Author Topic: Tale of Two BRods  (Read 17047 times)

rsterne

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Re: Tale of Two BRods
« Reply #120 on: January 22, 2018, 01:33:42 PM »
Today I worked on a new SSG to go inside the AR style stock adapter I made a while back.... I had to shorten up the adjusting bolt and spring guide so that it wouldn't interfere with the stub on the stock, as this is a paintball stock and uses a plastic slider.... This version uses a PRod trigger with a taller sear from Lloyd at AirGunLab to work with the thicker MRod tube.... The forestock is a simple piece of ABS plastic pipe slide over the main tube and secured with setscrews.... Here is what the finished gun looks like....



The 500 cc tank is heavy, so the all up weight is 9.5 lbs including the scope and rings, so the bare gun is about 8 lbs.... If you hold it with your hand just behind the tank block, it balances right there.... It uses the Cothan valve and the Cobra tank block with externally adjustable regulator.... It is currently tuned to shoot the 34.1 gr. JSB King Heavy MkIIs at about 950 fps (68 FPE), for about 40 shots per fill.... With the Cothran valve, the easiest way to adjust the velocity is with the regulator.... the pressure adjustment (currently 1900 psi) is a 10-32 setscrew on the bottom of the tank block.... The barrel is a LW Polygonal, 23.8" long, and fitted with a Hatsan Air Stripper.... The MROd receiver was bored out about 1.5" to accept the larger diameter barrel.... so the free floated barrel is very rigidly mounted to the receiver.... The bolt was fitted with a 3/32" diameter probe, making the chamber area the same as the barrel and transfer porting, which is 0.234"....

All in all, I am very pleased with the way this turned out.... It is easy to cock, with the SSG having 3 lbs. of preload on an 8 lb/in spring.... The 51 gram MDS (steel cored) hammer has 0.79" of stroke, and the SSG gap at this tune is 2 turns, so the cocking distance against the spring is 0.67", making the maximum cocking force less than 8.5 lbs.... About the only thing that would be a nice addition is a BiPod mount on the bottom of the tank block.... My thanks to Travis and Lloyd for their help in procuring the parts for this build, without them it would not have been possible....

Bob

« Last Edit: January 22, 2018, 02:08:47 PM by rsterne »
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oldpro

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Re: Tale of Two BRods
« Reply #121 on: January 23, 2018, 02:21:13 PM »
 Thats a nice looking rig with all the newest tech. installed.
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rsterne

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Re: Tale of Two BRods
« Reply #122 on: January 23, 2018, 07:15:55 PM »
Today I took apart the other lower, with the ART/SS valve in it, and cleaned it up and reinstalled the #87.5 jet that it came with.... After a couple of experimental settings of the hammer stroke and SSG preload and gap, I ended up using the 27 gram hammer with the striker set flush with the front face.... I lowered the preload to 0.37" (just 3 lbs.) and I was running low on my supply of 50 gr. JSBs, but I had a tin of the 45 gr. FX (JSB) pellets, so I used those to collect the data of velocity vs. preload, with the following results....



The combination of the stock jet and the very light hammer gave me a much nicer curve, with a full 200 fps of velocity adjustment before the valve fell off the cliff.... This is the widest range I have seen with the SS valve.... Once again, as soon as there was negative gap (preload) on the SSG, the gun became an instant air hog, and barked and kicked like a big angry dog.... However, as soon as there was any gap at all, the report was sharp and crisp, and just gradually became quieter as the velocity dropped, exactly like a conventional valve would do.... I set the SSG gap at 2 turns (just under 1/8") and tethered the gun to my 500 cc bottle with the output regulated at 1900 psi (actually just under).... I left the bottle tethered to the gauge of my Great White (tank valve closed), so that I could check the pressure drop to measure the efficiency.... I did two 7-shot mags. and got an average of 940 fps (88.3 FPE) with about a 1% ES, and the pressure dropped 250 psi for each mag., which works out to an efficiency of 1.18 FPE/CI.... Without changing any settings on the gun, I tried a 7-shot mag. with the 50.2 gr. JSBs I had remaining, and recorded an average velocity of 886 fps (87.5 FPE), but the pressure dropped only 200 psi.... Very surprised by that, I repeated the test, with exactly the same results.... That works out to 1.46 FPE/CI, which is pretty incredible for a .30 cal at that power level....

