Author Topic: fork end cap suprise  (Read 537 times)

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Offline tck

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fork end cap suprise
« on: 22 Apr 2020 at 15:44 »
I set up to do battle with the fork end caps. upside down in a vice ready to apply flame and force and noticed the hexagons were different! one of the end caps was made of brass or at least a non ferrous yellow I cant see cash strapped Douglas making them in brass (unless you know different) from the faded paint of poly blue under various coats of chipping paint I surmise it was off  a MK5  who knows what struggles I shall have undoing it, good job its a spare. I will try the aluminum cap first....

Offline tck

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Re: fork end cap suprise
« Reply #1 on: 23 Apr 2020 at 15:02 »
I should rename this thread mystery nail surprise.
While I wait for a suitable socket for the brass end cap I continued to dismantle the other leg after a lot of heat penetrating fluid all the parts are out except my  31279 abutment for spring and that's about half way down at the moment. However looking down the tube from the top through the top nut hole I can see at the top of the leg tube just underneath the bottom yoke at about 2 o'clock round the tube is a (say) 3/32" 'nail" (well it has a pointed end) and it sticks out into the tube about 3/4 of the way across the tube. i can feel where it is located on the outside and on the outside of the other leg at about 10 o'clock is another pin. Is it a rusted remains of part of the fork? its not in the parts list what does it do if anything?

Offline eddie

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Re: fork end cap suprise
« Reply #2 on: 23 Apr 2020 at 15:37 »
They are the locating pins that are inserted to keep all the parts in the correct position while the fork assembly is brazed up. At the point of construction, they would also have stuck out on the outside - after cooling, they were then ground back flush.
   At the top of each fork leg, you will also find a hole about " dia - this is where the brazing rod was fed in to fill the joint.

   Regards,
                  Eddie.

Offline tck

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Re: fork end cap suprise
« Reply #3 on: 23 Apr 2020 at 18:52 »
Thanks Eddie
Because the piston was so gummed up I had to help it down with a rod down the center of the spring and I was worried about the 'nail'
Incidentally the spring is a parallel round section with equally spaced coils I have seen the dimension of 10-1/4" banded about as the appropriate length but not sure if that apply's to just the rectangular graduated spring or this round parallel one as well
Also I am sure I have seen mention of a 'bump stop' where a taper forms a hydraulic lock and prevents a hard bottoming of the fork however when I take the cap off the end  (which has a flat inner face) the next thing I can see is the nut on the stud of the restriction washer which is what I assume is correct from plate 24 so I am nor sure where this bump stop is?

Offline eddie

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Re: fork end cap suprise
« Reply #4 on: 24 Apr 2020 at 10:21 »
OK, just taking into account what you have said in your last 2 postings, I would guess that the heat you have applied to the end cap (SA1515) has resulted in the cap expanding, with the damper plug staying in the bottom of the fork leg. Having an SA number means the cap is a sub-assembly and the individual parts are not numbered separately - hence the damper plug is not shown on it's own in the spares list. If you now look at the bottom of the fork leg, there should be about .100" of plain dia before the start of the fine thread (for the cap). If you have more than that, the damper plug is probably still jammed in the bottom of the fork leg. If the damper plug is still in the bottom of the fork leg, the bore will be about 1 3/8", whereas the bore of the actual fork leg is more like 1 7/8". If the damper plug is in place, then you will need to tease it out in order to get access to the piston - there will be a thin copper/asbestos washer(31356) between it and the bottom of the fork tube. When you get both end caps off - check carefully that both damper plugs are the same length - when we were involved with club spares, the Club had 3 different lengths available (probably something to do with the different damping required on Plus and Comp models!).
  Moving on to the springs - originally, there were 2 options available, both being wound from 5/16" square wire - the lighter, standard spring(32256) being taper ground on it's OD. The heavier spring (36298) was parallel on it's OD. I have also heard of some forks having round section springs, but, at a guess, I think they may have been supplied as the only alternative after the production run of original springs had ended.  10" long is the sort of odd figure Douglas would have specified, but I have heard of the abutments being stacked to obtain better ride heights - so the actual length may not be too critical - providing they don't become coil bound.

   Hope some of this helps,
                                           Regards,
                                                           Eddie.


P.S. The overall length of the standard damper plug is 1" exactly.

Offline tck

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Re: fork end cap suprise
« Reply #5 on: 24 Apr 2020 at 11:39 »
FYI
Thanks again Eddie
The damper plug is out! its internal diameter is 1.265" and its length including a very flat copper washer is 1.2" ( ie the under flange length)
I will keep an eye on the other length when I get the cap off
and the spring is a rounded one but it is 10.250" length
I have  attempted a picture but i cant seem to preview it at the moment but its in the attachment box


Offline hoejmark

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Re: fork end cap suprise
« Reply #6 on: 24 Apr 2020 at 16:42 »
Hi TCK
Good to see a picture of your front fork springs as it seem, that my MK5 does have the same type of coil springs out of round wire instead of square wire ones.
I thought mine were non original ones! The dimensions og mine look like the same as yours.

https://www.douglasmotorcycles.net/index.php?topic=5961.msg21893#msg21893

Regards hoejmark


Offline tck

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Re: fork end cap suprise
« Reply #7 on: 24 Apr 2020 at 17:06 »
Having read that thread I have located a square spring calculator https://www.newcombspring.com/resources/compression-springs-rectangular-wire
now if I had a square spring   :roll: perhaps i could make a comparison between those spring rates

Offline Neville Heath

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Re: fork end cap suprise
« Reply #8 on: 24 Apr 2020 at 19:07 »
Here is a picture that went with a Spannerama article I wrote several years ago. It may help. If you need anything measuring let me know.
Neville

Offline tck

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Re: fork end cap suprise
« Reply #9 on: 25 Apr 2020 at 10:52 »
Yes it seems that rectangular springs for a given space will exert more force than round ones but are more prone to stress (as I found out all those years ago when the spring came out in a hundred pieces)
I think Eddies reason for the round springs as a replacement for the original when stocks ran out is true (perhaps even from the last of the factory output?) as I note on the web  the figures for economic batch sizes of non standard square springs are in the thousands because of the wire cross section

Somewhere in my cavern I have a strong spring balance that I used for calibrating valve springs, with some wood adaptions I should be able to come up with a figure for lbs for the first inch of compression

I know that often Vincent fork springs were the same for comets and twins but there was an option for beefing them up by the insertion of an additional spring inside the first so thats an option perhaps

I was stripping these forks as a stop gap as i waited for other parts for prior projects to arrive, I do have another set of forks on the bike I am going to rebuild  as soon as I get a spare bench, so I need to inspect what springs that has before I take this further

Offline tck

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Re: fork end cap suprise
« Reply #10 on: 26 Apr 2020 at 11:19 »
Addendum
The other cap is off the other spring is the same round coil  and the damper plug is the same length at 1.2" it was also stuck up the tube and needed a lot of tapping to get it out the link seal face is badly scored on both links so I will have to try Eddies shim and aradite trick. even if the club spares was open i dont suppose they do smaller link bushes so I will have to turn some up in P bronze. all good fun.

Offline phil1

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Re: fork end cap suprise
« Reply #11 on: 26 Apr 2020 at 16:51 »
Hi tck
            While your doing front fork end caps I thought I'd mention my alloy plugs turned approx 3/4 Dia x 1/4" inserted in the inner recess to give a little more thickness for a 1/4 BSF drain plug so I'll never ever have to unscrew those troublesome end caps again, hope this maybe some food for thought. Regards. Phil.