AA Mistral O ring problem

[EdSIII]

I have an AA Mistral in .22 I picked up that was shooting about 7ftlb, so I assumed a new seal and mainspring should bring it back to @ 11ftlb. I stripped it and found it full of thick brown grease, but the mainspring wasn't compressed or canted and was actually about 1/4" longer than the replacement mainspring I bought from Chambers. I had bought a replacement piston seal O ring from Knibbs as I was ordering some other parts from them. Anyway, when I got the seal it sat loose on the piston, like the inside diam was to large. I decided to use the old seal which didn't look to bad, but after reassembling the gun it only shot 8.2 ftlb. So I restriped and put the Knibbs O ring on it thinking it may still work better than the apparently worn old seal, but now its shooting about 7.8ftlb. I am thinking of ordering an O ring from Chambers hoping its a better fit than the Knibbs o ring, and I can get it at least up to 10ftlb. One thing during the disassembly I noted is that in order to get the piston out I needed to use a screwdriver and a rubber hammer and pound out the piston by the cocking ratchet teeth, it seems the cylinder at the rear end is tight where the rachet teeth are when the piston is in the correct orientation. I rotated the piston and the piston goes right in upside down so it must be something with the teeth. I guess that could be impacting the piston movement, but it seems that's further back than the cocking movement area of the piston in the gun, pretty frustrated I cant get any decent power out of it, hopefully a better O ring will help, anybody had any experience with the AA sidelevers and a similar problem?.

[piggy589]

How about a dry build without seals, to check piston to chamber clearance?.

A smaller O ring beneath the new replacement perhaps?, to take up the clearance in the piston rebate.

[Shed tuner]

yes, good idea on checking clearances above. The piston should not be anywhere near that tight. The Cardew's experiments yielded around 1kg force to move the piston as optimum. Yours sounds more like 20kg, which will kill the power.

[EdSIII]

Yes, I did try fitting the piston without the seal and it would stick at the rear end of the piston just going into the cylinder, I assumed it was primarily because of the cutouts in the cylinder at that end and demurred it a bit but still was tight. I am going to disassemble and look a little closer at the cylinder and piston and see what's going on. I have done tunes on a lot of Airguns and never found one this tight.

[joffy]

i had a similar problem with my park rh93 (that and a front spring that was binding) i had to slightly alter the rear piston by grinding a high spot done a little bit and honing out a tight section in the cylinder ....now it cocks easily and shoots sweetly (ground the front spring down by a few mm and polished the ends)

[EdSIII]

Jiffy, that's looking like what I am going to have to do. I have used a straight edge along the piston and there seems to be a high spot along the side where the sear and ratchet teeth are. Hard to believe it left the factory like that but I guess so.

[Shed tuner]

easy fix though...

[EdSIII]

Well it looks like I found the reason for the tight piston, the point where the cocking arm makes contact with the piston had flared a bit from use. I filed it down with a small diamond file and now the piston moves freely in the cylinder! I will reassemble tommorow with hopes of some normal velocity now that I have that sorted out.

[joffy]

Well it looks like I found the reason for the tight piston, the point where the cocking arm makes contact with the piston had flared a bit from use. I filed it down with a small diamond file and now the piston moves freely in the cylinder! I will reassemble tommorow with hopes of some normal velocity now that I have that sorted out.

It doesn’t take much to gum up the works ....hope it works now

[Shed tuner]

yeah, a tight o-ring can easily cost a few foot-pounds, so a tight piston body could easily do the same.

[EdSIII]

Just an update on my Mistral seal problem. My original replacement seal came from Knibbs and was definitely oversized for the piston on the internal diameter, so I decided to try one from Chambers. Well the Chambers seal fit much better and after installing it the gun is now shooting at close to 10ftlb with JSB RS pellets. The gun is very accurate even though the trigger pull is pretty heavy. Anyone know how you set the trigger pull weight? Is it the tiny spring in the receiver block sort of like the webley tracker, vulcan etc adjustment? If so that may be my problem as that spring is pretty squished.

[Mr. Gen]

Trigger weight is the spring you have located. They are often damaged over time. Also check sear face and piston (where sear locates) for scoring as this is often a reason for heavy trigger pull...