Picked up first Gen. Diana 34 ** TUNED & NEW CHRONO NUMBERS **

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Also, inside the piston is a plastic sleeve. I'm assuming that's a spring guide within the piston and may also dampen spring twang/vibration?? Is that removable or not ?
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That part I honestly have no idea. I have an older D34 .177, but mine is a 2002 version and I've never had it apart. Maybe try shooting Motorhead a PM and see what he says??
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I'm pretty sure it's a guide/damper of some kind. The Dremel worked sweet on all the edges and other sharp spots. I don't think I even have to worry about the piston seal during assembly. I still need to give some attention to the runs of the cocking slot. The large keyhole opening where the cocking arm foot enters has been dealt with. Next I'll deal with the spring guide. The muzzle end of it is also a 90 deg sharp edge which I don't like. I'll bevel that slightly and then polish it up.
 
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You might want to look into the bottom of the compression tube and make sure there isn't a blob of solder or weld or something in there. If that's a flat spot and not a gouge in the piston seal, I'd want to know what caused it before I put it back together.
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I'll examine that spot on the bottom of the tube that corresponds to where that flat spot was. Don't know what to do if there is an issue though 😕 
 
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I checked that area of the tube and don't notice anything out of the ordinary. I got the Vortek seal and it's way to tight. Even after repeated sanding it would probably take a solid 40 lbs of force to get it to slide into the bore. Even with Moly on it. I'm going to get a stock RWS seal.
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Can you measure to know the difference? Mike the old seal and compare it to the new? It's hard to measure the bore without telescoping gages or something. 

If the seal goes in you can't be too far off. Back the sandpaper with something flat👍
 
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I checked that area of the tube and don't notice anything out of the ordinary. I got the Vortek seal and it's way to tight. Even after repeated sanding it would probably take a solid 40 lbs of force to get it to slide into the bore. Even with Moly on it. I'm going to get a stock RWS seal.
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Can you measure to know the difference? Mike the old seal and compare it to the new? It's hard to measure the bore without telescoping gages or something. 

If the seal goes in you can't be too far off. Back the sandpaper with something flat
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I reshaped the old seal and got that flat spot out. Just for the heck of it I lubed everything and reinstalled the seal. Only got 818 fps versus 839 before breaking rifle down. I think I used to much and the seal isn't right. Umarex is sending me a new piston seal and breech seal free of charge. I did completely disassemble and put back together the trigger block. It was by accident though. My short retainer pin fell out and some guts on the trigger went caddywampus. I ended up having to take it apart to get it back together correctly. Just had to study the components and figure what they all do.
 
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Well, finally got the old girl tuned (my first one !) and I'm pleased. Power is going up after 30-40 shots. As I mentioned in another thread I tried a Vortek piston seal and they're way to tight. Went with the RWS seal and glad I did. The gun likes heavier pellets as far as velocity efficiency goes. I get nearly the same velocity with 8.6 gr (4.52mm) JSB's as I did the same pellet weighed at 8.3 gr. It's definitely shooting a little stronger than it was before the tune.

Sig Venom 7.87 gr (weighed @ 7.9 gr) 880 fps

JSB 8.44 gr w/4.52mm head (weighed @ 8.3 gr) 843 fps

JSB 8.44 gr w/4.52mm head (weighed @ 8.6 gr) 840 fps
 
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