Another excellent joke, Mr. Douglas

[cardan]

Anyone who has played with Douglases is familiar with the joke that is the Douglas thread system. Pretty weird.

But I've just come across something even stranger. The hardened steel tappets in the RA (with the cam follower foot on one end, and adjustable ball end on the other to engage with the bottom of the push rod) pass though a brass/bronze tappet block. To stop them rotating, a hole is drilled through the tappet, and a pin pushed through so that is projects from either side. The pin engages in a slot in the brass tappet block.

The pin is 0.525" long and (wait for it) 0.121" diameter! The tappets and tappet blocks in my engine are fine, but the pins are worn flat where they've been sliding in the slot.

Any suggestions about replacing them? Maybe the shank of a 3.1mm (0.122") or a No. 31 (0.120") drill bit might work? A thinner-than-average 10g spoke (0.124")? Pay to have four 1/8" needle rollers ground to size (or find some appropriately worn ones)?

Tell me, Mr. Douglas, what's wrong with 0.125"? What does the drawing say?

Aghh...

Leon

[Doug]

Leon,

I am not aware of a drawing for the tappet. The measurement I came up with was 0.1195 for the pin. This would suggest they were leaning more towards the size of a #31 drill. It does not seem to work out to any British or American wire gauges, which was my other thought. Since the tappet did not belong to me, I did not press the pin out to see if they made the center section slightly larger - a stepped pin - to make it easier to press together.

You can buy hardened drill blank material for number drills, but it may be cheaper and easier to source four drill bits. Regardless, if you holes turn out to be 0.121, you will need that size pin for a tight fit or ream the hole to the next larger size that you can get a pin for (while remaining below the slot dimension.)

I have no idea why they picked a bastard size. One might say they started with readily available 1/8 inch wire and gave it a light pass through the centerless grinder. But they had things like 1/4 and 3/16 rollers where they had to start out with oversize material to clean up at the finish diameter, so that does not seem to be a iron-clad reason.

I am facing this same problem with my RA project, though I do not have any original tappets yet to refurbish. There were two thoughts. One was to replace the pin with an oblong key to provide a better wearing surface. This would be easy with a wEDM machine to cut the parts, alas I no longer have access to one. The second was to upgrade the tappet assembly to the DT which got rid of the cross-pin, the slot, and the two-part tappet body and used instead the head of the tappet to keep it from rotating. Not as original (for the RA), but lacking original parts it is a viable plan.

-Doug


[Fix typo. 19Jun14, Doug]

[cardan]

Thanks Doug. The pin I took out didn't seem to be stepped or tapered. I'll press out the other three, measure them up, then pursue the drill bit path.

Leon

[Doug]

For those that have not seen one, pictures of the RA tappet and tappet guide.

-Doug

[Douglas52]

Messrs Douglas certainly like to take the scenic route when it comes to detail design.
With regard to the pin, a thou interference  is a good hammer-in  fit for around 1/8" pin and #31 is a standard drill size.
So I guess Mr Douglas figured t was cheaper to use a standard #31 drill and make an oversize pin than to make a special drill and use standard pin stock. Thank heavens he didn't use the two standards  and swage them together.
 Still waiting for a rational explanation of the EW350 crankshaft design ..............
Cheers
Steve

[Hutch]

Hi Leon and Doug,

I just grabbed a circa 90 year old Douglas 11 gauge spoke at random (yes Leon you know where that may have come from  ) I cleaned off the grime and old enamel paint trying not to abrade the surface and measured the diameter. Depends where you measure, but the diameter on the one example I have measured comes out between 0.1141" and 0.1185". Majority of the diameters are about 0.116" The spoke is well used and who knows how stretched it may be, or how new the dies were when it was made. If I get a chance I'll measure some others.

The nominal size for 11 gauge Birmingham Wire Gauge (BWG) appears to be 0.120"

Could this be the source of the pin material or is the diameter a bit small compared with your pin ?? Would have been cheap and available to Douglas in the 20s and the 11 gauge dies were probably worn out by then!

Cheers

Ian

[Bob M]

You fellers are a deep abiding comfort to me. I occasionally visit both here and the Scott site whenever I feel I've gone a bit too far downmarket now I play with Villiers stuff.

I get depressed because I just have these boring bog reliable little motorbikes with no style at all.

Then I read about the latest Scott woes and strange Douglas engineering decisions and my depression lifts.

Keep it up, you're doing a good job. Thank you very much.

Bob M

[Hutch]

Hi Bob,
Glad we provide you with some entertainment ! Must be time for you to get an early Dougie and join us in the fun (and frustration..)  .

I have been looking up some histories of wire gauges and realised I've mixed BWG and British SWG. oops. The Douglas 11 gauge spokes appear to be  SWG, so 11 gauge = 0.116" as I measured.

So is your pin 11 gauge Birmingham Wire Gauge, Doug, as it is close to 0.120" ? Would be interesting to see how Leon's pin compares to this as it was measured at only one thou over?

BWG still appears to be used as a standard for medical equipment i.e. needles and catheters.

Cheers

Ian

[cardan]

Well I removed the other three pins. You guessed it, they are all slightly different sizes: 0.121, 0.122, 0.122, 0.124

I can only think that the holes a made in the tappets after they have been hardened, then the holes drilled with a tipped tool or even ground. To get the appropriate press-in fit, I can imagine a bench with a fitter selecting pins of different diameters (arranged in jam tins, I reckon) to suit the tappet in front of him. As the tool for boring the hole wore, so smaller pins were fitted?

My plan is to open the holes out and press in 1/8" rollers as guide pins.

