Gibson crankpins

[hollybeau]

Can anyone tell me please what the exact size/specification are Alan Gibson crankpins? They are stated to be M1 but elsewhere in the catalogue it refers to them as 14 BA. I know that BA technically is metric but there are subtle differences in pitch or some such which does not mean they are interchangeable.
The reason I ask is that I invariably have problems fitting the crankpins to his brass-centred wheels. I drill out for the M1 tapping size (0.75mm) then run an M1 tap through. However, when screwing in the crankpin it invariably gets tight after a few turns. When I first encountered this problem I snapped off a crankpin in the hole and have since learned to run a fine tapered broach through the hole, alternating that with the tap until the crankpin screws home. The result however is a slack fit, or there is a risk of the threads being stripped. I then find myself playing safe with some Loctite.
I have concluded that there is a mismatch between the crankpin and the tap. I have bought two M1 taps over the years (both from Eileen's as I recall so the quality should not be in doubt) and the problem manifests itself with both taps.
The ever-helpful Colin at Gibsons tells me that the crankpins are sourced from Switzerland and as far as he can guarantee are M1 screws. He suggests using an "engineering quality" tap such as made by Dormer.
My other thought is that this "problem" may not have shown itself before now simply because most people use the plastic (nylon?) wheels which are more forgiving when it comes to screws cutting their own threads.
As a final question is there more than one M1 thread size? I have seen references to M1 x 0.25 (which I assume to refer to the pitch). Does that mean there are other M1 screws at different pitches?
Many thanks in anticipation of your collective wisdom and knowledge.

Bryan

[dal-t]

The M1 screws you're most likely to encounter will be either coarse, or fine. Fine has a thread pitch of 0.2, coarse 0.25. Sorry I don't know which Colin's crankpins are, or which taps you've got, but there is at least the possibility you've got a mismatch. Have the taps been used much? My 'bush engineering' first resort would be to run them through something harder than brass a few (dozen) times, in the hope of inducing enough wear to make them compatible - but before you try that watch out for the flocks of 'real' engineers bearing down at flank speed to tell you how to really do it ...!

[David Knight]

I had problems with Gibson crankpins and M1 nuts that I had made up for a special job. Even after repeatedly running the tap through the nuts would jam. I sorted the problem by running a slightly larger tap drill through whereupon everything worked fine. The moral of the story, if there be one, is to check all options.

Cheers,

David

[Philip Hall]

Whilst I know that there is a difference between M1 and 14BA I have intermixed them on occasion with no problem, at least on crankpins. My most common use is an Ultrascale flush crankpin nut on a front axle behind slidebars, and many times these crankpins are AG.

Philip

[Tim V]

Do you "need" to tap them?

I never have (though I don't use Gibson's crank pins, make my own), they can be screwed up tight. For the outside cylinder pin, I use two half length crank pins, the first can be tightened up against the wheel, the second tightened up against the first.

[Alan Turner]

The thread-form is different between BA and metric.

BA has an included angle of 47.5deg and metric 60deg.

A 14BA has a pitch of 0.23mm. the 1mm has a pitch of 0.25mm

regards

Alan

[Terry Bendall]

The thread-form is different between BA and metric.

And those differences will be sufficient to make it difficult to try and interchange the two successfully. There is also, as dal-t has said a difference between the metric fine and metric coarse threads. For those who may be wondering the pitch of a screw thread can be defined as the distance that the nut moves along the bolt when rotated once.

They are stated to be M1 but elsewhere in the catalogue it refers to them as 14 BA

Perhaps not all parts of the catalogue have ben updated?

Have the taps been used much? My 'bush engineering' first resort would be to run them through something harder than brass a few (dozen) times, in the hope of inducing enough wear to make them compatible

err ... an interesting suggestion but one which I would doubt would work, unless you use something which has been hardened, in which case the tap would be useless afterwards.