There is something different going on with this valve, that may be the key to some of the high efficiency numbers we are seeing.... I don't know if it has to do with the reduced velocity, or the slower pressure decay that causes in the valve throat.... but I have never seen such a drastic increase in efficiency before with only a 10% increase in pellet weight.... Normally you would expect the valve to release slightly less air with the heavier pellet, because it "uncovers" a shorter barrel length during the valve dwell, which essentially stays the same, and that results in a slight increase in efficiency.... For some reason, with the heavier pellet, this valve used less air.... a LOT less air, a drop of only 200 psi with the 50 gr. pellet compared to 250 psi with the 45 gr.... I am at a loss to explain why.... Perhaps the higher the pressure in the exhaust port the faster the valve closes, increasing the efficiency.... I don't know, but I like it....  8)

I would like to increase the velocity with these two pellets a bit, so I think I will add a thin shim to the regulator to bump the setpoint a bit.... I would like to be able to adjust the velocity of the 50 gr. between 900-940 fps and the 45 gr. between 940-970 fps just by changing the SSG gap, and not have too big a gap (or too small).... I wish I had another Cobra tank block, with the externally adjustable regulator, but it is more important for that to be paired with the Cothran valve, which has no velocity adjustment expect by pressure.... At least I will have no worries about getting enough hammer strike with this valve.... the gun is very easy to cock, and I have lots of preload adjustment available, with only being at 3 lbs. at the moment....

My biggest problem now, is that I have been burning through pellets like crazy.... and so much air I had to rebuild my ShoeBox a couple of days ago (I got 90 hours on the HP O-rings in my Freedom 8, not bad).... Still, doing lots of shooting and collecting lots of valuable data is worth spending a few bucks, right?.... I hope my wife agrees....  ::)

Bob
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rsterne

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Re: Tale of Two BRods
« Reply #123 on: January 24, 2018, 02:28:20 PM »
I had a bit of an epiphany last night about how to tune this valve.... When I looked back over how far below the plateau velocity I generally tune my regulated guns, it turns out that about 3-4% is typical.... By the time you are 5% or more below the plateau, the velocity starts to show a significant increase when the pressure drops below the setpoint.... If you tune less than 3% below the plateau, the gun tends to use more air for not much increase in velocity.... and of course at the plateau it becomes an air hog.... So, this morning I hooked by regulated 500 cc bottle to the .30 cal BRod with the ART/SS valve in it, and did some experimenting.... but I made one (hopefully last) change first.... I made a slightly heavier hammer, that weighs 39 grams instead of the 27 gram I was running yesterday.... This is in between that and the 51 gram hammer I installed in the BRod with the Cothran valve.... The reason for the slightly heavier hammer is that I could not quite get to the plateau velocity with the 27 gram hammer without decreasing the SSG gap below zero (ie adding preload).... which wasted HUGE amounts of air.... I wanted to be able to get to the plateau at zero gap, and adding a bit of weight to the hammer seemed the best way.... and it worked great....

