Venom Maglaza - piston washer failure.

[Rustam Bana]

As the title suggests, I am experiencing piston washer failure and these are the events leading upto it.

Rifle was stripped down as I wanted to use pre-load washers. However, the main spring was slightly bent (been there since 1999), so I decided to replace the main-spring with a new one and forget about the pre-load washers. I also put in a new breech seal and piston washer.

The internals were degreased and then re-greased with Venom moly grease as per Venom's instructions.

I have done this service in the past and have found that the rifle works faultlessly from the very first shot without any hint of dieseling.

Having assembled the rifle, the M.V.'s recorded on a chrono started on an unusual high and then started dropping. The chrono readings were 841, 794, 817, 788, 775, 771, 764 & 759. There was also quite a bit of dieseling in the first few shots. This did not give the impression that the rifle would settle down in the future and in any case was quite unlike the rifle I know it to be.

The rifle was stripped again and I found that the piston washer did not hold it's seal. Also, there was a mark on its face corressponding to the transfer port. I haven't seen this mark on the older washers. I assumed that Venom had sent me a piston washer of the wrong size. I replaced this with a used piston washer in good nick, degreased the cylinder, added a little moly to the main-spring and reassembled the rifle. The rifle now fired the next three Lazapells at 737, 727 & 716 ft./sec. which was a lot less than it's original M.V.. I assumed this was probably due to the extra grease on the main-spring (having re-checked the grease on the main-spring this morning, I find that it isn't as excessive as I imagined it to be last night). However, when I opened the action this morning, I found that this piston washer had more or less lost it's sealing capacity and this too had the transfer port mark visible on it's face.

I cannot think what I am doing wrong here, except perhaps that the choice of nail polish remover as a degreaser could be affecting the piston washer. I have used Acetic acid as a degreaser in the past without any ill effects but it does tend to strip the blueing away if one is not very careful and hence, the choice of nail polish remover.

I have one old piston washer left which still seals properly and before I use that, I would be grateful if one of you experienced fellows could tell me what is going wrong.

Thanks.

Rustam

[Rustam Bana]

No one?

Well, in a couple of minutes we will know if it was the nail polish remover or not.

[Terry D]

Originally posted by Rustam Bana
I would be grateful if one of you experienced fellows could tell me what is going wrong.

It's fairly obvious to me, mate. Tendulkar just isn't on form, is he? I mean, if it wasn't for your bloke's maiden century - your neighbours would have hammered you!

HTH

Terry

[corkscrew]

we've not got out of bed yet rustam.
and yes it could be that acetone (with added oil of some sort i beleive) you have used.

[Jonathan]

Rustam, my guess is that there was a very small amount of your nail polish remover left within the confines of the cylinder. This would cause the gun to diesel violently due it's highly combustable nature. The result could be a scorched piston crown which, when heated - even once - can quickly lose it's ability to give a satisfactory seal.

If you're going to use this degreaser, perhaps best to leave it for a few hours after the cleaning to allow the thinners to evaporate completely.

Interested to know what others think...

Jonathan.

[woody2906]

Rustam,
Nail polish remover is acetone which dissolves most plastics!

I found this to my cost once when i 'melted' the internals of a parker pen in an attempt to clean it.

Clean it out thoroughly and use methylated spirit (meths) instead.

[Rustam Bana]

Cockscrew, Jonathan and Woody go to the front of the class.

Terry, you may remain where you are. Technical Editor my left foot.

As you may have guessed, the rifle is shooting well again. The velocity spread from 37 shots was 10 ft./sec. (772 to 762). Not great for a Venom but it has always given me a 10 to 11 ft./sec. velocity spread when I have used Lazapells. However, the down range groups have always been the best with this pellet.

I merely degreased it by washing the cylinder with hi-pressure water and Pril. Rinsed out the Pril and dried it thoroughly.

Now to raise the velocity a tad. Should I go for a preload washer/s or a nice stainless steel top hat, a la Air Arms?

Rustam

[Jonathan]

If it were me, I'd use the washers to get the right thickness that I was going to need, then turn myself a nice neat fitting spring guide / tophat to suit the spring so that everything was in one solid piece.

If you've got access to a gunsmith or engineer that's what I'd suggest.

Jonathan.

[Rustam Bana]

Thanks Jonathan.

Will need to make the top hat only as I have the Venom spring guide. However, I find that this spring guide is a tightish fit. The OD is more than the springs ID. I did turn it down a tad but it still isn't what I would call a 'slippery fit'. Would this later type of fit increase the MV?

Incidentally, just shot a string (10 pellets) of the old 15.5 grain H&N FTT's and got a velocity spread of 4 ft./sec. (764-768 ft/sec.). Pity, you only get the lighter versions these days.

Rustam

[Terry D]

Originally posted by Rustam Bana
Terry, you may remain where you are. Technical Editor my left foot.

Editors edit, Mushtam. That means they transform the hieroglyphics submitted by techies into that which can be deciphered by real people.

Mind you, even I would have known better than to have poured half a gallon of nail varnish stripper into my airgun!

What WILL Ivan say when I tell him?!!!

Terry

[Rustam Bana]

Originally posted by Terry D
Mind you, even I would have known better than to have poured half a gallon of nail varnish stripper into my airgun!

What WILL Ivan say when I tell him?!!!

Terry

LOL. Live and learn, I suppose.

[Jonathan]

Originally posted by Rustam Bana
Thanks Jonathan.

Will need to make the top hat only as I have the Venom spring guide. However, I find that this spring guide is a tightish fit. The OD is more than the springs ID. I did turn it down a tad but it still isn't what I would call a 'slippery fit'. Would this later type of fit increase the MV?

Incidentally, just shot a string (10 pellets) of the old 15.5 grain H&N FTT's and got a velocity spread of 4 ft./sec. (764-768 ft/sec.). Pity, you only get the lighter versions these days.

Rustam

With a MV spread like that Rustam I'd be inclined to leave it as is. Re-machining the guide will only realistically give you another 1-2fpe (max). It seems to have settled nicely the way it's running now (and I bet, with a fit like that on the spring guide, there's no spring noise), so I'd just leave it the way it is.

If you are going to machine the guide, tread carefully with about .004" (0.1mm) at a time off the diameter. As you know, it's easier to take metal off than to put it back on.

Jonathan.

[Rustam Bana]

Originally posted by Jonathan

(and I bet, with a fit like that on the spring guide, there's no spring noise)
Jonathan.

I am afraid you loose. There is a definite spring twang that was never there before. A little more grease on the guide, perhaps?

Here are some more velocity spread figures (10 shot strings);
Webley Venom Lazapoint: 3 ft./sec. (726 to 729 ft./sec.)
AA Hunters: 3 ft./sec. (731 to 734 ft./sec.)
Eley Wasp 5.6mm: 14 ft./sec. (760 to 774 ft./sec.)

Will leave the spring guide alone but am going to experiment with a ss top hat. If that gives me 800 ft./sec. with a 14.5 grain pellet, without the twang, I'll be a happy chappy.

Thanks for all the advice.

Rustam