How to provide a retaining fit for a parallel pin on a 5″G loco valve gear

[GreensandsParticipant @greensands]

Hi All – I am looking for a means of securing a parallel silver steel retaining pin in part of the valve gear of a 5” gauge steam locomotive.  The 3/16 diameter pin is designed to provide a smooth running fit through the middle hole of a combination lever which is supported either side by corresponding holes in the valve cross head assembly.

Following the “words and music” given in the build notes the hole in the lever has been reamed 3/16” and the holes in the valve cross head have been drilled using a No. 13 drill (0.185”) to provide a retaining fit in the valve guide.

In practice my No.13 drill has drilled slightly over sized and instead of providing the required friction fit for pin retention the drill has resulted in a freely rotating fit. Now in any other situation the problem could be overcome by putting a head on the pin and a retaining collar at the free end but the current location does not provide the space for this approach and so I am left with looking for other possible solutions.

My thoughts have centred on using a Loctite retaining compound but if so I would like to be able to drive the pin out at any time for possible servicing/replacement of parts without having to use too much brute force (or excessive heat) but I am uncertain as to which would the best choice of Loctite (or similar) type to use.

There may of course be other possible means of providing the required interference fit for the pin and I am open to all such suggestions.

Hoping that someone will be able to come up with a working solution.

[Michael GilliganParticipant @michaelgilligan61133]

May I suggest Loctite ‘Bearing Fit’

… the characteristics look an excellent match.

MichaelG.

 

[JasonBModerator @jasonb]

Can you drill and ream one of the members for a taper pin, these are  a 5mm pin so not far off your 3/16″, fisrt one the taper pin just cuts half way into the side of the pivot pin. Second it goe sthrough th emiddle

[JAParticipant @ja]

LBSC on Speedy (5″ gauge loco) appears to have used 3/16″ silver steel pins with nuts on each end. Mind you, that was long before the discovery of Loctite.

I will follow this post with interest since I am building a Speedy (as my main project).

JA

[GreensandsParticipant @greensands]

 

The location of the cross head guide bars would make the use of a taper pin difficult and at this stage I would rather pursue the idea of an easily removable Loctite retaining compound.  The 3/16 diameter pin is shown in the second photo as currently fitted with a through hole.  As can be seen, the presence of the guide bar pillars make it impossible to fit a pin with an integral head and collar arrangement

 

ytty

 

 

[Andrew CrowParticipant @andrewcrow91475]

Could I suggest using a 5mm hand reamer which will have a taper lead.

If you carefully ream the hole but do not go all the way through, this will leave material in one side to press fit a 5mm pin rather than the original 3/16 one.

A bit of a bodge but may save you having to make a complete new part.

[Clive FosterParticipant @clivefoster55965]

Can you not just go up to a plain 5 mm diameter pin using a no 10 hole on the retaining side to get the interference fit. The difference between your oversize no 13 and no 12 is likely to be so small that simply twiddling the drill by hand should reamer the hole out enough to twiddle a no 11 and then 10, which are only a couple of thou difference, to end up with the correct size.

Theoretically 5 mm pin is 0.1968″ Ø so no 10 at 0.1935″ ought to work albeit with a touch more interference than the 3/16″ pin which will have 2.5 thou in perfect no 13 hole (0.1875″ in 0.1850″ nominal) whilst 5 mm in no 10 gives 3.3 thou. But at those sizes odds are that the drill will cut oversize, as you have found.

Clive

PS Andrew posted whilst I was typing.

[GreensandsParticipant @greensands]

Yes, refitting with 5mm pins is definitely food for thought but I think it would require dismantling of the gear to ensure satisfactory installation.

[Andrew CrowParticipant @andrewcrow91475]

I think that any satisfactory solution would require the parts to be dismantled,  as either drilling or reaming in situ would prove to be very difficult to control accuracy.

[Martin ConnellyParticipant @martinconnelly55370]

Do you have a press that would allow you to slightly upset the head of the pin?

Martin C

[GreensandsParticipant @greensands]

I have thought of using an upsetting approach (shades of Swindon loco shops where I gather they had experts at the task) and I do have a small mandrel press that may be up to the job but I have never tried the technique myself. Again, food for thought.

[GreensandsParticipant @greensands]

The downside of using upsetting would be difficulty in controlling the degree of upset and the problems of possibly having to use excessive force should the need arise for any subsequent dismantling.