Leon

[Hutch]

Sounds like a good plan Leon. Looks like number/metric drills can be bought as blanks which might help if you didn't  decide to go that way;
http://www.tapdie.com/html/drill_blanks_hss_metric_drill_blanks_-_hss_inch_drill_blanks-_hs.html

[Doug]

My plan is to open the holes out and press in 1/8" rollers as guide pins.

Or use dowel pins. Just as hard and easier to get in longer lengths.

It is surprising the variation for that small a diameter, it means they had to use a tedious selective fit/assembly technique. This measurement I presume is in the center of the pin, where pressed through the tappet? If so, then no need to check the hole sizes as a 0.003 inch range in pin sizes means the holes had to have been different sizes. For that diameter tolerances would have to be in the order of 0.0002 inch to maintain a consistent press fit from pin to pin. Otherwise I would suggest the holes are all reamed just under, say, 0.124 inch and the pins have the ends slightly reduced to make it easier to press into the holes. I.e., slightly barrel shaped, with some tapered down more than others?

I am not sure the Sports used the same tappet orientation arrangement; it looks as if they used the head of the tappet, like done with the tappet roller on its predecessor the 4hp. The early OW/OB was intended to use the same tappet and guide (according to the handbook), but whether it actually did and for how long is unknown. So the tooling should not have been too worn out, in effect being introduced new for the RA.

Otherwise, out of ideas as to what Mr. Douglas was thinking!

-Doug

[eddie]

I once suggested to Bill Douglas that his great grandfather must have been a cantankerous so and so - fitting his bikes with these odd size threads. To which Bill replied, "No, he wasn't cantankerous, he was a canny Scot who knew he couldn't get very much for his bikes, so he made sure owners went back to him for their spares!"

  Another case of  'It's no good getting old, if you don't get crafty!!!'

  Eddie.

[oily bloke]

This is exactly how the Japs make their money.
Sell em cheap but make sure the spares are model specific, have annual dimension and colour changes to thwart the pattern part makers and make them very pricey.
There really is nothing new!! Our ancestors were already on the ball.

[Douglas52]

Interesting topic. According to several references the standard tolerance on a hole this size would be minus a thou to plus half a thou. Drill s are often made undersize to offset their tendency to drill oversize - hence the minus tolerance. So the tolerance on a "standard" hole is one and a half thou (probably greater in the 1920s). But the required interference fit range is 0.7-1.0  thou; less than the hole tolerance. This means either the hole (and possible the pin) will need to be resized, or the commonly used technique of selective fitting could be used. A range of pin diameters will be held and the pin which has the correct "feel" will be selected. So the pin that fits the hole is selected  instead of spending time and money producing components which have the required interference. 

[cardan]

OK: the first one is done, and I'm very pleased with the outcome. 

A "standard" interference fit is something like 1 thou per inch of diameter, so the if the pin is to be pushed in and stay there the 1/8" hole has to be just a fraction of a thou smaller than the pin. Fiddly.

My procedure was very low tech, and is the sort of thing that can be done in most sheds with very little equipment. I made a very slightly tapered cast iron cone on the lathe, about 0.120 at the pointy end and 0.125 at the thick end, about an inch long. The plan was to use this with valve grinding paste as a lap to increase the size of the hole just a few thou. I ended up doing the lapping by hand. (Doug don't read the next bit!) I mounted the lap in the jaws of my Vice Grips, and held the Vice Grips in my bench vice so that the lap was vertical and well away from the vice so the foot of the tappet would clear the jaws of the vice. I then just applied grinding paste and worked the tappet back and forth on the lap as if grinding a valve. The centre of the tappet is hollow where the hole is drilled, so it was more a task of opening out two rather shallow holes rather than one deep one. Just flip the tappet over regularly to work on the other side.

The progress of the hole size was measured by how the far down the cone the tappet moved. I also stopped regularly to wash the grit off and make independent measurements, using my various drill shanks and the tapered shaft of my scribe, some engineers blue and a micrometer. As I got close I offered up the roller (from an friend who has Indian motorcycles - "help yourself"). I had one gentle try at pressing the pin in (using my vice), but it was too tight. Open the holes out just a bit more and it pressed in nicely.

On assembling the tappet in its block, it feels excellent. Glides in and out, no free play when oil is present, a tiny bit of rotation (a couple of degrees probably) as the pin rotates in the slot but no roughness if I rotate hard in each direction and slide the tappet in and out.

Time? A few days thinking and measuring, a few wasted hours, then about 90 minutes of work. Cost? Nothing. Satisfaction: unmeasurable.

On to the next three. Thanks for all the helpful suggestions.

Leon

[Doug]

Leon,

I have nothing against hand lapping the holes. Though I might have also suggested electro-chemical machining or laser ablation...   

-Doug


"... unmeasurable", pun?

[cardan]

Doug - it's just the embarrassment that comes with the very amateurish setup. I'm sure you don't even own a set of Vice Grips! I feel totally inadequate not using a centreless grinder as part of the fix...

Answering an earlier question: yes the unworn centre sections were measured, and the holes were very different sizes. The largest hole took a 1/8" roller after about 2 minutes' work. The smallest hole took an hour or so. Anyway, I've done my own "fitting" now and all four tappets are snug, but not too snug, and smooth in action.

Bob - pleased we supply you with some amusement. I wonder if the future of the vintage movement is not in one-make clubs, or at least informal groups. Douglases are interesting bikes, but of more interest is that people here take interest in models that are not their own. I guess it's not such a big step to consider a different model of the same marque? I have a feeling that "general interest" in old bikes is flagging.

Leon

[Doug]

Leon,

Why of course I have Vice-Grips. I use them all the time on my Harley-Davidson!

-Doug

[cardan]

You sure know how to hurt a guy :-)

[Daren W Australia]

we need a like button as per face book you make me laugh Doug