1913 crankcase hole

[Steph1960]

I am putting the final bits back on the bike now and I don't have anything to put in this 1/4 threaded hole in the crankcase, it's only 3/8 deep so I would guess it backs onto the skirt of the front barrell. The part book looks to show a bolt with a special head that may go in here, part number 473,  but I am not sure. Does anyone know what the hole is for and what goes in there please?

[LENBOY]

Hi
The shouldered bolt that fits is to take the front fitting of the primary chain guard which is of a hooked shape and fits over the bolt.
Best wishes
Len.

[Steph1960]

Thanks Len
In that case I am missing the primary chaincase then. Does anyone have pictures of this where the belt drive is on the left please?

[cardan]

You can see it in the catalogue illustration of the Model N https://www.douglasmotorcycles.net/index.php?topic=9427.msg37496#msg37496

Looks like a more-or-less flat strip of metal that loops around the outside of the chain. The single-speeder is pretty unusually, so most people won't have seen the chain guard for it.

Leon






Snip added from Leon's link above - Dave, 24Mar2023

[Steph1960]

Thanks Leon
I'll put a bolt in the hole for now and do some research over the summer, if all else fails there should be some single speed Douglas at the Pioneer run in October.

[Hutch]

Hi Steph,

Further to Len and Leon's information, here is a picture of the stud that goes in hole on the drive side crankcase half for the primary chain guard for a 1915 2 speed 2 3/4hp (from the 1916 "The Handbook of the Douglas Motorcycle").

(BTW, watch out for the tread pitch in the crankcase hole tho', as I think on some of the ones I have it is 25 TPI and easy damaged (I will double check on the weekend) - the rest I have been butchered in their previous lives unfortunately.)

There appears, in the Douglas booklets, to be two methods they used to attach the front of the primary chain guard to the crankcase - one is the "hooked" method, as Len suggested, and the other I will call the "hole" method. (I will put some pictures together of this when I get a chance). I don't know if the stud goes with the hooked guard or the shouldered bolt goes with the "hole" method or the other way around etc. and will see what I can find out.

The primary chain guard is in two parts (front and rear) that bolt together (can just make it out it in Leon's / Dave's picture above) and as Leon says, is made up of flat sheet that has been curved a little bit in the direction 90 deg. to the chain. On the two speed 2 3/4hp's I have seen, there is a cast bracket that holds up the rear of the primary chain guard and it is held in place by one of the studs to the gearbox and looks (fairly obviously) like that method will not work with the Model N.  In the picture from the 1916 booklet for the primary chain guard (only one type pictured) it shows a bracket 651D. Part 650D (no picture) is listed as "Chain Guard Stay (single speed models)" and Part 651D (shown) is listed as "Chain Guard Stay (other models)"

The strange thing is that 651D doesn't look anything like the ones I have seen for the "other models" so I am wondering if the stay shown is the single speed one not the two speed variety??? - (i.e. typo's by Douglas in the parts list are not unheard of!)  Not sure at all and will have to investigate further.

Cheers

Hutch

[Steph1960]

Thanks Hutch
I think the chain guard will be reasonable  to reproduce if I can find one to take accurate measurements from, work in progress for the summer. I unsuccessfully tried to find a temporary bolt for the hole today, I cant measure the TPI of the thread but it looks like it's a 19/64 thread size, hopefully you will be able to confirm the exact size next week.

[Hutch]

Hi Steph,

I'm still trying to find out what the thread in the hole for the primary chain guard was originally on the crankcase half - I suspect the veteran ones started life as 17/64" 25 TPI but no proof found yet.

I have 5 crankcase halves and 3 of them are the later type which didn't have the hole drilled and tapped, as the chain guard attaches in a different way, (they still have the cast in boss where the hole would be tho'). On my 1915 version the threads are completely missing but it "might" have been 5/16" (TPI unknown) at some time in its life before it came to me. One other crankcase half I have has a rough-ish thread left in it and it appears to be 5/16" but hard to get an accurate measurement of the TPI with the thread gauge as the threads are not full depth anymore. Might be 25 TPI but cannot be 100% sure.

Unfortunately, the outer threads are a bit mangled due to the primary chain hitting the little cast in boss (so it appears the engine was run for some time without the chain guard and no bolt / stud in the hole) so I cannot simply thread a bolt in to see if it fits. I will attempt to clean it up and see if I can get a better measurement. A 5/16" 25TPI bolt I tried makes a start but I don't want to force it. (I might try the method of filling the hole with silicone and then unscrewing the plug when it has set and use for measurements. I have done this before a long time ago - this time tho' I must remember to use a release agent before filling with silicone tho'! :-)).
 
Edit - of course anything could have happened to this hole in the life of the crankcase half - it could have started life as 17/64" and is now 5/16" !!