Terry Bendall

[sebring115]

Easy way to check the tap pitch relative to the screw is to put them next to each other and try an get the threads to interlock, (you may need magnifcation) but if they go off over the length of the screw/tap you know the pitch is wrong.

[David B]

Might I suggest contacting Colin and asking him?

[Le Corbusier]

I have concluded that there is a mismatch between the crankpin and the tap. I have bought two M1 taps over the years (both from Eileen's as I recall so the quality should not be in doubt) and the problem manifests itself with both taps.

Perhaps we could ask Derek at Eileen's to experiment with a gibson crankpin/nut on his various taps and dies to see if any are compatible. If the pins come from Switzerland wouldn't they be metric?

[Alan Turner]

The thread-form is different between BA and metric.

And those differences will be sufficient to make it difficult to try and interchange the two successfully. There is also, as dal-t has said a difference between the metric fine and metric coarse threads.
Terry Bendall

I have yet to come across a fine thread 1mm. It may exist but I have never seen it. Perhaps in the watch making trade - but then they seem to use their own thread forms.

regards

Alan

[David Thorpe]

I bought some M1 screws on ebay. They take AG crankpin nuts quite happily.

DT

[martinm]

I drill out for the M1 tapping size (0.75mm) then run an M1 tap through. However, when screwing in the crankpin it invariably gets tight after a few turns.

Bryan,

I haven't got there yet, but in the catalogue, the instructions say "but for wheels with cast metal centres the crankpin holes will need to be drilled out with a No.66 (0.85mm ) drill and then tapped M1/14BA."

This doesn't help clear up the whole M1 or 14BA question, but does seem to match the suggestions that you need a larger hole,

regards,

martin

[dal-t]

Actually raises a further question - why? - since ISO standard is 0.75 for coarse, 0.80 for fine!

[hollybeau]

My thanks to all who have replied. In an effort to help others and liking the idea of a close up of the tap and crank pin side by side I have taken a photo (attached). This is with a decent macro lens and as close as I can get. In retrospect I should have used a deeper depth of field as the tap is out of focus (but not the dirt on my cutting mat!). Unfortunately when setting this up the tap rolled off the bench and, being brittle, the end snapped off. (I now remember why I have two taps since this is what happened to my first purchase). I may try to grind the end down to a taper since there is enough left to get through a wheel or I may buy another. If anyone can recommend a supplier that would be helpful. (No disrespect to Eileen's but given the problem, and the fact that I cannot change the crankpins, then the only variable that potentially can be changed is the tap).
If anyone can see how the two differ then your eyes are better than mine. I'll try a new photo with a better depth of field when next in the workshop.
Bryan

[derekrussan]

Perhaps we could ask Derek at Eileen's to experiment with a gibson crankpin/nut on his various taps and dies to see if any are compatible. If the pins come from Switzerland wouldn't they be metric?

Swizerland produce all my smaller BA taps and many of my BA smaller screws, so not relevant.

Sorry, I do not have any Gibson Crankpins.

On a thin nut you will probably be able to interchange M1 and 14BA as the pitch difference is over a short distance. As the thread length increase so does the degree of interference, so I am not surprised that an Ultrascale thin nut fits, not that greater problems occur running through wheel thickness.

If I read it correctly then the problem is solved by easing with a broach or using a larger hole, which implies that the ID of the thread is slightly under, so may be problem with thread shape. I do not see how this could be a problem with tap wear. However the solution of either starting with a bigger hole or broaching it out seems to be the way to go.

[CornCrake]

The reason I ask is that I invariably have problems fitting the crankpins to his brass-centred wheels. I drill out for the M1 tapping size (0.75mm) then run an M1 tap through.

Should you not be drilling out with a 0.85mm drill as you have metal centred wheels?

See current catalogue

but for wheels with cast metal centres the crankpin holes will need to be drilled out with a No.66 (0.85mm ) drill
and then tapped M1/14BA.

