Line SI blow-by?

[Glovebox Operator]

I just picked up a Line SI Bushmaster in pretty good shape off of craigslist. I didn't take it apart yet but I did give it some test shots. No leaks but it does puff some gas up the feed tube when fired. Any idea what seal or seals would cause that? I know they aren't that complicated but I'd like to know where to look in case I do need to buy parts. I've got a good oring set, but anything past that and I'll have to order parts.

Thanks

[Jonnydread]

Any blowback up the feedneck is likely due to the o-ring on the front of the bolt being worn. Replace that and it should cut down on it.

[Ecapnation]

Any blowback up the feedneck is likely due to the o-ring on the front of the bolt being worn. Replace that and it should cut down on it.

Bushmasters should have delrin bolts not sure if they have orings as well

Are you holding the pump while shooting?

[Socalpumpballer]

All the ones Ive ever had came with stainless bolts with a front oring

[Glovebox Operator]

I'm not sure if I was holding the pump. Test shots tomorrow.

[Myrkul]

@socalpumpballer

I always thought the stainless "o" ring bolts were the Lapco upgrade, and that stock it was "o" ring less delrin? I could be very wrong

[BOOSH]

It's possible that it's something crazy like a power tube that's too short. You'd have to pull it apart to know for certain - It could also be air blowing out of the valve seat because the power tube doesn't align or is too small for the seat. Or a power tube that is bent/crooked <----most likely this.

[Ecapnation]

Socalpumpballer
All of mine have these internals.

[Glovebox Operator]

Delrin bolt with no groove for an oring, very little sign of wear on anything. Nothing seems bent or misaligned. I'll put it together once the baby goes to bed and check length of the tube.

[Glovebox Operator]

It's got too be leaking around the bolt. The tube does extend a little bit into the back of the bolt when assembled and when a little air pressure is applied down the muzzle it leaks out the feed neck.

[Ecapnation]

Wait by blow by are you meaning a leak??

Because your wording sounds like you are having air escape into your feed neck while firing.

If it's constantly leaking your cup seal is shot.... Nelson bolts and hammers have nothing to do with valve leaks

[Glovebox Operator]

Only when fired. It holds tank pressure just fine but when fired a little puff of air blows the next balls up the feed neck. I put some light air pressure down the muzzle with everything assembled and I could feel a little air escaping up the feed neck, nothing out of the pump arm bolt slots or anywhere else.

[Ecapnation]

Are you holding the pump forward?

[Glovebox Operator]

Yep.

[flyweightnate]

That bolt looks far smoother and newer than any I've seen. The crooked side of my brain says it must be an undersized aftermarket bolt front, because all the original bolts I've owned were scratched all over and I haven't ever heard of this issue.

[Glovebox Operator]

Could be, but the barrel also has very little wear on the anodizing from stroking the pump. This thing is in great shape overall. I'll have to test it with a hopper and see if it's actually an issue. The 10rd tube that was i used to test it earlier wasn't even close to a tight fit, gravity is about all that kept it in, and a single ball placed in the feed neck got puffed out too. I guess i should mic the bolt face and barrel ID.