Hw77 Breech Seal Power Reduction Problem

[Halfinch 51]

I purchased a "well used" Hw77 today, and the guy in the shop says it was shooting at about 10.4 ftlbs, Quite low for a hw77, but it was very cheap, i thought it can be sorted.

When i got it home and put it on the crono, it was actually between 9.7 and 10.3ftlbs, so apart it all came. It had obviously been worked on at some stage because it had a top hat, a nicely machined tight fitting spring guide, and one preload washer, the piston seal looked like an original, as did the spring, but i'm not 100% sure.

 

I de-greased everything, polished all the parts that needed it and put it all back together, back on the crono and it was now reading between 10.1 and 10.6, better, but still low and quite a lot of variation Trying to think where the power could have gone, and why it was so variable, i remembered reading somewhere, to test a breech seal, use a bit of tissue paper over the loading port, well sure enough the tissue was blown in the air, i didn't have a spare seal, so i reversed the one that was in it, did the paper test again and it was fine

I put the crono back on and tested it again, a lot more consistent, but now it was firing 13.1, to 13.3

 

Obviously It's coming apart again, and i'm guessing the spring will need cutting. Is it better to do one coil at a time or should i take say 3 coils off and up the preload if necessary

 

Also Can a breech seal really make that much difference?

[andyfr1968 772]

A breach seal can make a hell of a difference, sounds like your's was on the verge of total failure. Turning them round on HW's often does the trick as they're the same which ever way round you fit them but if it's started to compress other than just a bit of damage, it's best replaced for the cost.

 

Spring wise from what you've said, go for one coil at a time but remember that if the gun's been totally stripped and re-lubed etc, it will take it a little time before it settles itself down to a reliable reading.

 

Hope this helps.

[Halfinch 51]

Thanks Andy, I will get a new breech seal, i only turned it round as a temporary measure tonight, to see if it made a difference. I didn't expect THAT much difference.

I'll get and fit a new seal tomorrow, I'll throw a few hundred pellets through it, then start on the spring if it doesn't come down.

[moxy 617]

I would use a blowtorch and a pair of long nose pliers and heat and compress the first coil of the spring. As Andy says it may take time to bed in and settle.

You can't put back what you have cut off but you can re stretch and temper the spring if you need to.

[moxy 617]

Thanks Andy, I will get a new breech seal, i only turned it round as a temporary measure tonight, to see if it made a difference. I didn't expect THAT much difference.

I'll get and fit a new seal tomorrow, I'll throw a few hundred pellets through it, then start on the spring if it doesn't come down.

I would put a few brands of lead through it. Even them felt cleaning fellas. It will give a variance in pressure tolerances within and help settle things down.

[andyfr1968 772]

I would use a blowtorch and a pair of long nose pliers and heat and compress the first coil of the spring. As Andy says it may take time to bed in and settle.

You can't put back what you have cut off but you can re stretch and temper the spring if you need to.

 

I'd tend to agree with you here, Moxy, about heating and re-tempering the spring but most folks don't have the skill or kit to do this and in my experience with most of the springs that seem to be around these days they tend to break as often as not after this has been done. Maybe I've just been unlucky or my metal work skills are a bit crap!! LOL

 

Cutting a spring is a bit gash, I'll be the first to admit this but so long as it's finished off properly and preferably seated on a stainless steel washer it should be OK.

[barrywhite 282]

I can not see how you can harden and temper a large coil spring with out an oven .i used to make springs [not coil springs] and harden and temper pins in a forge by hand .I would not like to heat a finished spring I would cut the spring and put the cut end into a top hat.I would like to see a lot of these heated collapsed springs after time .I am not saying it is not possible but I connot see how it is not a bit hit and miss .I would try to get the correct spring and polish the ends for a top job .but cutting or heating the spring yourself you will get to know a bit more one way or the other.