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Thread: Spring Gun tuning - Top Hat advice

  1. #1
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    Spring Gun tuning - Top Hat advice

    Hi Folks,
    Iíve been tweaking and sorting an old BAM DB4 over the last few weeks ( some may have seen my crazy snakeskin sprayed stock )
    I got a chance to try it at a longer range than in the garden last week, and itís reasonably accurate at 25yrds. ( Open sights ) However, although I originally stripped the spring, piston and compression chamber, cleaned and greased it all up, it still Ďtwangsí quite a bit

    Itís running at an average of 8.7ftlbs with an average spread of 10fps, which Iím happy with ( itís not a hunting gun ). I was reading a bit about top hats, spring guides and sleeves and having a lathe and some suitable Delrin rod I was wondering about
    making a top hat for it and whether it would help with the twang ?
    I havenít done any measurements yet but i seem to remember when i took it apart there wasnít much room between the spring and the piston so Iím not sure if thereíd be room for a liner.

    The questions I have are, if I do make a top hat, how thick does the end have to be, I assume were it goes into the spring it has to be a reasonably tight fit so the spring rotates on the delrin, also if itís a reasonable thin end do I have to remove anything off the spring or live with a bit more pre-load? There isnít a large amount of preload anyway.

    i know itís only a cheap chinese job but from what Iíve read on here and the UKCAF forum they can be turned into a reasonable plinker so any advice would be very gratefully received.

    Norm
    Now retired I planned to concentrate on my shooting & other hobbies, but my better half seems to have other ideas - she's on page 4 of the list of things to do www.thatsmypicture.co.uk or www.thatsmypicture.ddns.net

  2. #2
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    A spring guide snug in the spring reduces most of the twang.
    The top hat helps with the twist of the spring when compressed and uncompressed.
    about 3mm proud of the spring would be usual for a top hat. Doesn't have to be that long about 12- 15 mm overall.
    Make sure that spring will still compress without the top hat touching the guide. This is normally done by assembling the rifle with no spring, cocking the gun and making sure the sear engages correctly.
    Dear Spell checker, that's not what I was trying to say.

    I am getting tired of your shirt.

  3. #3
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    Quote Originally Posted by Neil54 View Post
    A spring guide snug in the spring reduces most of the twang.
    The top hat helps with the twist of the spring when compressed and uncompressed.
    about 3mm proud of the spring would be usual for a top hat. Doesn't have to be that long about 12- 15 mm overall.
    Make sure that spring will still compress without the top hat touching the guide. This is normally done by assembling the rifle with no spring, cocking the gun and making sure the sear engages correctly.
    Thanks for the info Neil, thatíll certainly be useful , sounds like i wonít have to take anything off the spring which will be handy !
    Now retired I planned to concentrate on my shooting & other hobbies, but my better half seems to have other ideas - she's on page 4 of the list of things to do www.thatsmypicture.co.uk or www.thatsmypicture.ddns.net

  4. #4
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    As Neil says, Norm.

    Your post didn't make it clear, but if you haven't home made a rear guide already (and if the standard guide is a sloppy fit) do make a rear guide. Also, try and make the rear guide as long as you can (ie so that the guide and top hat don't touch nod interfere with each other) to give the unsupported section of the spring more, er, support.
    Last edited by TonyL; 31-05-2020 at 09:51 AM. Reason: "Mad" to "and"!
    THE BOINGER BASH AT QUIGLEY HOLLOW. MAKING GREAT MEMORIES SINCE 15th JUNE, 2013.
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  5. #5
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    Quote Originally Posted by TonyL View Post
    As Neil says, Norm.

    Your post didn't make it clear, but if you haven't home made a rear guide already (and if the standard guide is a sloppy fit) do make a rear guide. Also, try mad make the rear guide as long as you can (ie so that the guide and top hat don't touch nod interfere with each other) to give the unsupported section of the spring more, er, support.
    Hi Tony,

    Blimey, I thought I stayed up late !

    The rear metal block that holds the spring in does have a short ( 1Ēish ) guide to centre the spring but itís nowhere near long enough, so yes Iíll try and extend that somehow. To be honest initially I just wanted to get it cleaned and lubed so that i could put it together and check out the accuracy, unfortunately lock down put a bit of a damper on checking it at anything other than 15yrds in the garden.
    It was enough to convince me that it wasnít too bad and I then concentrated on the stock.

    Now iíve had a chance to try it out at a slightly longer range I wanted to try and Ďrefineí it a bit, if thatís possible.