I had checked the output of my 500 cc regulated bottle, and it was actually 1850 psi instead of 1900.... Using that, I plotted a new velocity vs. preload curve with both the 45 gr. and 50 gr. pellets, and got a plateau of 972 fps with the 45 gr. and 945 fps with the 50 gr.... The valve fell off the cliff at 9 turns of gap, with an adjustable range of about 150 fps before it did that.... I tried a few gap settings around 3-6 turns out, and found that at 5 turns of gap, if I let the tank pressure fall below the 1850 psi setpoint I was getting a slight bump in velocity, just as I expected, peaking at about 1780 psi before declining.... That is EXACTLY what I was looking for, my normal tuning point where the shot count is extended a bit below the setpoint pressure, but with a velocity increase of less than 1%.... The problem was, it was occurring at about 940 fps with the 45 gr. and about 880 fps with the 50 gr. pellets, and I wanted a bit more than that.... The solution was to drain the bottle and increase the setpoint pressure a bit.... On the third try I got the shim stack just right, and the new setpoint is exactly 1950 psi.... I repeated the testing with this higher setpoint, with the following results....



As you can see, the plateau velocity is higher, as you would expect with the 100 psi higher setpoint.... Is it now 988 fps with the 45.0 gr. pellets (97.6 FPE) and 954 fps with the 50.2 gr. (101.5 FPE).... and I can get there with a turn of gap in the SSG.... I found that with 4 turns of gap the velocity didn't increase below the setpoint, but it didn't fall off immediately either, giving me another 5 solid, usable shots below the setpoint.... the velocity didn't drop off until the pressure fell below 1800 psi.... What I had done was tune the gun to act like it was unregulated below the setpoint, with the velocity peaking about 100 psi below the setpoint.... but the curve is so flat at that point, the slight bump in velocity is lost in the SD.... The fact is, I have the gun tuned at 1950 psi, but I can use the tank down to 1800 and never even notice it.... all the shots stay within the normal ES, which is just over 1% (about 12 fps with unsorted pellets)..... The velocity with the 45 gr. pellets is just over 3% below the plateau, and with the 50 gr. pellets it is right at 4% down.... Once again, my preferred tune on a regulated PCP is right around 3-4% below the plateau.... I did one 7-shot mag. as an efficiency test with the 50.2 gr. pellets, and it worked out to 1.25 FPE/CI at an average of 916 fps (93.6 FPE).... This is not as good as yesterday, but the velocity is 30 fps higher, so pretty much expected.... I should still get about 35 shots at nearly 94 FPE with this tune.... and by using a 4500 psi bottle that would more than double....

Bob
« Last Edit: January 24, 2018, 02:31:49 PM by rsterne »
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rsterne

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Re: Tale of Two BRods
« Reply #124 on: January 24, 2018, 04:33:12 PM »
Here is what the .30 cal. looks like assembled....



It weighs 3 oz. more than the AR version.... The stock is heavier, but the barrel is lighter because of the CF sleeve to bring the diameter of the TJ's liner up to the same as the LW barrel (16mm).... all up with scope and rings it is 9 lbs. 11 oz.... I'm quite pleased with the appearance and ergonomics.... and it balances well, right where you would hold it near the front of the wooden stock....

Bob
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Alan

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Re: Tale of Two BRods
« Reply #125 on: January 24, 2018, 05:09:50 PM »
Too bad you don't own Crosman/Benjamin!
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Alan

I have an Omega compressor. If you're a fellow Guild member, and you pass through Roswell, NM, I'll fill your portable tank as a courtesy.

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Re: Tale of Two BRods
« Reply #126 on: January 24, 2018, 05:46:31 PM »
Too bad you don't own Crosman/Benjamin!

AMEN to that!!!

Great work Bob and excellent data sharing on the SS valve. Especially since I just ordered one.

Chris
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rsterne

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Re: Tale of Two BRods
« Reply #127 on: February 03, 2018, 11:34:38 AM »
I spent the last 3 days casting bullets from all the moulds I got from NOE over the summer.... I went through about 20 lbs. of 40:1 lead:tin alloy from Rotometals, and cast bullets in .224, .243, .250 and .257 cal, and one in .308.... One of the bullets I cast was the heaviest BBT designed for .25 cal airgun barrels.... It turned out to be 58.3 gr. in that alloy for the FN version, and 55.0 gr. for the HP.... Here is what one looks like after pushing through the LW Polygonal barrel on my BRod....



