Registered User
I'm looking for some advice on lapping a conical loading tap, as it's something I'm not very good at. The gun is an early Webley Osprey. I have made some attempts, but at the end the tap is very prone to binding (suggesting high spots, I would guess).
Do you guys lap the new tap from the very start, or do you size it down a bit first, such as on a drill/lathe with emery or wet and dry?
What do you use for lapping paste? I've tried valve paste, but found it very harsh, then rottenstone in oil, which seemed much better.
Do you do it all by hand, or by mechanized means?
What kind of finish do you aim for on the final surface?
All tips and advice appreciated!
Mushroom Man
I would be very interested in the advice as well.
I must assume that the oversize tap is oversize to an extent that it does not sit such that the pellet bore is not in line with the barrel; in fact the tap sits too far out from its housing. The skill is such to get the tap seated such that everything lines up while maintaining a good air seal.
Personally I only ever 'lapped' car valves ....
So ... let's hear it from the engineers here.
Cheers, Phil
Registered User
If you lap the tap by any great amount, you will possibly start off-setting the pellet bore from the barrel bore.
Thanks - Geoff.
Registered User
Perhaps I should have been clearer - I'm talking about lapping in a new, oversize tap to fit. Aligning the pellet seat in the tap with the bore is part off the lapping process.
Mushroom Man
Agreed ... how is this done? I had always assumed the use of valve grinding paste and a slow approach by hand ... but this would, of course, remove material from both tap and action. Does that matter? Maybe not.Originally Posted by bill57
Cheers, Phil
Registered User
Provided that the bore is sound and not scored my suggestion would be to find where the high spots are by cleaning both tap and bore carefully and then using engineers blue (or a good felt tip pen if not available) and inserting the coated tap "dry" into the bore and gently pushing home and rotating. Inspection of the tap would them show high spots as the blue would be removed by contact with the bore. You could then gently "ease" these areas with a very smooth file followed by "wet or dry" and keep repeating the process until alignment and friction improve sufficiently. The trouble with lapping in is that it's a one way process, go too far and the bore is ruined and in any case, might from then only ever work with that one particular tap. However, it might be that the bore does need a bit of finishing off and in that case It's possible that extra fine grinding paste could be used sparingly on the tap followed by chrome cleaner or brasso.
Regarding hand or machine, I'd go with hand processing and being very gentle and patient when doing so.
Just a suggestion and no responsibility accepted if it either doesn't work or undue damage occurs to either part.
Vic Thompson.
Administrator
The last one I did was well oversize so I used a fine cutting paste to lap the tap in, rotating the tap through a full rotation clockwise and then anti clockwise.
Cleaning everything off and checking frequently for alignment is a must as it's very easy to lap too far using cutting paste.
When I got near I finished off by lapping with Solvo Autosol Chrome polish, again with frequent checks for alignment --- you can check the alignment by pushing a pellet skirt first down the barrel from the muzzle back down into the tap or by simply looking through the cylinder at how the tap aligns with the bore.
I found that care must be taken when rotating a dry tap in a dry cylinder as the two can ring together as they're so closely matched.
The tap does feel a bit lumpy when dry but a smear of moly grease makes them silky smooth.
Tap lapping's a pain to get right, but when you get it right it gives an airtight seal and you fully understand why BSA changed to parallel taps.
All the best Mick
Registered User
At the factory they used a special reverse spiral tap on MKIII's and Osprey's, it removed tiny amounts of metal and left a highly polished tapered bore, this was done by ''experience'' using a bore size gauge to determine when the tap as almost in line; then it was lapped in with coarse paste and finally fine paste until it lined up with the bore. If you have a ''big'' oversize tap it will take a long time with just paste, but coarse first will help.
Registered User
That's exactly why I asked about a bit of mechanical reduction first, but I could see that introducing unevenness that might be impossible to lap out (or in). All sounds pretty much as I thought, I suppose it's just down to practice and a bit of good fortune. It's the main reason I tend to avoid older Ospreys, this is the only one I have.
Registered User
A little more info regarding tap fitting; I wouldn't advise reducing the tap unless you can fit it into a lathe and set the correct taper angle on the top slide; even then you are probably talking of microns of removal, as even a tiny reduction in size moves the tap a long way in the housing; emery or anything non solid could produce a ''wavy'' surface, also most likely produce tiny grooves and ruin the tap. I have correct reamers and paste plus bore gauges, so I am able to repeat factory processes. The factory grinding paste is ''CHEMICO'' valve grinding paste, in a double ended tin of coarse and fine paste; from the County Chemical Co. Chemico Works Birmingham. Probably no longer in existence as the tin is dated is dated 1949.
abellringer
Registered User
So if I understand this right, once a tap has been seated correctly in its bore and the pellet hole bored in precise alignment with the barrel, any further lapping process of the two parts that have already been lapped and bored as a matching pair is going to misalign the pellet hole in the tap with the lead-in to the barrel.
If that is the case, the only solution for a leak proof set-up is to build up some material on both tap and its mating bore and re-lap till the pellet hole lines up with the barrel lead-in.
Having said that, I have a tap-loading Falke that leaks a bit from the tap, but it still shoots plenty strong enough, so its not the end of the world if a tap loader leaks a bit. I managed to reduce the leaking by using bike chain lubricant, it is rather sticky and fills in any scratches in the bore while allowing the tap to rotate as it should.
Airgun Alchemist, Collector and Scribe
There's not much written about it, but you can electroplate iron onto steel. I have built up loose threads successfully this way. So this offers a possible way of building up the tap prior to lapping, with the advantage that the layer could be finished off by case hardening if required. .
Registered User
That sounds good . Is it possible to electroplate the tapered bore in which the tap rotates, without having metal deposited in the leadin to the barrel or the barrel itself?
Registered User
The loading tap could be chrome plated too and then lapped in.
Regards Graham
Airgun Alchemist, Collector and Scribe
It would be tricky but just about do-able. You would need to be able to confine the elecrolysis bath to a small volume that encompasses just the tap recess area and not the rest of the gun, otherwise you would need a huge bath to accommodate everything. You would then need to protect the areas where you do not want build up with an easily removable coating. I have used a solution of common or garden polystyrene foam in acetone as a suitable lacquer for protecting areas during hot salt bluing or electroetching. Completely untouched by strong chemicals and a perfect insulator, but comes off straightaway with an acetone rinse. You could plug the barrel lead-in bore with a plug of polystyrene foam.
I visualise lying the gun on its side with the tap recess vertical, sealing the across bottom of the recess with tape/lacquer/wax, in fact anythng that makes it water tight. The upper end of the recess would be fitted with some plastic tubing pointing vertically, sealed in place again with anything that gives a watertight seal. The net result is a narrow vertical tube that can be filled with the electrolyte, the bottom few cm of the tube being the tap bore itself. For the electrolyte you can use a solution of ammonium ferrous sulphate (cheap, off Fleabay) in water (17.5 grams in 50ml water), acidified with whatever acid you can find to a pH around 3-4. Wire the barrel to the negative terminal of a battery. Insert a large iron nail into the upper part of the tube with this wired to the positive battery terminal. The battery should provide only about half to 1 volt - you don't want to see much bubbling from the tap recess. Significant build up of iron on the exposed metal area should occur in an hour or two. In my experience with threads, the coating feels gritty at first, but it wire brushes readily to a smooth finish.
Obviously you would need to do some experimenting first with bits of scrap metal before commiting the gun itself to the process, but at least when building something up it is easy to reverse if things don't work out - unlike processes that remove material!
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