fire"
> IMHO - Induction hardening of the sears could solve 'wear & tear' considerably - some food for thought
Briha

[bennedose]

Unfortunately rounding off the trigger sear as indicated above did not work. So I had another blob of brass deposited on the trigger sear and filed it to the configuration below - leaving thick wads of metal both on the piston sear and trigger sear. This is working absolutely fine as of now. (Image below) I have fired about 60-70 shots with no change in feel. Will see how it goes. In the meantime new issues have cropped up which I will discuss separately - they are interesting enough for a separate discussion

repairedsear-sm.jpg