Spring Gun tuning - Top Hat advice

[Mr.Fixit-Norm]

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 ( 1”ish ) guide to centre the spring but it’s nowhere near long enough, so yes I’ll 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 wasn’t too bad and I then concentrated on the stock.

Now i’ve had a chance to try it out at a slightly longer range I wanted to try and ‘refine’ it a bit, if that’s possible.

I’ll get started on it next week as the missus has Sunday and Mondays off so that’s us time rather than me time

Thanks for the advice,

norm

[TonyL]

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.

[Mr.Fixit-Norm]

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.

I’ll have to take before and after pics and post them. I really should’ve paid more attention when I took it apart but you know what it’s like sometimes

Looks like I’ve 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 I’m hoping to buy a s/h springer they have for sale, which I’ve taken a fancy too I’m hoping I can buy it and get it shipped to a RFD friend of Unframed Dave’s
I find it annoying that as individuals we can send sub 12ftlb airguns to each other by post quite legally, but an RFD can’t

It’s weird, other than the Original 65 & 75 that I refurbed and eventually moved on, I haven’t really had a hankering for springers, having got several PCPs but it’s taken this lockdown to rekindle an interest

Norm

[Mr.Fixit-Norm]

Right, I’ve 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

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

If I’ve 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 won’t be coil bound.

Also I’m 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 couldn’t 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 doesn’t have any major bends in it but it is slightly out of true.

Norm

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

[Septic]

Nothing wrong with improving a Bam....
It’s actually very satisfying...

[evert]

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

[Mr.Fixit-Norm]

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! I’ve got two different diameters of delrin rod on they’re 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