Ok, here it is, probably the final entry for my B-51 tune up using ‘The Book’.
When last we saw our hero, he was very frustrated in that his shot string had a pretty much straight down-hill slope and there were sporadic (8 shots out of 50) in which the muzzle velocity of the Crosman Premier Heavies (10.5gr) dropped by about 100fps from the general string run of values. Not good said our hero and he sought greater wisdom from the ancients (the Darkside Forum experienced tuner’s).
Our hero received many great and timely comments and suggestions. The wise ancients, DanoInTx NS SpySir bestowed the latest leafs of wisdom on our hero and the result was a shot string that was perfect from our hero’s goal in the B-51 tune up process in the first place.
As you will recall, our hero sought wisdom on the B-51 tune up from the Darkside forum and stumbled upon ‘The Book’ in which much wisdom be many ancients was inscribed. Our hero jumped in, feet first and brain last, and managed to obtain a gun that shot in the mid-600’s fps.
Our hero followed ‘The Book’ in that he dissembled the B-51, dissembled the trigger assembly and hammer spring mechanism and modified or replaced various components. On inspection, our hero found that the cocking bolt mechanism was a bit stiff and gritty feeling. Also, on removing the breech, our hero found that the top transfer port o-ring had about 25% of its ring missing (no bits and pieces laying about, just missing. On inspection of the lower transfer port o-ring, our hero found that this o-ring had a classic failure indication, a concentric split on the inside diameter of the o-ring (a typical failure found in AA S400 breech seal o-rings). Additionally, our hero found that the breech screws were not very tight. Finally the bolt probe o-ring was worn looking.
The first step that our hero took was to clean the barrel and breech since he could run a bore snake all the way through both the barrel and the breech bolt areas at once. Goof-Off on a pad run through 3 or 4 times finally came out clean. Our hero then used a bore mop soaked in bore oil to lube up the gun bore (not the breech).
Our hero then polished the bolt and used white grease from Maccari to put a light coat of lubricant on the bolt. Polishing the bolt was accomplished by lightly sanding with oiled 600-grit wet-dry sand paper followed by oiled 1000-grti sandpaper.
Then came the hammer and hammer striker. Our hero chocked the hammer into his wood lathe and used the same 600-grit and 1000-grit oiled sandpaper to polish the hammer mechanism, paying particular attention to the grooved area of the hammer where the trigger sear must engage and smoothly disengage. Also, our hero polished the spring guide and the inside of the hammer so that the spring guide would turn smoothly in the hammer when the gun was cocked and fired (the spring wants to rotate). Since our hero was replacing the stock hammer spring with a less stiff McMaster Carr spring (ala ‘The Book’) he turned in the hammer striker inward screw so about a millimeter of the striker screw would be exposed. He did this knowing the hammer spring change would give velocities in the 700’s rather than his desired roughly 900 fps. Increase the travel, increase the momentum and hence the strike force on the discharge valve stem and a higher resulting velocity.
Our hero then committed a big mistake; he used a thin coat of white grease lube on the hammer and hammer chamber.
Finally, our hero used a diamond stone and 1000-grit oiled-sandpaper to polish the trigger surfaces that would come into rubbing contact with other trigger or hammer components. Then he used that wonderful white grease to lube all trigger rub and pivot points.
Upon reassembly, our hero tried a fill to 2000# and fired a shot off, the pellet did not even come out of the barrel – oops, I had gas lock due to the low tension McMaster Carr spring. DanoInTx warned me he had had valve lock at 2300 psi on his first try and our hero thought 2000 psi would be low enough – no dice. Additionally, on assembly, our hero was very careful with the safety spring and ball bearing, so careful, he lost them while trying to rotate the safety during reassembly – he should have used the plastic bag method mentioned in ‘The Book’.
Our hero then slowly bled down the reservoir pressure to 1700# and was able to get a pellet to fire out the barrel. Unfortunately, our hero could easily follow the pellet trajectory and watch it arc gracefully up and then down to hit the floor well before the target 12.5m away. Hmmm, maybe I need a little more spring tension (remember that grease on the hammer? Don’t forget it now).
Our hero then opened the breech assembly one more time and decided to add three -111 sized o-rings on the spring guide to pre-tension the spring a bit more. Comments abounded from the wise ancients about the possibility that these o-rings would wear out and that an annular spacer in the breech plug or stacked metal washers on the spring guide might be better – our hero had o-rings, and they were handy.
Upon re-testing the gun, by sound, our hero noticed a definite increase in pellet velocity – oh boy, now we are getting somewhere.