    Iíll get started on it next week as the missus has Sunday and Mondays off so thatís us time rather than me time

    Thanks for the advice,

    norm
    Now retired I planned to concentrate on my shooting & other hobbies, but my better half seems to have other ideas - she's on page 4 of the list of things to do www.thatsmypicture.co.uk or www.thatsmypicture.ddns.net

  6. #6
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    Hi Norm, having a longer rear guide that fits the spring properly is the main part of the equation to reduce the twang. Be interesting to know how that existing "guide" is fixed in. Don't know if you can post pictures? Hopefully you'll be able to remove and substitute it for a custom, properly fitting guide. Or maybe use the existing "stump" as a base for the new guide?

    Hope this helps and it'll be nice to hear how it turns out.
    THE BOINGER BASH AT QUIGLEY HOLLOW. MAKING GREAT MEMORIES SINCE 15th JUNE, 2013.
    NEXT EVENT :- Summer / Autumn 2020,. BOING!!

  7. #7
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    Quote Originally Posted by TonyL View Post
    Hi Norm, having a longer rear guide that fits the spring properly is the main part of the equation to reduce the twang. Be interesting to know how that existing "guide" is fixed in. Don't know if you can post pictures? Hopefully you'll be able to remove and substitute it for a custom, properly fitting guide. Or maybe use the existing "stump" as a base for the new guide?

    Hope this helps and it'll be nice to hear how it turns out.
    Iíll have to take before and after pics and post them. I really shouldíve paid more attention when I took it apart but you know what itís like sometimes

    Looks like Iíve got a bit of work to do, mind you it should be worth it

    Also, once the gun shop on Guntrader that I was looking at reopens Iím hoping to buy a s/h springer they have for sale, which Iíve taken a fancy too Iím hoping I can buy it and get it shipped to a RFD friend of Unframed Daveís
    I find it annoying that as individuals we can send sub 12ftlb airguns to each other by post quite legally, but an RFD canít

    Itís weird, other than the Original 65 & 75 that I refurbed and eventually moved on, I havenít really had a hankering for springers, having got several PCPs but itís taken this lockdown to rekindle an interest

    Norm
    Now retired I planned to concentrate on my shooting & other hobbies, but my better half seems to have other ideas - she's on page 4 of the list of things to do www.thatsmypicture.co.uk or www.thatsmypicture.ddns.net

  8. #8
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    Right, Iíve made a start this afternoon.

    I stripped out the spring and have taken some measurements, which hopefully will allow me to make a rear spring guide and get it right first time !

    Perhaps someone could double check my calculations for me

    The spring measurements are

    Length uncompressed = 256mm
    OD = 18.1mm - as near as I could measure it with a digital calliper
    ID = 12.1mm, again as near as I can measure it.
    The spring thickness is 3.0mm
    Number of coils = 37

    I had an idea to measure how much the compressed length of the spring is by cocking the gun then pushing the spring back in and measuring the amount of spring sticking out of the end of the action.

    The amount it was sticking out was 94mm

    The Delrin plug/spring guide has a 49mm guide and the plug is 31mm long
    Therefor when the gun is assembled as is, By my calculation the spring compresses down to 131mm ie

    256mm - 94mm ( the bit sticking out with it cocked ) = 162mm - 31mm ( plug length ) = 131mm compressed length

    Iíve made a top hat out of Delrin, 3mm long and 14mm overall and 0.3mm bigger than the dia. of the spring. Iíve squared and polished the ends of the spring to the best of my abilities as well

    If Iíve got my calculations right, the springs coils take up 111mm add to that the new top hat = 114mm. Take that away from the compressed length of 131mm would equal 17mm of overall gap, which should mean with a thin slip washer at each end of the spring it wonít be coil bound.

    Also Iím assuming I can make a spring guide of no more than 120mm, although 100-110mm would probably be safer.

    Sorry for all the figures but I couldnít think of any other way to show the working out !

    Three quick questions,
    What can the slip washer for the top hat end be made of and does it have to be a tight fit in the piston
    I can order some 12mm Delrin for the guide, would that be the right size to get or should I go slightly smaller ? The spring doesnít have any major bends in it but it is slightly out of true.

    Norm

    Spring, existing plug/guide and my homemade top hat.



