BSA mainseal o ring to cylinder ,how tight fit

[original45]

Thank you all for sharing your knowledge.
I really wasn't aware the o ring needid room in the slot.
So will address this
I'm not using the original plastic piston head as it was split .

It was cheaper and seemed a better idea to use the t r Robb stainless head as you get o rings ,buffers and it tightens up by nut on thread .

Head has clearance of cylinder but from info above it could be as I noted o ring sits pretty tight in its groove.

I will try and obtain a slimmer o ring .
The scorpion spring seems to look same as a airsporter supersport spring but shorter .

Probably had to compress 2 coils to slip pin through so was pree loaded on 're fitting .

Will have another bash when I get rings.

[Shed tuner]

Probably had to compress 2 coils to slip pin through so was pree loaded on 're fitting .

Sounds about right, but mines doing 5.5 FP with a crappy old spring at that preload (.22) then again I did reline the barrel which added nearly 1 FP as the pellet actually sealed, rather than rattling an inch down

[original45]

Sounds about right, but mines doing 5.5 FP with a crappy old spring at that preload (.22) then again I did reline the barrel which added nearly 1 FP as the pellet actually sealed, rather than rattling an inch down

I must add I noticed pellets are loose in barrel,and the transfer seal needid some removing off the back as the barrel wouldn't lock up right .
Transfer seal is 100%.
Shoots beautiful just needs this little issue sorting and will be great once consistent .
Hopefully.

[Papa g]

BSA scorpion pistol cylinder is also 28mm diameter Mr G; they are monsters

Blimey I never realised they were that big! I always thought they were the same diameter as the meteor with same diameter spring only shorter, must agree Jon they are beasts!
Regards Graham

[original45]

Had it apart last night and what I found was since fitting the new o ring and it slid into cylinder nicely that now I struggled to get piston out.
On inspecting o ring it appears it has enlarged somehow.

It was sloppy in piston head grouve and bulged out on one side.

Very confusing ,so I went through several others but it seems the 3.53 is the daddy .
Now for the question why did said o ring bulge out. I'm using same size from same pack same fit again .

Now don't call me a twit but could it be the peloil I used to lube o ring or Molly grease in cylinder ?.
.totaly cleaned off peloil .
Gonna do some testing latter but I'm confidant it will now be ok and guessing it was the pelgunoil oil .

Didn't have this problem with my airsporter I did same ring same Molly just no pelgunoil .

Don't like these o rings , it appears the rws 28 mm seal will fit nicely but a lack of suitable adapters has put this on hold for now .

Fingers crossed.

[Shed tuner]

hmm, that's weird.

Is the piston head itself quite a loose fit in the cylinder bore (with no o-ring) ? i.e. is there too much clearance / insufficient support for the o-ring ?

[original45]

No john it's very close.No rattle I'd have guessed if it wasn't bought on line ide have said he had measured cylinder and was very fine tolerances.
To use a next size down o ring with same 3,53 size use have to use ptfe tape under o ring as it just drops down under own weight.
O ring size is 20,22 x3,53 .n b r 70°shore A.
O ring had excellent support if I place edge of o ring into slot it needs a nudge to pass and go in.

[oliver13]

It's probably down to the material the O ring is made of, if it's the wrong compound it swells - & they all look the same...
Personally I prefer ones typically based on a fluorocarbon rubber like Viton, which aren't hard to get hold of, all the ones on your car are probably something similar in the fuel & oil systems. You're best getting one from a reliable Factor who species the material used, & they're still only pence each.

http://www.applerubber.com/chemical-...ibility-guide/

[original45]

It's probably down to the material the O ring is made of, if it's the wrong compound it swells - & they all look the same...
Personally I prefer ones typically based on a fluorocarbon rubber like Viton, which aren't hard to get hold of, all the ones on your car are probably something similar in the fuel & oil systems. You're best getting one from a reliable Factor who species the material used, & they're still only pence each.

http://www.applerubber.com/chemical-...ibility-guide/

Yes I believe the pelgun oil was the cause that somehow it kind of swelled the o ring at least it seemed a to be bigger in dia as it was bulged out of slot on one side and w as so tight in cylinder.

I will need to get some of these o rings as you mention once I'm clear on size.
At present the 3,53 seems perfect but will only know over crono latter on or tomorrow.

