HW95 Short-stroke fiddling

[sherlock1963]

I have a .22 HW95 which now has a 10mm polyurethane o-ring piston nose extension. With 10mm of pre-load it now runs at about 11.00 ft/lbs. It shoots well and recoil is less than before.

I am wondering whether it is worth opening the transfer port to 3.3mm? (I happen to have a reamer in that size)

Also, is it reasonably simple to lighten the piston? I have a plan to drill and tap the piston and use nylon grub screws as buttons ......assuming that isn't a stupid idea....

I don't have access to a lathe.

[Shed tuner]

Good work

26MM bore I assume ? What is the stroke / TP length / TP current dia ? Piston weight all up ?
Have you tried it with lighter and medium pellets, see if there is a trend in terms of power favouring one or the other ?

[sherlock1963]

Good work

26MM bore I assume ? What is the stroke / TP length / TP current dia ? Piston weight all up ?
Have you tried it with lighter and medium pellets, see if there is a trend in terms of power favouring one or the other ?

yes 26mm. Stroke now 75mm (I assume factory setting is 85mm) TP is 3.00mm, length is standard (15mm? not measured). I bought it secondhand and I'm pretty sure it hadn't been opened up before I got my grubby mitts on it. Piston is a fairly hefty 250g, I wanted the poly short-stroke nose to keep weight down.

I realise now that I should have weighed and measured everything before asking technical questions. The o-ring piston seal is a bit of a pain to slide into the cylinder as it catches on every cut out and needs to be coaxed past the end of the cocking slot.

15.9 and 14.66 pellets give very similar power

[shauny]

dont drill and tap,that will mean going right through the cylinder wall.
get some 6mm delrin rod, a 6mm drill bit, and a 6mm milling bit.
mark where you want the holes with felt tip,punch,then with the drill
just go deep enough to use the hole to pilot the mill bit,then mill
out the hole so it has a flat bottom, but be careful not to go all way
through.then stick in delrin,use a dremel to cut back to rough size
then sand for a good fit.ive got some delrin rod if you want a bit.
pm me,and ill tell you which glue to use. s.

[sherlock1963]

dont drill and tap,that will mean going right through the cylinder wall.
get some 6mm delrin rod, a 6mm drill bit, and a 6mm milling bit.
mark where you want the holes with felt tip,punch,then with the drill
just go deep enough to use the hole to pilot the mill bit,then mill
out the hole so it has a flat bottom, but be careful not to go all way
through.then stick in delrin,use a dremel to cut back to rough size
then sand for a good fit.ive got some delrin rod if you want a bit.
pm me,and ill tell you which glue to use. s.

Thanks for the advice. I am usually reluctant to do any fiddling that can't be reversed back to original spec (the TP is likely to remain unmolested for a while too)
I have seen a piston lightened by a series of holes drilled through. I assume from your response that you don't think that is a good idea.
The buttoning project is one for the future, I find it a pain in the arse to dismantle/rebuild compared to my AirArms.

[Shed tuner]

yes 26mm. Stroke now 75mm (I assume factory setting is 85mm) TP is 3.00mm, length is standard (15mm? not measured). I bought it secondhand and I'm pretty sure it hadn't been opened up before I got my grubby mitts on it. Piston is a fairly hefty 250g, I wanted the poly short-stroke nose to keep weight down.

I realise now that I should have weighed and measured everything before asking technical questions. The o-ring piston seal is a bit of a pain to slide into the cylinder as it catches on every cut out and needs to be coaxed past the end of the cocking slot.

15.9 and 14.66 pellets give very similar power

OK, cool... so stroke sounds perfect for .22, maybe even a bit generous, but that's fine. Port looks a fraction tight - my 80mm stroked .177 PE has been opened up to 3.2mm, and has a 240g piston, and still bounces a fraction. You have .22, and less stroke, hence the lower bounce.

Short answer is that opening the port up to 3.2 or 3.3 sounds a great idea, BUT you will almost certainly have to reduce the piston weight by around 20-30g to compensate or it may get a bit slammy. I'd reduce the weight first and then weight, before attacking the port. When you do both it will have less recoil, less surge, and probably need barely if any more spring.

