LGU owners, got a question for you?

[bigtoe01]

How many of you have seen a power drop with the LGU's?

Mine to start went warm, i swapped the spring to a 5mm shorter LGV spring and with 2mm additional preload it was back at 11.2...however tested today it had dropped again to 10.7 or so.

Now i think I know whats causing it, but i was wondering if anyone else had found their LGU had lost power or if any RFD's have seen guns back with low power issues?

[landymick]

Not yet its only had 1200 so far & still at 11.3 in .177 with RWS Superfield not too bad with these i use AA Field in the LGV.

[Arthur John Smithsplease]

Mine has dropped power.

It was too close to the limit for comfort when I got it, but power had dropped to less than 11fpe with H&N FTT, and a lot further with RWS Superdome when I last tested it.

That may be because I accidentally left it cocked in my cabinet for 48 hours. I'd meant to leave it that way for only 24 hours, but forgot.

I'll chrono it again tomorrow with H&N FTT and JSB Exacts 4.52 and let you know the results.

It seems to prefer Exacts but I don't yet know what power they're running at.

[melling]

Only had 500 pellets through it , loves rws superdomes , when i first chrono ed the rifle it was 10 .6, will chrono when put a few more tins through it .

[Arschilecki]

Spring broke in. In all my new HW rifles I noticed power drop after 500-1000 pellets.

[bigtoe01]

The LGU is lubed with grease that does burn, this detonation does add to the power and does account for some of the decrease in power some are seeing as its burn off...but thats not what I have found.

You guys with LGU's, shoot some 10gr pellets for me, bismags or exact heavy, whats the shot cycle like, does the action buzz and vibrate a little?

A word of warning, do not shoot to many at this weight, just a few and report back.

[Snooper601]

The LGU is lubed with grease that does burn, this detonation does add to the power and does account for some of the decrease in power some are seeing as its burn off...but thats not what I have found.

You guys with LGU's, shoot some 10gr pellets for me, bismags or exact heavy, whats the shot cycle like, does the action buzz and vibrate a little?

A word of warning, do not shoot to many at this weight, just a few and report back.

Detonation knackering the spring? or is it damaging the seal lip?

Cheers

John

[bigtoe01]

Detonation knackering the spring? or is it damaging the seal lip?

Cheers

John

neither....I think the issue is the transfer port...too small.

Whats happening imo is the peak pressure is to high, this fatigues the spring over time, so to start the gun is ok, the lube burn off and it settles, then depending on what weight pellet you shoot the spring starts to fatigue quite quickly.

White chatting to Darrin XTX today, he mentioned his LGU had lost power...again, he has also had to reset a customer gun that had lost power. I mentioned mine went warm (within 50 pellets) so i changed the spring to a 5mm shorter lgv spring and dialled in back to 11.2 with exacts...thinking the LGV spring may also suit his gun better. Anyway, i asked what pellets he has been shooting, he had moved over to FTT as they were proving very accurate out to 55yards, they are close on 9gr in weight. So I got curious.
I fettled my LGU a while back now and shot it at length last week, possibly 200 pellets or so, and another session before with around 150 pellets, I had set power to 11.2 with exacts, today however...10.6. So I went out and pushed some Bismags thru it...piston bounced like crazy, exacts heavies...piston bounced like crazy...lots of vibration.

So my theory is this, the SCR is seriously high with the 2.6mm port, the spring is having to overcome this with minimal bounce to give a sweet shot feel, this however depends on the second valve within the rifle...the pellet. I feel mid weight pellet look to be borderline on causing piston bounce, a lightweight is fine, especially a falcon...but push a 10gr pellet in there and the peak pressure duration stretches out long enough for the piston to rattle back and forth. Now...heavy pellets fatigue springs, however so do to small transfer ports, add the 2 together and i think you are asking for issues. Darrin told me he has heard of a few LGU's losing power, Armex feel there is not an issue..i however think there is an issue...the issue is you wont see it from a new gun, only a shot in rifle, especially one thats been used with 10gr pellets all its life.

So because im brave LOL I just whipped out the comp tube and followed some advice Wonky gave me a while back and opened the port out..not by much...just to 3.1mm. I think Dave went to 3.2mm, i will recheck with him soon. Now remember the gun has an LGV spring, its 5mm shorter than a stock LGV and LGU spring (I think they are the same anyway) and I added 2mm of preload say 2 weeks ago to tweak it up where i wanted it. tested today the velocity pre 3.2mm TP had dropped in 2 weeks from average 775fps to 750 to 755..very consistent however. So..lathe drill clean lube rebuild...790fps with exacts, so strip,remove preload, rebuild and im back at 775fps, shot cycle way sweeter. So now i move onto the heavies...shot cycle way different, you would think its shooting an 8gr pellet, very sweet, velocity up from 650 to 685 to 690fps, 11.2fpe peak just like the exact 8.4's. Pushed some FTT thru it...11.2fpe, again super sweet shot cycle. The piston bounce had totally vanished, i did not add a sleeve, or any grease to the spring, just added a little extra ultimox type lube to the seal and comp tube bearings to replace what I had removed...this lube does not burn remember.