The back of the nose measured 0.250" and the driving band 0.252" before pushing through.... After pushing through the choke, the driving band measured 0.251" across the grooves, and 0.246" across the "lands", but they did not drag on the center portion of the bullet, exactly the way it was designed to work.... At 1950 psi the HP just broke 800 fps, but I wanted to find out what it would do with the SS Valve at 3000 psi, so I swapped parts around to test that.... With the SSG adjusted for a turn of negative gap (preload) I got 952 fps with the FN (117 FPE) with a huge BRRAAAPPPP of air, but I assume that is the plateau velocity.... With a whisker of gap (less than 0.020") the report was normal, nice and crisp (although pretty loud) and I recorded 930 fps for the FN (112 FPE) and 950 fps for the HP (110 FPE).... Which is pretty good for the stock length LW barrel of just 23.8".... This gives a pretty good idea of what the SS valve is capable of.... The only way to get more would be to use a retracting bolt to provide unobstructed flow in the chamber, as right now it is the equivalent of 0.234", and I am actually using that for a transfer port diameter, as there is no sense in going larger....

This experiment gives significant insight into what could be done with the ART/SS Valve in .257 cal with a longer barrel, using a 4500 psi bottle regulated down to 3000 psi.... With a 28" barrel, 130-140 FPE should be quite possible, which would enable using the venerable Lyman 257420, which is the benchmark bullet in .257s.... My Hayabusa .257 (3000 psi fill, unregulated) with those specifications (with retracting bolt) reached a peak of 160 FPE, and can be tuned for a very nice string at over 130 FPE.... and that could be increased a bit if regulated to a solid 140 FPE.... There is no reason that the SS valve could not produce the same results, and be a LOT easier to cock as well.... My Hayabusa uses a 115 gram hammer, with a stroke of 1.2", and an 18 lb/in spring.... Cocking force at 160 FPE is 24 lbs....  :o

The experimenting I have done here provides a roadmap for anyone wishing to build a wonderful Varmint or long range target rifle, using proven components.... A 1/2" OD TJ's 25-20 barrel that is 28" long, with a 14" twist, the MRod action (modded for a retracting bolt), with an ART/SS Valve, and a 500 cc CF bottle regulated down to 3000 psi would supply the power.... Tensioning or sleeving the barrel to stiffen it, and maybe a harmonic tuner, should provide the accuracy.... Some decent optics, and an SCBA tank (for refills or tethering), and you would be good for a whole day of serious long-range Varmint busting....

Bob

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sixshootertexan

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Re: Tale of Two BRods
« Reply #128 on: February 03, 2018, 04:10:20 PM »
Which .308 did you cast? Hope to see your test findings with it.
CCS 2300, CCS 2400, Custom Built Regulated .25, Custom Built PRod Clone, .308 Bullet shooter, XS46U .177, Custom regulated .177, 850 Hammerli .177

rsterne

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Re: Tale of Two BRods
« Reply #129 on: February 03, 2018, 06:30:37 PM »
It was the new 106 gr. straight sided NOE airgun bullet.... came out at 104 gr. FN and 98 gr. HP....

Bob
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rsterne

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Re: Tale of Two BRods
« Reply #130 on: February 04, 2018, 02:20:24 PM »
Ran some test today with the .25 cal BRod tethered at 3000 psi.... still running the 12 lb/in spring with only 3 lbs. preload.... I was able to just reach the plateau at zero SSG gap as this pressure.... using my 47.5 gr. BBT HPs I got 1002 fps, and with the 50.4 gr. BBT FN I hit 977 fps (106-107 FPE).... At 3.5 turns of gap that dropped to 100 FPE with a nice crisp report.... I didn't check the efficiency.... but those HPs at 970 fps would sure lay a smackdown on anything up to Coyote size, I think....

