Taking weight off a hw97 piston

[buttloaves]

I cant work you out "buttloaves".

Are you trying to say i have to work it out for myself?

Just thought some might like to share abit of knowledge to save a poor man ruining an air rifle.

Sorry Elmer, i was more asking a question than answering one, my thinking was if most of the weight was in front of the spring with a light skirt behind it would travel more true to the bore, a bit like a fire works rocket with the weight at the front and a long stick to staballise it, but i could be wrong, oops there i go again.. Please don't ruin your air rifle.

[ElmerFudd.177]

Thanks guys...
Very helpful info.

Ive got use of a lathe if i need it, so was thinking other than drilling i could take weight out alround through machining the center out, or half a mm all the way along to say 2.5cm from each end. Maybe reducing metal to metal contact.

I have a short stroke extension so im not going to modify piston straight off, ill try spring mods first and transfer port fiddling last.

[Dannyboy759]

As said its a balancing act. Too much off and the piston looses efficiency.
The standard 77/97 piston is about 250-265g depending what size you are using i.e. 25/26mm. Ive only ever used 25mm as the volume the 26mm gives you is really too much for UK levels.
I had an early Venom 77 Lazaglide that the piston in 25mm was only 170g, the trade off for this was the transfer port was over 4mm and the spring had a lot of preload to get the power due to the light piston.
The ultimate way to go with a 77/97 or tx200 for that matter is 22mm sleeved using a skirtless piston and then you can take the weight of the piston down to 150-170g and the tp between 3.5-3.8 and the gun has hardly any recoil and the sight picture stays on target. It puts anything else ive had or shot to shame.
But staying on theme if you wanted to reduce the weight of a 97 piston the easiest way is to machine down the thickness of the skirt on a lathe. you should be able to get it down to about 225-220 and open up the transfer port to about 3.3mm to start with as ive found that you cant put a set figure on this as every gun performs and acts differently so start with this and see how it goes and feels.
Drilling bloody great holes in the side is another way to do it, used to be the old way Venom did it back in the day but it looks uncouth and a bit primitive.
The other way to do it depending on what skills and equipment you have is to remove the piston skirt on a lathe and replace it with aluminium but this really does make it a bit too light and has detrimental effects on other things.

[ElmerFudd.177]

If i was to take weight off the piston skirt do i take it from the inside of the piston?
I was thinking i could cut off a small amount from the rear of piston?

Ill see how a polish, deburr and regrease with new drop in kit and short stoke extension goes first if im happy and its under 11.4 ill leave it.

Thanks guys

[TonyL]

Is surface area of the piston face a factor, less force pushing back on the smaller piston.
Is it possible to sleeve down the comp tube to accept a smaller piston?

As I understand it, yes, the surface area of the piston would be a factor.

It most certainly would be possible to sleeve down the comp tube. This has been done for years, especially with the HW80, reducing the bore from 30 to 25mm. Latterly, some of out more adventurous "shed tuners" have reduced the bore on the smaller bore guns, too. Obviously, thickness of the sleeve / comp tube has to be taken into consideration and also availability of suitable material, usually hydraulic tube. So, 26 down to 23 and 25 to 22 have been undertaken. And with excellent results in many cases.

[TonyL]

If i was to take weight off the piston skirt do i take it from the inside of the piston?
I was thinking i could cut off a small amount from the rear of piston?


Thanks guys

As mentioned up above, it would be a far better idea to machine away the outside of the piston, leaving the front and rear one inch sections unmolested.

[deejayuu]

For the shed tuner with no machine tools I think slots identical/similar to the cocking slot equally spaced around circumference
would be easiest option. Basic tools/skills only.
Spoken like a man that's never tried it. Done lots of other tampering tho'.

[bmp01]

As mentioned up above, it would be a far better idea to machine away the outside of the piston, leaving the front and rear one inch sections unmolested.

Err. ...you sure about this?
From what I remember about the HW97, it uses all the piston OD, ie over the whole length to guide the piston into the cylinder. ... Piston doesn't have rear bearing like the TX200. Making a step (or two) in the piston OD therefore doesn't sound like a good plan to me. Am I missing something?

BMP01

[tinbum]

Holes or slots are the way forward with the 97/77. But having said that, just the parts you have will work fine with that piston weight. Opening the TP to 3.6-3.9mm could help if you get any bounce.

[Mark A]

Err. ...you sure about this?
From what I remember about the HW97, it uses all the piston OD, ie over the whole length to guide the piston into the cylinder. ... Piston doesn't have rear bearing like the TX200. Making a step (or two) in the piston OD therefore doesn't sound like a good plan to me. Am I missing something?

BMP01

The rear of the piston is flared out slightly, so that the rear is a slightly larger diameter. The front and middle dont touch the compression chamber, just the skirt.


You really dont want to be going anywhere near the skirt, unless fitting delrin bearings as it'll become loose and you will remove the case hardening. Also you dont want to be touching the inside of the piston as the spring wil then be a loose fit inside the piston which can increase twang.

[Mark A]

Holes or slots are the way forward with the 97/77. But having said that, just the parts you have will work fine with that piston weight. Opening the TP to 3.6-3.9mm could help if you get any bounce.

Any tips on how to go about it without a lathe or milling machine etc?

[buttloaves]

The rear of the piston is flared out slightly, so that the rear is a slightly larger diameter. The front and middle dont touch the compression chamber, just the skirt.


You really dont want to be going anywhere near the skirt, unless fitting delrin bearings as it'll become loose and you will remove the case hardening. Also you dont want to be touching the inside of the piston as the spring wil then be a loose fit inside the piston which can increase twang.

Whats the diameter difference between the skirt and piston body? it must be slight as i didn't notice a flare at the skirt end, it looked parallel to me, very smooth metal though it looked kind of flecked, what is it?

[Rickenbacker]

Clearly some of you know how a 77/97 works, and some don't.

[buttloaves]

Clearly some of you know how a 77/97 works, and some don't.

Tell us then..

[Mark A]

Whats the diameter difference between the skirt and piston body? it must be slight as i didn't notice a flare at the skirt end, it looked parallel to me, very smooth metal though it looked kind of flecked, what is it?

I think I'm getting confused with the hw95 piston. Think Rooti McNote was refering to me .