[sebring115]

Put the tap and screw so the threads actually engage, then it will be clear if the pitch is the same or different

[Philip Hall]

Bryan

Both your tap and the screw seem from your photograph to be rusty! I doubt though that this would make a difference in the thread cutting. To return to the original question, clearly there is a mismatch between the tap and the screw, that can be the only reason.

If it is not something you can sort out, your other option is simply to buy some 14BA screws and a 14BA tap and try those. The outside diameter of a 14BA screw is the same as a M1, or near as makes no difference.

Philip

[hollybeau]

I have made a little progress having followed some of the comments and advice on here. The main difference has been to increase the size of the initial tapping drill size from the 0.75mm I have used previously to 0.85 mm. Having run my 1mm tap through this the crankpin does screw through albeit with some stiffness and needs to be handled gently (inserted a few times). I have at least a workable solution.
I have taken a (better) photo (below) and am now satisfied that the tap and screw match at least in threads per cm. The pitch seems to be correct as well but in all honesty I would need a microscope and better measuring equipment than I have to be sure.
My conclusion is that the tap I have (which is some years old and been used on many of these wheels) has lost its "bite" on the outer diameter (probably because it has been working too hard on the smaller tapping size). I won't know for sure until I have a new tap (hope to go to the Stafford show next month and will see Derek at Eileen's) and tried that with the larger tapping hole. I don't think we can mark this thread SOLVED as they do elsewhere but we are getting there and that is thanks to you.
Philip - yes it does look rusty in the photo but I suspect that is me having got the white balance wrong on the first photo. As I say the tap is rather old and it lives in my (normally unheated but dry) workshop.
The real question is why there is different advice as to the "correct" tapping hole. Colin has it correct in his catalogue (should have looked there first!) but even Derek advises (on the Eileen's web site) that the tapping hole is 0.75mm - and he is not alone.
Here is the photo:

[sebring115]

Just enlarging your picture I'm not so sure the pitch is the same. On the left by the head you have peak to root between tap and bolt, but on the right you have peak to peak which says a different thread pitch, close enough that a nut will go down but not close enough for a deeper thread where you will get binding due to the different pitch.

Might be worth taking your bolt and tap to the show to look at and compare to a new tap.

cheers

Mark

[Phil O]

Can I suggest that you interlock the threads and hold them up to the light, This was how I was taught to check a thread and tap, if you can see light, then there is a mismatch.

Phil.

[derekrussan]

A lot has been said about the tap, but not much about the drill or the material. There is interesting stuff about drilling and hole sizes in the dormer catalogue.

For example in a quote from the Dormer catalogue https://www.dormerpramet.com/Downloads/2015_NAFTA_Dormer_Catalog-English.pdf:

"The tapping hole size depends upon the material being drilled, the cutting conditions selected and the condition of the equipment being used. If material is pushed up at the thread entry by the tap and/or the life of the tap is too short, select a slightly larger drill diameter. If on the other hand the profile of the thread formed is insufficient, then select a slightly smaller drill diameter."

So, I see no mention of what quality of drill is being used, is it top quality e.g. Dormer Branded A100, or a budget quality, or well worn? All have an influence.

[PeteT]

Whilst I know that there is a difference between M1 and 14BA I have intermixed them on occasion with no problem, at least on crankpins. My most common use is an Ultrascale flush crankpin nut on a front axle behind slidebars, and many times these crankpins are AG.

Philip

Hi Philip, this is a bit tangential to the topic but I thought might be of wider interest so went for a quote rather than PM - when using the ultrascale flush crankpin, do you recess the wheel so as to accept the crankpin as it is, or shorten the crankpin so as to sit flush against the flat boss on the wheel?

Thanks, Pete

[Philip Hall]

Pete,

I shorten the flush crankpin bush and use a thinner small washer over the steel pin to space the rod away from the moulded boss. I also file two small flats on the head of the nut to provide a place for a pair of flat nosed pliers to grip when tightening.

Philip