Interestingly, this is very close to the same SSG setting I was using for the .30 cal BRod shooting 45-50 gr. pellets on 1950 psi.... and only a couple of turns less gap than the .25 cal shooting 34.1 gr. Kings on 1900 psi.... Once you find the sweet spot on the ART/SS valve it doesn't seem to require a lot of adjustment.... even with large changes of caliber or pressure....

Bob
« Last Edit: February 04, 2018, 02:28:26 PM by rsterne »
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rsterne

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Re: Tale of Two BRods
« Reply #131 on: March 21, 2018, 12:18:54 PM »
I have more information that would apply to someone building a 6 mm or .257 cal BRod, based on the results from this winter's PCP projects....

http://airgunguild.com/pcp-c02-and-helium-powered-airguns/any-interest-in-6mm/105/

The 6 mm with a 29" barrel, regulated at 2800 psi, is getting in the mid 900s with the 64 gr. bullet.... Since the Lyman 257420 is the same Sectional Density (0.155), this means that the idea I talked about above, using the SS valve in a regulated .257 BRod with a similar barrel length can be considered a proven concept using a 500 cc CF 4500 psi bottle.... Starting with a 1/2" OD TJ's barrel with a 14" twist, and sleeving it up to 5/8" with a CF tube inserted into the front of the receiver just shy of the 4-40 screws is completely feasible, although you would have to single load bullets of that length (maybe)....

This brings up a new idea I am working on for longer magazines to fit the MRod receiver, for up to .357 cal.... There are several ways to do this, and here is the first prototype I am working on....



The magazine is shown mounted in a 5C collet in a hex block to enable indexing to drill the holes for the pellet/bullets.... You can see a JSB .35 cal pellet on the right, and a 154 gr. .357 cal bullet on the left (they are sitting on a couple of pieces of dowel to raise them up for the photo).... The material is a piece of 1.25" OD 2024-T3 aluminum bar stock, and it is long enough to make 2 mags.... I am not sure yet how I will mount the mag. and it will likely have to be indexed manually, but one way it could be mounted is on a retractable pin set into the MRod receiver, similar to the way a Hatsan mag. is located.... I checked the width of the mag. slot in a .25 cal MRod receiver, and it is long enough to accommodate that 154 gr. bullet if there is nothing else in the slot, and the barrel is not protruding.... This isn't my plan, I would never want to shoot anything that heavy.... and likely I will make the mag. wheel thinner to help mount and index it.... but it does shown some possibilities....

The other way to make a mag. for a .357 cal. is to copy the MRod .25 cal mag. but with a larger wheel in a similar housing, possibly made by a 3D Printer.... I have drawn one up and sent it to Travis for him to look at.... It would be the same inside length (0.45") as the .25 and .30 cal mags, but that would fit the JSB 81 gr. pellets, the EPP-UGs, and my 109 gr. BBT.... so likely all the length you would need for most .357s based on an MRod.... The advantage to this method is that it would be self-indexing....

Bob
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rsterne

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Re: Tale of Two BRods
« Reply #132 on: March 21, 2018, 07:01:00 PM »
Well I got sidetracked on the long .357 aluminum mag project.... I had another look at the .25 cal MRod mag. and came up with a different way to modify it for .35 cal and still use the original housing, albeit modified somewhat.... The first step was to make a .35 cal wheel that would fit inside the .25 cal mag. and the only way to do that was to reduce the radius that the holes are on that hold the pellets from 0.50" to 0.45".... This would normally raise the center of the pellet 0.050" above the boreline, but I realized that if I inserted the magazine further to the left, while keeping the feed hole at the same height (which it must be to align the pellet with the bore), I could just rotate the wheel a bit more counter-clockwise (when viewed from the back) to get the height to work out.... here are some photos of the work so far....



