Hello all,

I'm the current custodian of  one of the Bert Thorpe alloy motors.

A friend of mine who is an extremely competent Engineer has split the crank and has expressed his concern that it might be beyond use. It has been skimmed to accept oversize rollers previously in its life. The conrods are home made alloy rods 

It's engine is 68mm stroke and 68mm bore and from its engine number I've been told it's a 1925 TT jobbie.

I'm hoping to fit a new crank and a pair of new rods in this engine, is anyone making new cranks and rods?

Cheers,

Ed

Such a lovely day I took the T6 out for a short twenty mile ride and the more I ride it the more I am falling in love with it, it's so smooth, vibration free and handles like a dream and as I live in Wales there are a lot of hills but the T6 laughs at them so much torque.

 Am hoping next week to get my head around the front brake, I will post results as and when.

Ride safe, Colin

Sandro,

Yes a picture is worth a thousand words as they say! (..or I my case maybe two thousand    ).

I have a similar one but it is for 2 3/4HP not 4 HP. It is loosely attached to a gearbox and I can take some pictures of it disassembled for you tomorrow (as it is the evening here.)

These "old traps" may challange us, but worth the effort I think 

I apologise for taking a long round about way to answer your question but I hope you are close to getting it apart with out resorting to the lathe solution.

Cheers

Hutch

good morning, this is the photo of the half cater with clutch plate still mounted; it is different from the one in your photo and should be a 1916/1917.
In fact, the serial number 4836 is stamped on it.
Today I will try to disassemble the piece again, but tomorrow I will use the lathe to widen the internal hole of the gear to the depth of the bearing and then I will disassemble everything.
I am starting to hate these old traps!!!! 

Sandro,

Patience is often required when dismantling these old machines that have parts that have been together for a very long time ! :-). Good that you are able to re-make the gear if things don't go to plan but I think it should still come apart without being too violent with it!

I am wondering if you have the earlier cork inset clutch type? I don't have any good pictures of the 4HP version but I found this picture of the fitting 642N it uses to attach to the 605N gear. On the 2 3/4HP this is a left hand thread and I assume the 4HP version is the same (I don't have one to check).

(Note that the clutch basket is not fitted on this gearbox). For comparison part 2206N is shown and this is used for 1920 onwards 4HP models. Which do you have?

Because the kick start acts directly on 642N for these earlier clutches via a ratchet, it could be tightening on the 605N gear when the engine is kick started and the bike is not in neutral. This means that it can be very tight on the 605N gear. and hard to undo. It may require a long stiff bar and a quick shock to loosen it (left hand thread)

(The 2 3/4 HP models with 3 speed and flywheel clutch have a similar problem with the kickstarter tightening up the nut that holds on the cush drive to the input gear).

If you get a chance please take a picture of what you have. and best of luck with it on the weekend.

Cheers

Hutch

Thanks for your advice Ron

Good morning and thank you for reply, next Saturday I'll try to look at the whole assembly again.
I tried to unscrew the right or left parts, heating them up to 400 degrees without success.
I tried to separate the two parts with a 12mm (1/2") diameter threaded pin, always heating the hollow stem, but the thread tore.
If I don't succeed at the next attempt, I think I'll completely turn the gear head and then rebuild an identical one.
I have no problems with the gears because I work in this factory:
www.biemmeingranaggi.it.
Thanks for the information, I'll keep you posted

Biellestanche,

I had a quick look at the The Handbook of the Douglas Motorcycle 4HP 1920 and here are the instructions for dismantling the 4HP Dry Plate Clutch of 1920.

note the section "To remove the boss on which the clutch is carried from the gear-box shaft it will be necessary to unscrew the nut with a box spanner"

(i.e. unscrew 2222D from 605N to be able to remove 2206N). Is this a right or left hand thread? I will not trust my memory and will check.

The pictures in the copy of the 1920  4HP Handbook I have are not good as they are a photocopy of a photocopy of a photocopy, but I have attached one from the 1920 Douglas advertising brochure ("Peace Programme") which appears to be the same diagram as in the Handbook. It does not show 2222N clearly. I have attached the diagram of the similar clutch from the 2 3/4HP Handbook from 1925 and it shows the position of the similar nut.

If you have removed 2222N and 2206N will not come off 605N then the application of some heat to 2206N may assist along with a suitable puller, or supporting 2206N and using a suitable soft drift (+ hammer) on the end of the shaft of 605N, being very careful not to damage the threads.

I hope this is the information you are after and I have the part numbers correct.

If there is no thread on 2206N and hence no nut 2222N I would be interested to see a picture of it. I'm not sure that would be what Douglas intended? (maybe another forum member may know more about this? I only have 2 3/4HP experience with these style clutches and no 4 HP experience)

-Hutch

A good welder could likely repair that.
It would need GOOD  penetration through that crack.
And be machined back to allow for the bearing function. (?).

The weld fillet on the outside would need to be retained - for extra strength.
And likely need to be stress relieved/heated, to avoid any difficulties with the heat zone of the weld.
Welders employed near light aircraft airfields would be your best bet.

Nothing to lose trying, could be tough to find another ?

Hi Biellestanche,

I can now see that I misunderstood your question and you are not having trouble getting the bearings out of the gearbox casings (which is the usual problem) but how the input gear is connected to the clutch. Sorry for the answer to a different problem (!) and any confusion caused.

I am assuming your gearbox and clutch is not the earlier cork inset version (1914-1919) but is the later circa 1920 plate version? - if not please let me know as they are assembled differently - a picture of the clutch would be useful to confirm this.

So, is your question, how to disassemble the clutch to get to the clutch body (2206N) off the input shaft?

I will see if I have a good cross section or diagram of the clutch assembly.

I note that part 2222N is listed as the nut securing 2206N. It does not appear to be shown in the 4 HP parts list. This would have to be unscrewed before 2206N can be removed from the input gear 605N.

I will see if I have a good cross section or diagram of the clutch assembly, including the 2222N nut.

Large hammer should hopefully not be necessary! :-)

Cheers

Hutch