I had a look at Chris Wright's excellent thread guide on this forum, but didn't spot the one for the primary chain mounting stud / bolt - maybe I missed it?

https://www.douglasmotorcycles.net/index.php?topic=1102.0

I asked a fellow forum member (Thanks Glenn!) and he found that a 1/4" 26 TPI bolt was a good fit in a crankcase half he had - would be interesting to see if the thread is actually 25 TPI as the 26 TPI cycle thread bolt will probably fit ok for the limited number of threads it has to engage?

So all I can suggest is to carefully try it with various bolts (i.e. the barrel bolts are 5/16" 25 TPI and the cross bolt that holds it in and the crankcase halves together should be 17/64" 25 TPI and 1/4" and 5/16" 26TPI  cycle thread bolts should be easy enough to find) and see what fits. Unfortunately the threads on this hole are very easily damaged and not many originals survive. I think this is why Douglas went away from this mounting method on the later models??

Edit: Looking at your picture again the hole does look like it is about the same diameter as the 17/64" bolts that hold the crankcase halves together ? Hard to tell.

I am still cogitating the best way to go about repairing my 1915 case as there is not much metal left and I wouldn't want to make it worse than it already is and I am also considering maybe not repairing it as it is too risky and attaching the primary chain guard (when I have made one) with a different method to what it was originally. I think I have only seen one original primary chain guard for sale in the last 10 years and it went for silly money of course!!

I will keep on looking into this when I get a chance. Sorry about the verbose post for such a "simple" question ! :-)

Cheers

Hutch

[graeme]

Hi Hutch

For your mangled hole - use Devcon to repair it, then tap it with whatever thread type you want! With the cover fitted, who will ever know?

Cheers, Graeme

[Hutch]

Hi Graeme,

Thanks for the info. on repairing the hole - a great suggestion. I have used Devcon before - the bronze putty version. Not cheap but did have some success on some jobs and mixed result with others. Heat seemed to be one of its enemies from memory (150C? - that is most likely ok for the location  of the hole tho'). I have been looking at some of the JB Weld products and maybe might try one of those instead as they seem to resist heat fairy well and are much cheaper last time I looked. I have investigated (and partly made) a internal thread insert to go in the hole (i.e. basically a special Douglas thread helicoil), but as there is not much metal left where the cylinder goes I was pondering how well it was going to work before I go ahead and tap the crankcase half hole out.....

I have a friend who can weld it up and then I can re-drill and tap the hole, but as it is  made of the usual Douglas "very variable quality alloy" (!) I was only going to attempt that as a last resort (I have already asked him to do some practice welds on a scrap crankcase in preparation for another tricky repair job I have to do sometime in the future on an OHV crankcase:-) )

So I may try the JB Weld route as it won't make the crankcase half any worse if it goes wrong and if it does then I can try the next method on the list out etc ! Luckily, as you say, it really can't be seen, but as quite a few of my "bodges" in the past on similar jobs have failed I'm keen to try and do a robust repair and hopefully it will last :-).

Cheers

Hutch

[Steph1960]

Hi Hutch
A 1/4 bolt goes straight through the hole and a 5/16 bolt doesn't grab. I turned a piece of mild steel down to 17/64 and it feels about right if it was threaded. The cost of a 17/64 x 25tpi tap and die set from the club is £277 + post and as yet I havn't been able to find another supplier. I wont be spending £277 to make one bolt so will probably drill and tap it out to 5/16, as Graeme says, you cant see it anyway.

[eddie]

You could always go metric - 7mm x 1mm pitch is only about 10 thou larger than the original 17/64" x 25tpi.

  Regards,
                Eddie.

[Steph1960]

Thanks Eddie, I have a 7mm die so I'll check the thread pitch. If not ill buy one and try it, probably only a tenner on ebay.

[Hutch]

Hi Hutch
A 1/4 bolt goes straight through the hole and a 5/16 bolt doesn't grab. I turned a piece of mild steel down to 17/64 and it feels about right if it was threaded. The cost of a 17/64 x 25tpi tap and die set from the club is £277 + post and as yet I havn't been able to find another supplier. I wont be spending £277 to make one bolt so will probably drill and tap it out to 5/16, as Graeme says, you cant see it anyway.

Phew.....at £277 I would possibly be using a 7mm x 1mm thread as a 7/16" x 25TPI substitute as well - great suggestion Eddie ( I have seen it mentioned on this forum as a possible replacement size for the big end bolts for the 2 3/4 HP),

 I have the special taps and dies from the club, but I bought them years ago when they were about £60 a set - I thought they were expensive then!. I am super careful using them as if I break or wear them out I probably won't be getting anymore......

Best of luck with it Steph,

Cheers

Hutch