[duncan webster 1Participant @duncanwebster1]

If the pin can be ever so slightly longer than the distance over the yoke, you could peen it over both ends, but not without dismantling to some extent. You only need to raise a burr. If that won’t work, drill a hole say 3. 5mm up one end, assemble to the yoke, then drive a hardened tapered punch into the hole to swell it up a bit. If you drill right through you can swell both ends,but then it’s harder to get out

[bernard towersParticipant @bernardtowers37738]

Use a roll pin.

[GreensandsParticipant @greensands]

The pin is there to provide a running bearing for the backwards and forward movements of the combination lever and in this application I don’t think a roll pin would provide the answer.

[bernard towersParticipant @bernardtowers37738]

In. that case Jasons solution is most probably the best one.

[Paul LousickParticipant @paullousick59116]

One option for fitting a pin in an oversized hole is to knurl the pin, making it slightly bigger. Can you do this on one end of the pin ?

[Howard LewisParticipant @howardlewis46836]

My thoughts, Paul.

Howard

[HopperParticipant @hopper]

I think I would turn up an oversized pin to be a press fit into the two existing outside holes. Then I would ream the inner hole out to suit using a 3/16″ adjustable or expanding reamer. (Or possibly your fixed 3/16 reamer with a slip of paper down one side of it to make it cut oversize, but test it on some scrap first.)

I would avoid Loctite as I always manage to get it everywhere and would end up Loctiting the inner hole as well as the two outers. And I am not a fan of knurling as the grip is a very small area on the edge of the knurl lines,which can fatigue quickly in a dynamic application such as yours.

If you do, however, proceed with the 5mm drill and ream solution, as you are now aware expect the drill bit to also drill a thou or two oversized and select your drill bit accordingly. Also measure your silver steel carefully. It often comes a thou or so undersize from nominal.

It can help to get a drill bit to drill on size if you very slightly round off the outer corners where the two cutting edges meet the flutes with an oilstone. Try drilling a hole in some scrap first to check though. Drill the hole first with a pilot drill, then with the next smallest drill bit than final size that you can get, so the final drill is removing absolute minimum metal,just a few thou etc. and acting more like a reamer than a drill.

 

[ChrisLHParticipant @chrislh]

Pin 2 mm dia, say, with slightly enlarged head. Crosshead drilled 2 mm through concentric with valve rod. 2 mm diameter of pin fits through cross hole in 3/16 pin and is retained by valve rod. Lots of disassembly and reassembly required though.

[Peter Cook 6Participant @petercook6]

How about drilling an M3 tapping hole 5-6mm deep in the outer end of the pin. Tap using a 1st taper tap. Slit the pin the depth of the hole, and then use a short M3 grub screw in the tapped hole. That will expand the slit portion slightly to give the grip. Could be done at both ends if there is Allen key access to the inner end. Remove by unscrewing the grub screw(s).

[duncan webster 1Participant @duncanwebster1]

Enlarging on Chris LH idea, the valve rod could have a reduced diameter pip on the end which locates in a groove in the 3/16 pin.

[GreensandsParticipant @greensands]

I am currently in the process of putting Peter Cook’s suggestion into action using a 6BA grub screw. What I like about the idea is that it does not require any dismantling or significant modification to the existing parts apart from an easily replaceable pin. Will report on the results when available.

[Andrew TinsleyParticipant @andrewtinsley63637]

I am with others on this. simply knurl the existing pin That will make it a tight fit!

Old ford cylinder heads did not have replaceable valve guides, They were simply bored holes in the head. When they wore oversize, the engine reconditioners used a tool that effectively knurled the valve guide holes. The valves were then a good fit! This trick lasted about 50,000 miles, if my experience was typical.

Andrew.

[GreensandsParticipant @greensands]

Some sign of success!  Having tried the 6BA grub screw into a pin with a split end without any joy I changed tact, re-drilled the hole 2.5mm, tapped out to 3mm and inserted a short 3mm c/s screw with the head turned down to 0.0170. Net result, the desired tight fit in the cross head without the need to do any dismantling and visually quite pleasing. Current thinking now is to put a plain screwdriver slot into the inboard end of the pin  enabling it o be inserted from the front whilst being held back with a modified screwdriver at the rear. Anyway, that is my current thinking. All good stuff.

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