Friday, 29 November 2013

The SMK B 45-3 Gets Sorted

The problems are finally solved,


After many, many months of trial and error i can say with confidence that the problems with the inlet valve are fixed, and the pump arm has a lot less side to side play. There were times i thought i should let it die with dignity, but with it's simple design there was no reason this shouldn't be a really devastating multi pump. Besides i had invested to much time to give up on it, i am especially pleased with the new stock i built it for starters.

The side to side play in the pump arm was caused by the pump arm being half a millimeter smaller than the the space allowed for it in the front sight assembly, 

Front sight assembly.

the only thing i could do to pack the sides evenly was use shims made from drinks cans. Two layers either side reduced play but didn't stop it, and three layers wouldn't fit with out them tearing apart. 

\\\\\\\\\\\\\\front sight assembly removed with relevant bolts and the handy metal washer to reduce side play,

I have washers that size so i could pack one side but i hadn't any thinner washers, and it works perfectly well just packed out on one side anyway. There was less play with one washer than with two shims either side, so one washer will do fine until i source thinner ones or thicker shim material.

The problem with the air inlet valve was that a raised collar cut into the rubber flap washer seal after about 50 full power shots, depending on the type and quality of the rubber. 

 The white rubber gets cut to shape and makes a good seal.

I found that the rubber that covers electronic cigarette refills was more durable than the thicker, softer rubber of an old car tyre inner tube. 

Inner tube rubber needs super glue to stop it twisting inside out.

After taking a mold of the inside of the valve i came up with two possible solutions.

Delrin seats measured against the mold.

Solution one was to make a delrin seal that would seat on the end of the spring and seal against the collar, but the three i made would not seal properly all the time. 

Turning seats and washers on the fonly lathe.

I am not able to turn things on my Fonly lathe to the tolorences required for an air tight seal, the slightest burr would unseat the seal as was the case.

12 mm for the outer wall.

Solution two was to stop the collar from cutting into the washer seal, by using the mold for the correct measurements i turned a delrin washer to sit outside the collar.

6 mm for the air inlet

 I chanced upon this idea when one of the previous flap washers had been completely cut, the outside part had wedged itself between the outer wall and the raised collar. However the violent motion of the compressed air exhausting itself from the valve unseated it and caused it to block the air inlet, but a ridged delrin washer would stay put and reduce the collar from a cutting edge to slight raised area to seal against.

The delrin washer to fit between the outer wall and the raised collar.

With the delrin washer in place i was able to use the tougher, thinner rubber from the refill caps, allowing more of the metal of the inlet seal retaining nipple to fill the 6 mm air inlet hole. The inlet hole is a lot of dead space for compressed air to build up in, which in turn can expand to push the pump head seal back down the compression tube. Now the pump arm locks in place and is not prone to springing out when holding 6 pumps or more, also it holds all of it's charge over night without any leakage though i only tend to store it with one or two pumps.

I was sold a pot of Logun silicone grease years ago and use it on all PCP rubber seals, i used it on the pump rod assembly so what ever works through is going to be beneficial to the valve seals.

I found that petrolium based grease and oil used to soften and degrade rubber seals, and this i'm sure was the case with this multi pump up until now.

Now i've said all there is to say on the SMK B45-3 i can use my spare time on some of my springers, they certainly need some more tweaking and TLC.


Wing Commander Sir Nigel Tetlington-Smythe.



  Now that's finished i hope to not see this lot for awhile.

All the tools you need to replace the inlet valve washer


The screw that joins the action to the stock.


\The screw that joins to the compression tube and barrel band as seen through the pump arm.


all the valve bits and bobs.


Measure twice, cut once.


Easy when you get to do the same thing over and over again.



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