You can reduce the middle section of the piston OD to save some weight, or drill some holes - but as slugger says, you don't want the bearings to pass clean through the piston walls ! Another bearing option is TX style - just cut a groove around the front and rear and slip on some delrin / nylon bearings

HTH - JB

[shauny]

Thanks for the advice. I am usually reluctant to do any fiddling that can't be reversed back to original spec (the TP is likely to remain unmolested for a while too)
I have seen a piston lightened by a series of holes drilled through. I assume from your response that you don't think that is a good idea.
The buttoning project is one for the future, I find it a pain in the arse to dismantle/rebuild compared to my AirArms.

drilling right through the piston to lighten is ok,but cant be put back when you have drilled,also a real
pain to de-burr inside the piston after drilling.but drilling right through to button
not a good idea,as buttons may push through.

[sherlock1963]

OK, cool... so stroke sounds perfect for .22, maybe even a bit generous, but that's fine. Port looks a fraction tight - my 80mm stroked .177 PE has been opened up to 3.2mm, and has a 240g piston, and still bounces a fraction. You have .22, and less stroke, hence the lower bounce.

Short answer is that opening the port up to 3.2 or 3.3 sounds a great idea, BUT you will almost certainly have to reduce the piston weight by around 20-30g to compensate or it may get a bit slammy. I'd reduce the weight first and then weight, before attacking the port. When you do both it will have less recoil, less surge, and probably need barely if any more spring.

You can reduce the middle section of the piston OD to save some weight, or drill some holes - but as slugger says, you don't want the bearings to pass clean through the piston walls ! Another bearing option is TX style - just cut a groove around the front and rear and slip on some delrin / nylon bearings

HTH - JB

Thanks Jon. I appreciate all the advice that so many on here give so freely. Good job i didn't drill and tap for buttons before I asked!. I followed advice and resized a LGU piston seal once on my ProSport and buggered it up. Lesson learnt....ask first.

If I can get access to a decent lathe operator then I will pursue the lightening before the TP change. It shoots much better that standard already and maybe needs a couple of tins of pellets through it before I change anything else.

I have got the PS shooting really well at -15mm stroke, The HW is nearly there at -10mm and I have a -3mm stroke on a MK1 TX which I fancy going a bit further with, maybe -10mm as it was at 12.5 ft/lb with no pre-load at all.

Too many things at once for a novice.

Regards Dave

[Shed tuner]

and I have a -3mm stroke on a MK1 TX which I fancy going a bit further with, maybe -10mm as it was at 12.5 ft/lb with no pre-load at all.

-10mm is too much for a mk1, unless you are converting to HW seal, and it's .22, and you are shortening the port, and reducing the piston weight

[maximus]

drilling right through the piston to lighten is ok,but cant be put back when you have drilled,also a real
pain to de-burr inside the piston after drilling.but drilling right through to button
not a good idea,as buttons may push through.

Why not make the buttons have a larger diameter on the outer surface so they cant push through. I think buttons may be better than having a full bearing.

[shauny]

Why not make the buttons have a larger diameter on the outer surface so they cant push through. I think buttons may be better than having a full bearing.

it doesnt matter what size buttons you you use if you drill right through, they still
gonna push through!!thats why you only drill half way through.

[maximus]

it doesnt matter what size buttons you you use if you drill right through, they still
gonna push through!!thats why you only drill half way through.

Not if they have a smaller diameter that locates in the piston body with a larger diameter "head" to form the button.

[sherlock1963]

drilling right through the piston to lighten is ok,but cant be put back when you have drilled,also a real
pain to de-burr inside the piston after drilling.but drilling right through to button
not a good idea,as buttons may push through.

I think that my lack of tools and ability will mean that the piston remains as it is.

[sherlock1963]

-10mm is too much for a mk1, unless you are converting to HW seal, and it's .22, and you are shortening the port, and reducing the piston weight

I intended to use a HW seal. It is currently -3mm with a poly o-ring seal from NickG. With the standard MK1 spring it is 12.5 ft/lb. I figured a further 7mm would bring it down to 10.5 ish . It is a .22.

It seems an incredibly efficient rifle, when I first tested it the combro showed 12.4 ft/lb (the breech seal was split, there was no top-hat and the collapsed coils were on the spring guide ). I am using my old PS/MK3 TX spring with 15mm pre-load to get 11.00.

It's just an experiment really, not much outlay to buy a piston nose. What's the worst that can happen........?

[shauny]

I think that my lack of tools and ability will mean that the piston remains as it is.

well i suppose if you used a stepped drill that would be ok, but still
less hastle to only drill halfway, no de burring to fiddle with inside
the piston.