The wheel was machined from a piece of MDS, with seven positions, one of which uses a glued in blanking plug that prevents you from closing the bolt when the mag. is in the empty position.... Drilling 7 holes was a challenge with a manual milling setup, the key was to use an online calculator to convert from Polar coordinates (radius and angle) to Cartesian (x and y) coordinates, and then just position the vertical and crossfeeds on the milling attachment on my lathe to drill the 7 holes around the 0.9" diameter circle.... It worked out great, although there was a lot of hand finishing to do, removing the milling flash where the holes broke through the outer diameter of the wheel.... You will notice that the recess in the back of the wheel for the torsion spring is smaller in diameter, so I will need to find or make a smaller OD spring to force the necessary rotation of the wheel to make the mag. self-index....



Here is a photo with the wheel in place, but I have not yet moved the feed hole over in the outer casing, or milled away part of the bump stop to make it stop in the correct place.... In the photo below I have machined a larger hole in the clear cover in the correct location.... You can see that it is shifted about 0.090" to the right of the original location.... and you will notice that part of the bump stop will have to be machined away when I mill the hole in the outer housing in the correct location to allow the mag. to slide to the left....



There are lots of things that will remain to be done to make the mag. functional.... the spring as I mentioned, and a new location for the small brass pin that rotates the wheel to allow loading pellets when you rotate the cover.... I will also have to figure out a way to hold the mag. in place, because the slot in the front for the barrel will lose the little bumps that allow the mag. to "snap" into place on the barrel stub.... First I have to get the mag. to function and line up.... then I will worry about the details....

Bob
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rsterne

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Re: Tale of Two BRods
« Reply #133 on: March 22, 2018, 03:51:15 PM »
I took me many hours and lots of hand work, but I now have a fully functional 6-shot .35 cal MRod magazine, made from a .25 cal.... Here are the photos....



The spring is a mess, cosmetically, but it works.... I made it by winding the stock spring around a 9/32" drill bit to tighten the coil enough to fit inside the 1/2" hole in the wheel.... Of course it came out a mess, so I spent a lot of time with two needle-nose pliers tweaking it to get it to work....  ::) .... I had to hand carve the groove for the pin in the cover that rotates the wheel with a tiny burr on a Dremel, and again it may not be pretty but it is functional....



The housing was milled out with a 3/8" end mill to move the hole over and increase it in size.... There is a 1/32" hole drilled through the filler plug in the wheel for the tail of the spring, which is installed in the photo....



The position of the feed hole, and it's associated recess that clips over the end of the barrel were the key to this project.... They are the same height as in the stock .25 cal magazine, but are moved 0.084" towards the centerline of the mag..... That was necessary for the hole to line up with the smaller radius of the wheel, which was necessary so that the pellets fit inside the housing.... I was concerned about how to retain the magazine in the slot, but as it turns out, that was a non-issue.... When I plunged the 1/2" mill into the front of the housing to create the recess, it left two tiny bumps that slide past the barrel when you insert it, exactly the same as on a stock mag.... The difference is that as you slide the mag. into place, you feel it "click" twice as it slides past the OD of the barrel, once for the stock bumps, and then for the new ones, which are actually tighter than original.... Once in place, the mag. protrudes that 0.084" past the left hand side of the receiver because it sits that much further to the left to line up....



Here is a photo of the loaded magazine.... You can see the brass rotating pin, which I had to relocate, and the enlarged, kidney-shaped loading slot in the clear cover.... It functions perfectly, indexing to the next pellet each time you push one out.... I even tried it sitting on the end of my .357 barrel, and the pellet just slides right into the bore, exactly as it should.... Now that I have figured out how to do this, I will order another .25 cal MRod receiver and bolt, and a PRod trigger group and AR stock so that I can complete a third BRod in .357 cal.... I already have the bottle, tank block, and a spare FLEX tube, and most of the parts to make a valve.... so it looks like a regulated .357 BRod repeater shooting pellets is the next project.... although it likely won't get done until next winter, I am quickly running out of time as spring is here and the Motel will soon be busy....

Bob
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Monkeydad1969

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Re: Tale of Two BRods
« Reply #134 on: March 25, 2018, 08:46:16 AM »
Nice work, Bob.

Joe
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