Our hero then charged the gun up to 1900 psi and started firing off pellets over his Beta Chrony. The shot strings, to say the least, were not impressive. The velocities started out in the low 600’s and climbed through 800. Our hero checked the fill pressure on his Hill pump and found it to be about 1700. The shot string looked good initially, but our hero noticed every few shots the velocities would drop by about 100 fps – not good (remember that lube in the hammer chamber?).
Now, our hero starts shooting round after round over his Chrony then realizes he is not being consistent in his angle over the Chrony. He changes and tries to free-hold more consistently but there are still ups and downs in the shot string velocities.
Frustrated, before stopping for the day, our hero dumps a bunch of 3in1 oil into the hammer chanber to reduce friction – so he thought. He fired off one more string of twenty and saw even worse up and down results. Our hero, beaten and bleeding, stops for the day and starts crying for help from the ancients at the Darkside Forum.
SpySir comes to the rescue the following day and tells our hero that lube in the hammer chamber is not a good thing – vacuum can build up behind the moving hammer, slowing it down unpredictably – can our hero get a big Duh-Huh!.
The next day, after our hero has a headache from beating his head against the wall, he opens his B-51 for the 4th time and cleans out the hammer cylinder and polishes it with both oiled 600-grit and 1000-grit sandpaper. He then cleans and dries both the chamber and the hammer assembly and puts on and then polishes off a coat of Molly Paste. Noting that his shots had a high of over 930 fps, our hero then removed one of the three o-rings he placed at the base of the spring guide to put more tension on the hammer spring; thus, he hoped, reducing the shot velocity to a value closer to the proverbial 900 fps he desired.
Our hero then re-assembled his B-51 and in the process replaces, again, the transfer valve o-rings and the bolt o-ring. He noticed that on his last assembly that the breech block did not sit uniformly (side to side) on the pressure chamber – be careful to alternately tighten the two forward screws while insuring that the seam on both sides of the breech is uniform.
Our hero then fills the gun to 1750psi and shoots off one shot – it sounds good and hard. He then sets up sand bags for his B-51 so that the placement of the gun relative to the Chrony will not change from shot to shot (he’s learning!) and he begins shooting and logging his shot strings (again – man this part is boring).
The results are impressive (at least to our un-tutored hero). Our hero wanted a string of consistent shots around 900 fps with 10.5gr Crosman Premier heavies with a fill pressure lower than the breath-taking 2700psi he was using. The shot strings started out at 854 fps gradually went up to 912 fps then gradually down to 859 fps in a 50-shot string, success at last - as follows:
Begin Fill Pressure: 1700
End Fill Pressure: 1100
Shot No. Velocity Shot No. Velocity Shot No. Velocity Shot No. Velocity Shot No. Velocity
1 854.1 11 892.2 21 911.3 31 903.7 41 882.2
2 858.7 12 895.3 22 908.8 32 896.0 42 880.1
3 865.0 13 896.3 23 912.4 33 898.5 43 875.9
4 876.2 14 904.0 24 908.0 34 898.5 44 877.5
5 869.0 15 901.4 25 908.9 35 896.3 45 868.9
6 882.4 16 907.6 26 907.6 36 892.8 46 870.3
7 879.4 17 903.4 27 909.3 37 896.2 47 866.4
8 879.4 18 907.2 28 907.6 38 891.4 48 867.4
9 881.5 19 906.8 29 905.5 39 889.4 49 858.9
10 890.0 20 910.1 30 905.1 40 884.0 50 859.6
Hi 890.0 Hi 910.1 Hi 912.4 Hi 903.7 Hi 882.2
Lo 854.1 Lo 892.2 Lo 905.1 Lo 884.0 Lo 858.9
Ave 873.6 Ave 902.4 Ave 908.5 Ave 894.7 Ave 870.7
Es 35.9 Es 17.9 Es 7.3 Es 19.7 Es 23.3
Sd 11.5 Sd 6.0 Sd 2.3 Sd 5.5 Sd 8.1
Our hero thinks that now he has an easy-pumping gun that requires only a 1700psi fill for 40 good, moderately high power shots.
Now, on a serious note, my thanks to all those at the Darkside Forum that gave such good support and provided such great inputs and suggestions. I am happy where I am at on this tune. Now, I think I do need a barrel re-crown to get close to the AA S400 one-hole strings that I can get on a good day.
Special thanks to:
Dan (DanoInTx) and John (SpySir) – this forum is the greatest!
Scott in Harwood (our hero).