[NickG]

No john it's very close.No rattle I'd have guessed if it wasn't bought on line ide have said he had measured cylinder and was very fine tolerances.
To use a next size down o ring with same 3,53 size use have to use ptfe tape under o ring as it just drops down under own weight.
O ring size is 20,22 x3,53 .n b r 70°shore A.
O ring had excellent support if I place edge of o ring into slot it needs a nudge to pass and go in.

Too tight in the width , the o ring needs room to do its job yours is crammed into the groove and wearing rapidly, shore 70 is fine with most lubes , you only need Viton when dealing with something corrosive , such as petrol or brake fluid widen the groove a little you want a bit of clearance, abot .007" in old money, about the same as the crush on diameter.

[Shed tuner]

I'd go with Nick's advice

Also Nick, another Q for you here:
http://www.airgunbbs.com/showthread....=1#post7088889

[original45]

Too tight in the width , the o ring needs room to do its job yours is crammed into the groove and wearing rapidly, shore 70 is fine with most lubes , you only need Viton when dealing with something corrosive , such as petrol or brake fluid widen the groove a little you want a bit of clearance, abot .007" in old money, about the same as the crush on diameter.

Yes it's too tight I'm sure would be better with some room for slight movement. I won't be buying another stainless head that's for sure.
If it shoots consistant for now I'm going to leave it as IDE rather use a rws 28 mm seal but was struggling to get a adapter made up.
I can just change the o ring head out as I can see I'm not going to get on with o rings and get the stainless head grouve widened for future use .

[Andy001]

Yes it's too tight I'm sure would be better with some room for slight movement. I won't be buying another stainless head that's for sure.
If it shoots consistant for now I'm going to leave it as IDE rather use a rws 28 mm seal but was struggling to get a adapter made up.
I can just change the o ring head out as I can see I'm not going to get on with o rings and get the stainless head grouve widened for future use .

I'm having similar problems to you, this thread has opened my eyes a little. It's raised a few questions about mine and whether the answers are correct or do my specs need changing.

Tw chambers do the alloy head for the scorpion, but it isn't a nut type. It still uses the Circlip!

One other thing I'm thinking of is the weihrauch type affair. Hw35 seal with the adaptor modified to fit the 28mm bore size.

It's 8 dollars in America from vortex, struggled to find a uk dealer!! or 12 from knibbs, and a seal. I'm looking at getting one myself, to see what I can do with it......

I'm sure I read on here you can size the hw seals down. So, if the adaptor is made to fit. Bolt it to the end of the piston, and the use a resized hw35 seal to fit the 28mm bore.
This is all if's, as it's literally popped into my head...some one else may have an idea.
Cheers

[original45]

Yes I have little experiance with o rings ,past use of them was limited and went great no problems.
Bought this piston head as plastic type o m part was bust . Seems grouve is too deep, not wide enough ,and o rings sent all drop straight in under own piston weight.

Yes I believe the 28 mm seal is the way to go .personaly love the rws blue seals . IDE guess with lathe would be a simple adapter to make too ,it could with longer bolt and collars to suit different Pistons be pretty simple to sort and fit varied use .

For the scorpion a simple adapter , bolt through with tapered head ,a collar to take up the room between bolt and piston hole As a full sized thread woul fill the hole nicely but would make no room for a nut to secure in place due to room for a spring without fouling it .
,a sized flat washer to bridge area over end of collar and piston base so a small enough nut could be used to secure adapter so as not to foul Spring .

I'm sure it could be designed simpler too.

I understand the cylinders are varied sizes too so one size o ring isn't best for all.

[original45]

Just had it over the crono . Had to check crono against an air rifle I have because I couldn't believe the results.
Under 300 fps 2.9 ft lbs. pretty much no differance at all.

Back apart and will be fitting a used nylon o m part that should arive tomorrow or Wednesday .
I read a post on the net stating when someone else used these stainless heads same as I have fitted it was ill fitting and power was low, as suggested he machined it reduced dia and with different o ring it worked ok.
Not one of the o rings supplied fitted they all just dropped in cylinder under piston weight even with finger over muzzle to trap air .
Has to be piston head as seals need to be over size from o.m yet grouve isn't wide enough to accommodate it .
re checked transfer seal all good.

Just a bit frustrated is a understatement.
Rant over .