So my theory was the peak pressure is to high with the small port, the piston is not travelling down the comp tube far enough to overcome this pressure with added momentum etc especially when the pressure release valve (the pellet) is a tight fit or heavy..or both so I dropped the SCR a little to allow the piston to fly further and gain a little extra speed, add in extra flow thru the port and it looks like i dialled out the bounce with the heavy pellets and with a bit of luck dialled out the premature spring fatigue along with it.

Im going to monitor this, shooting the gun again this Thursday, so i will chrono it before i drive to the club, while im at the club i will test it and when i get back I will test it again over my skan...then i will check it the day after and see if its settled down or still moving.

For now, im 99% sure the TP is the issue...its too small.

[TonyL]

Very interesting, BigT.

Will be even more interesting to see how it fares now with less SCR.

Would you say that the quality of the springs is in any way in doubt, or just that piston bounce is to the magnitude and that violent that it would affect any spring? And how does it feel whilst shooting in terms of perceived surge?

[bigtoe01]

Very interesting, BigT.

Will be even more interesting to see how it fares now with less SCR.

Would you say that the quality of the springs is in any way in doubt, or just that piston bounce is to the magnitude and that violent that it would affect any spring? And how does it feel whilst shooting in terms of perceived surge?

It was good when the spring was fresh, as i said mine went warm...(only with exacts and only just) but i do feel that would have changed had i just kept shooting it. I did note as the power had come down the bounce was getting worse. Now...is the spring to blame? I dunno, maybe, however they work just fine in all my 80's and the one in my LGV is working just fine also (although only 1k pellets thru it at most)
So now the port is larger...still small in relative terms mind at 3.1mm, and yet the shot cycle is superb, where i thought it was good before its now very very good, much like my LGV which i really like and my .177 AGT80.
The main difference was with heavies, way different shot feel, totally differing in every way to how it was with the small port...and big gains in power with the heavy pellets making them a usable alternative if they prove accurate.

The SCR is 1100+:1 Tony with the 3.1mm port...still very high I work on 550 to 600:1 on a 77k so nowt to worry about here.

[Shed tuner]

Hi Tony, I'm sure we said it needed a 3.2-3.5 port when we looked at the specs before they even arrived

Agree completely with your diagnoses - this is why ally piston guns can also knacker springs if not set up right - too much bounce too early - and what's worse with ally pistons is the lack of inertia increases the accelerative forces acting on the spring.

(Note to anyone else reading - a well setup ally piston with lower SCR etc can work very well... you have to balance the two...)

Obvious question to non altered LGU owners - shoot some JSB 8.4s, compare the power to what you get with JSB 7.3s or 7.9s and report back.
(reason for these is the pellet is much the same fit and lead mix, but the weight differs)

PS Tony - is the spring you are using lighter (less mass) than the stock LGU spring ? If so this would accentuate the problem by effectively lightening the piston.

Rgds - JB

[bigtoe01]

Jon

It was obvious to me with a heavy pellet the valving was to tight and the 255g piston was bouncing bad...

Now Darrin has fitted a Titan spring and is finding the added stiffness has the light weight pellets bouncing and the heavies shooting nicer...this with the stock 2.6mm port.

So..stock spring looks to need a larger port, the stiffer spring...im thinking less piston weight

[Arthur John Smithsplease]

Tony, what size is the transfer port on the 177 LGV?

I ask this because I took mine to the range today. It was smoother and grouped better with JSB Exact Express (7.8) and the JSB Exact RS (7.33). I'll try these again tomorrow with my LGU.

My LGV is the Challenger Pro, imported from Germany. It has no rear or front sight and an aluminium cylindrical barrel weight, but, although it's mildly front heavy, it seems to suffer from either muzzle flip or excessive piston bounce. I'm going to put some weight in the butt of the synthetic stock and a heavier scope up top, but I also think it would also benefit from something like a Parker Hale steel silencer at the front - not for the sound reduction, but for the extra weight.

[melling]

Ok , so if there is an issue with the transfer port being to small , how difficult is it to open it up to 3.1 mm , what would i need to open the port, ie would a pillar drill be suffice or should i give it to someone who knows what they,re doing .
Any help would be appreciated .
cheers mark.

[john bailey]

SCR what is this? yes I have a LGU and following this thread with interest.

ATB John.