Piston scuff marks

[rg627]

Hi

My hw99 has developed some scuff marks on the metal piston. I assume it's from metal to metal contact, is this anything to be wary of?

I stripped it down to degrease as it was dieseling a bit, i then saw the marks and raised my concerns.

Is this common in hw break barrels?

Thanks

[tinbum]

It happens all the time and comes from the cocking stroke, not firing. Put the piston into a cordless drill by the latchrod and spin it up gently with some medium wet and dry til the witness marks have gone, then some fine, then fine with oil. Finish with Autosol or similar, relube carefully, rebuild and ignore for next 5,000 shots.

[Hsing-ee]

I found that the REAR of the compression tube (not the bit that is actually involved in the chamber) is often rough as a badger's arse that has been roughened with badger-paper of the coarsest grade. You can smooth this down with a thick dowel and some fine emery paper, or with a cylinder hone if you are brave and don't get the hones involved with the cocking slot.

Then follow Mr Bum's advice.

[rg627]

Ok thanks

So if I polish the piston and hone the rear of the cylinder it should prevent it.

Funny you say the cooking action, it does sound a little gritty if that makes sense.

Am I right in assuming that as long as the piston seal and bore are a good fit, the rear doesn't matter if a tad of metal has been removed/ honed?

I am going to use bisley gun grease, is this still a good option for rebuild? I'll do a smidge on the piston seal and a smidge in the rear of the piston. None on the spring. Is this still the agreed way to re lube?

[TonyL]

The Bisley Gun Grease will do fine.

A VERY small amount (hardly visible) on the edge of the seal - none in front.

I'd use a fairly generous amount on the rear inch or so of the piston, rubbed in. Then, with the piston pushed fully forwards, before you fit the spring, apply some more of the grease to the inside of the cylinder, around the circumference, behind the piston, using a stiff plastic straw or similar.

Depending on guide fit, I usually DO grease the spring. A moderate amount "worked in" around the whole of the inside and outside and also the spring ends.

[junglie]

I've found maxigrip fitted bsa'a a bugger for this, careful de-burring of the fixing slots & the cocking slot stops it happening again.

[robs5230]

Ok thanks

So if I polish the piston and hone the rear of the cylinder it should prevent it.

Funny you say the cooking action, it does sound a little gritty if that makes sense.

Am I right in assuming that as long as the piston seal and bore are a good fit, the rear doesn't matter if a tad of metal has been removed/ honed?

I am going to use bisley gun grease, is this still a good option for rebuild? I'll do a smidge on the piston seal and a smidge in the rear of the piston. None on the spring. Is this still the agreed way to re lube?

Cant see any point in lubing the seal unless its been sized. The smooth sides of the standard unsized seal will not retain lube and you'll end up with dieselling to some extent.
Some lube will be needed on the guide and top hat, the rest of the spring can remain dry

[rg627]

Ok cheers

Interesting point about the no grease on piston seal, I have not heard that before but may try.

I shall have a look at the cylinder to see if there's corresponding marks.

Ideally I don't want any dieseling at all, I think it originally dieseled hence hence the poor consistency in muzzle velocity.

Seemed to follow a string if consistent shots and then every few through in a much higher or lower number.