HW80 Swiss cheese piston

Rhodeboy

Rhodeboy

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An HW80 I bought was struggling to make power and suspected the seals were old. The breach seal was still slightly raised but looked old and which fell apart when being extracted. Suspected the piston seal was also worn out, so pulled it apart to give it a service. What I saw was a piston that looked like Swiss Cheese. Was it done by a guru of the airgun tuning world?
old HW80 piston.webp

I have put a new piston into the HW80 which it has responded to very well and just love the strong recoil and keeping an eye on the chrono as the seal beds in. I am planning to experiment with the Swiss Cheese Piston as it looks like some time has been spent on it and to see how it changes the characteristic. Will it make the mighty HW80 any better I wonder? Any comments, let me know.
 
Though the machining looks well done (and the 80 has a real lump of a piston, that would benefit from shedding a few grams) the wear on the piston rear skirt suggests a less than good tuning solution, overall, likely caused by excessive preload and a badly supported mainspring

No reason, with a good tune, the piston couldn't be used successfully

Its a bit unclear from the pic, but it looks like the OD of the piston body has been reduced. The rear bearing area should still be the original OD and wider than the rest of the piston body. If the rear bearing area has been reduced, it will really need a delrin band fitting to support it properly in the cylinder
 
Riggs said:
How much does the piston weigh? Have you checked if the transfer port has been messed with?
Click to expand...
The swiss cheese piston is 248g where there are 39 holes at 10mm dia.

I haven't checked diameter of the transfer port yet, but all is initially looking good at the moment across the different pellet weights. I have been shooting the PCP for quite a bit, so it was like coming home to mumma's cooking. Suspect it will need at least 1/2 coil spring chop before it comes out to play.
 
P8X said:
Though the machining looks well done (and the 80 has a real lump of a piston, that would benefit from shedding a few grams) the wear on the piston rear skirt suggests a less than good tuning solution, overall, likely caused by excessive preload and a badly supported mainspring

No reason, with a good tune, the piston couldn't be used successfully

Its a bit unclear from the pic, but it looks like the OD of the piston body has been reduced. The rear bearing area should still be the original OD and wider than the rest of the piston body. If the rear bearing area has been reduced, it will really need a delrin band fitting to support it properly in the cylinder
Click to expand...
I was wondering if the rear skirt had been sanded down (getting rid of the burrs in the drill holes) as the scrape marks are on the opposite side to the cocking grove. There wasn't excessive preload and it is still slightly wider than the rest of the piston body, so put this down to poor lube. Will take a measurement of this for you when I get my verniers out of the garage.

The rear skirt is 0.9cm from the cocking grove and the length of the piston to the edge of the seal (backside of seal) is 13.6cm which hopefully the piston length has not been tempered with.

The spring support is the original metal version with the splayed feet and a washer to better support the spring.
Thanks!
 
The piston sleeve has gone missing.
Make one of steel to bridge cocking slot with an inner PTFE sleeve, suitable for a reduce diameter spring.
The original 23 mm OD spring with thick wire is unnecessary for a sub 12 HW 80. but a heavy piston can seem sluggish with a softer spring.
Lightening the piston goes some way to resolve this issue.
The 125 mm depth inside piston skirt ,leaves ample space for a compressed spring, so length of spring/ number of coils will rarely be an issue.
Consider a Dagda spring, supplied by forum member @WelshWilly ,coming with plenty of length to cut and fettle for a balanced solution.
Will require a guide to suit internal ID of less than 14 mm.
 
Rhodeboy said:
I was wondering if the rear skirt had been sanded down (getting rid of the burrs in the drill holes) as the scrape marks are on the opposite side to the cocking grove. There wasn't excessive preload and it is still slightly wider than the rest of the piston body, so put this down to poor lube. Will take a measurement of this for you when I get my verniers out of the garage.

The rear skirt is 0.9cm from the cocking grove and the length of the piston to the edge of the seal (backside of seal) is 13.6cm which hopefully the piston length has not been tempered with.

The spring support is the original metal version with the splayed feet and a washer to better support the spring.
Thanks!
Click to expand...
Good news if the rear bearing is still its intended OD.

Bin the 4 legged guide and washer and get a delrin guide, top hat and washers made to suit whatever spring you use
 
P8X said:
Its a bit unclear from the pic, but it looks like the OD of the piston body has been reduced. The rear bearing area should still be the original OD and wider than the rest of the piston body.
Click to expand...
I can't compare the new piston dimensions as it is currently in the gun, but there is approximately 0.03" height difference between the main body (1.140") area and the rear bearing (1.170"). Is this in the ball park of the OD?
 
shergar said:
The piston sleeve has gone missing.
Make one of steel to bridge cocking slot with an inner PTFE sleeve, suitable for a reduce diameter spring.
The original 23 mm OD spring with thick wire is unnecessary for a sub 12 HW 80. but a heavy piston can seem sluggish with a softer spring.
Lightening the piston goes some way to resolve this issue.
The 125 mm depth inside piston skirt ,leaves ample space for a compressed spring, so length of spring/ number of coils will rarely be an issue.
Consider a Dagda spring, supplied by forum member @WelshWilly ,coming with plenty of length to cut and fettle for a balanced solution.
Will require a guide to suit internal ID of less than 14 mm.
Click to expand...
I have removed the sleeve where it was just a ptfe sleeve and have made a metal one for it. The OD spring was used and is approx 23.5cm long. Have a Titan#2 spring in the wings if it is needed.
Measured the internal piston length of 5.1" (127mm) which was a little difficult to do, but it looks like the piston length hasn't been reduced.(y)
 
P8X said:
Bin the 4 legged guide and washer and get a delrin guide, top hat and washers made to suit whatever spring you use
Click to expand...
Will this modification just quiet down the action or is there a performance advantage with a delrin guide? It might be worth doing this even with the new piston.
 
A well fitted steel guide will be very similar to a delrin one, I had one in a Mk1 HW80, I thought it was a delrin one until i cracked it open - I’ve also had one with electrical tape wound around it, that was surprising smooth and quite too
 
Rhodeboy said:
Will this modification just quiet down the action or is there a performance advantage with a delrin guide? It might be worth doing this even with the new piston.
Click to expand...
Steel needs to be an extremely good fit to be anywhere near as quiet as a well fitting delrin one
The old 4 leg guides aren't the best for allowing a slip washer to do its job. On top of that, they break
 
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