    Now retired I planned to concentrate on my shooting & other hobbies, but my better half seems to have other ideas - she's on page 4 of the list of things to do www.thatsmypicture.co.uk or www.thatsmypicture.ddns.net

  9. #9
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    Cool

    Nothing wrong with improving a Bam....
    Itís actually very satisfying...
    Where be your gibes now? Your songs? Your gambols? Miserable bugger!

  10. #10
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    You really need a longer rear guide! Short guides are one of the main reasons springs get kinked and worn.

  11. #11
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    Quote Originally Posted by evert View Post
    You really need a longer rear guide! Short guides are one of the main reasons springs get kinked and worn.
    Yes, especially on such a long thin spring! Iíve got two different diameters of delrin rod on theyíre way from the bay which should arrive next week

    If it works out right the one I make will be just over twice the length of the existing one.

    Norm
    Now retired I planned to concentrate on my shooting & other hobbies, but my better half seems to have other ideas - she's on page 4 of the list of things to do www.thatsmypicture.co.uk or www.thatsmypicture.ddns.net

  12. #12
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    If you whip the piston out, Norm, and just make that rear guide to a length so that the guide and top hat don't clash once any spacers / washers etc fitted. Re top hat bearing, maybe a nice, polished steel one that's not a tight fit in the piston, allowing easy removal if required.
    THE BOINGER BASH AT QUIGLEY HOLLOW. MAKING GREAT MEMORIES SINCE 15th JUNE, 2013.
    NEXT EVENT :- Summer / Autumn 2020,. BOING!!

  13. #13
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    P.S. Norm, most people would go for a guide that is a nice, firmish, sliding fit to reduce twang. Not so tight that you have to hammer it in, though. The diameter you mention may be a little loose? But, if you don't mind going "old school", moly grease will help dampen the twang. Plus that extra length will help.

    I'm guessing you may have to go for the original type of guide fitting, ie into the end block, as a guide with a flange at the rear may not allow the piston to come backwards by the required mount to allow cocking? You'll need to have a little play to see if this may be the case. If a design with a flange would work, you'd need a wider diameter delrin bar to start with, larger diameter to suit the cylinder ID and the smaller diameter to suit the spring ID. Does the piston have a latch rod or does the trigger sear engage with a cut-out in the piston body?
    THE BOINGER BASH AT QUIGLEY HOLLOW. MAKING GREAT MEMORIES SINCE 15th JUNE, 2013.
    NEXT EVENT :- Summer / Autumn 2020,. BOING!!

  14. #14
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    Quote Originally Posted by TonyL View Post
    P.S. Norm, most people would go for a guide that is a nice, firmish, sliding fit to reduce twang. Not so tight that you have to hammer it in, though. The diameter you mention may be a little loose? But, if you don't mind going "old school", moly grease will help dampen the twang. Plus that extra length will help.

    I'm guessing you may have to go for the original type of guide fitting, ie into the end block, as a guide with a flange at the rear may not allow the piston to come backwards by the required mount to allow cocking? You'll need to have a little play to see if this may be the case. If a design with a flange would work, you'd need a wider diameter delrin bar to start with, larger diameter to suit the cylinder ID and the smaller diameter to suit the spring ID. Does the piston have a latch rod or does the trigger sear engage with a cut-out in the piston body?
    Hi Tony,
    Yes I was going to try both sizes I’ve ordered ( 10mm & 12mm ) and see which works best in terms of slideability

    When I said a flange, I got that slightly wrong, I meant I was going to cut the old guide off flush with the end of the plug then if say the 12mm rod is suitable I’ll drill an 10mm hole in the plug and machine the end of the rod to slightly bigger than that so it’s a tight push fit in the plug

    I’m assuming it’s best to still use moly grease on the spring to make it slide over the guide even easier.

    The trigger sear engages with the piston body directly.

    Just got to wait for the rod to arrive, should be here anytime after Friday this week.

    Norm
    Now retired I planned to concentrate on my shooting & other hobbies, but my better half seems to have other ideas - she's on page 4 of the list of things to do www.thatsmypicture.co.uk or www.thatsmypicture.ddns.net

  15. #15
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    With the spring ID being 12.1mm, Norm, use the 12mm rod; the 10mm one will give too much clearance.

    And, yes, use the moly grease. A little on the guide and a little smeared over all surfaces of the spring - ends, ID nod OD.
    THE BOINGER BASH AT QUIGLEY HOLLOW. MAKING GREAT MEMORIES SINCE 15th JUNE, 2013.
    NEXT EVENT :- Summer / Autumn 2020,. BOING!!

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