Author Topic: Bang! What do you think?  (Read 12667 times)

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Offline tck

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Bang! What do you think?
« on: 12 Jul 2007 at 20:46 »
Douglas T35 bought with 29 miles on clock after what looks like a good rebuild.check oil and settings started first kick every time
Gentle runs out and all OK however at 52 miles at about 35mph it suddenly locked up with a bang. I pulled in the clutch and stopped. I thought it was the rear wheel or transmission because it was so sudden. A piston seizing (and I have had a few) gives a little warning. The drive to the wheel was ok in neutral but nothing moved if I tried the kickstart.If I pulled in the clutch the kickstart would depress but only with a fair amount of resistance.
Now I have a racing bike to rebuild for the next meeting so I know I shall not get around to finding out what the problem is for a week or two
So lets see what you all think it could be, and then I will let you know what I find
(The bike is for my wife to ride in Manx week so I shall have to get it sorted!) :mrgreen:

Offline graeme

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Re: Bang! What do you think?
« Reply #1 on: 12 Jul 2007 at 23:30 »
Not knowing of postwar Doug gearbox quirks, I would be thinking a bottom end seizure. I had a big end seizure once on a dirt bike I was riding - and there was no warning, just an instant lock-up. Freed up to the point where you could kick the machine over with a lot of effort, but was obviously not right. Sounds like the same symptoms you have  :(

Offline KiwiJohn

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Re: Bang! What do you think?
« Reply #2 on: 13 Jul 2007 at 01:28 »
Crankshaft has become disconbobulated, or something.  They are only squeezed together and apparently it is rather tricky to get everything back without it later coming adrift. :cry:

Offline Doug

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Re: Bang! What do you think?
« Reply #3 on: 13 Jul 2007 at 02:25 »
It does sound as if the crank has shifted out of alignment. The rear main shaft and clutch may be skewed, such that releasing the clutch enables you to kick it over, but the clutch plate and spline are still binding due to the misalignment. Try slacking off the cap head screws holding the trans to the engine to see if it will free up.

The press fit of the post war cranks is a fiddly thing. It needs about 0.002" interference, which is hard to press up square without permanently stretching the eye and ruining the fit. Factory refurbished cranks were honed and pressed up with a 0.004" oversize crank pin. Apparently they felt the cranks were only good for one press assembly per fitting.

The post war Dougies will solider on with ruined crank pins, as I personally found out. They just feel rough, as if you had too much ignition advance. Even to the extent where the rod is so tight it will support the weight of the piston.

-Doug

Offline Ian

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Re: Bang! What do you think?
« Reply #4 on: 13 Jul 2007 at 02:26 »
Wow, I have never heard of a cracnkshaft being discombobulated !!

Discombobulate
Verb

1. Cause to be confused emotionally.

2. Be confusing or perplexing to; cause to be unable to think clearly:

Offline eddie

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Re: Bang! What do you think?
« Reply #5 on: 13 Jul 2007 at 06:25 »
There are a few 'Cranks' in this world that are 'Disconbobulated' - and their not all Douglas owners!!!

Offline Ian

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Re: Bang! What do you think?
« Reply #6 on: 13 Jul 2007 at 06:42 »
although I guess when looking at broken Douglases the Douglas owners would be very much that way !!

Offline trevorp

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Re: Bang! What do you think?
« Reply #7 on: 15 Jul 2007 at 05:39 »
Eddies pdf file see here explains it all in detail , this file allows and good machine shop to follow his instructions
Doug too has done a great article here

Offline tck

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Re: Bang! What do you think?
« Reply #8 on: 17 Jul 2007 at 12:48 »
Hmm.... Well its still at least a week or so before I can look but I think the crankshaft moving after a rebuild  fits the picture
I can see that its not a 'put it in a strong crate and send it to Alphas' job
I am the proud possessor of a Douglas stationary engine the barrels will fit a T35 so if its crankshaft is good and true will that fit an early T35?

Offline Doug

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Re: Bang! What do you think?
« Reply #9 on: 17 Jul 2007 at 13:16 »
Not without alterations. I will leave the details to those that know the T35 better, but I seem to recall two points mentioned in the NCR in years past. First that the barrels are not the same length as the motorcycle, but I forget if longer or shorter. Also, there is something about the front journal of the generator engine that needs to be machined to fit/suit the motorcycle engine. It can be done, as I have heard of several generator engine cranks being used in T35 and Marks.

-Doug

Offline eddie

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Re: Bang! What do you think?
« Reply #10 on: 17 Jul 2007 at 15:58 »
The crank from the generator engine does not have an oil thrower for the rear main bearing, so a thin spacer may be needed in order to position the conrods centrally in the bores. Also the plain portion on the front end that carries the timing gears is a little shorter than that of the Mark series. This can usually be overcome by turning a shoulder on the retaining nut (so that the spigot fits inside the outer timing gear). Another problem that can arise - and is not immediately obvious - is that the crankpins may not have as large an interference fit in the crank webs as the motorcycle cranks - so may be even more prone to twisting. I found this out the hard way - I experimented with fitting an electric starter (driving the front of the crank) and this was powerful enough to twist the crank as it attempted to get the flywheel spinning. I ended up producing crankpins 1 thou oversize to overcome the problem (still keeping fingers crossed).
        Going back to the original problem of the engine suddenly siezing - maybe the rebuilt engine had a front main bearing that was too neat a fit on the crank journal. In the original T35 handbook, Mr Douglas gives the reccommended clearance as .003"! Less clearance than this usually results in the bush siezing on the crank and turning in the cases - the locating peg then rolls the flange back, taking up all the endfloat - and more! so the crank may not be twisted but just binding on the distorted front main bearing. Alas, a full strip down is needed if reliability is to be restored.
                                            Regards,
                                                       Eddie.
                               

Offline tck

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Re: Bang! What do you think?
« Reply #11 on: 23 Oct 2007 at 18:58 »
Well
It was a snapped valve
here is the picture
now I need a T35 OFFSIDE CYLINDERHEAD
Perhaps it was not such a good rebuild....

« Last Edit: 24 Oct 2007 at 00:24 by alwyn »

Offline Doug

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Re: Bang! What do you think?
« Reply #12 on: 24 Oct 2007 at 01:37 »
Ouch! Well that would do it, so much for mis-aligned or seized crankshafts. Though after such a catastrophic stop, it would certainly pay to check to see that the crankshaft did not inadvertently shift position, when you are doing the rebuild. It may also have indented the races, so you will want to see how smooth the big ends feel.

-Doug

Offline eddie

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Re: Bang! What do you think?
« Reply #13 on: 24 Oct 2007 at 06:47 »
I guess it's inquest time. Was the valve an original Douglas valve or was it one of the new generation valves that are intended for use in engines running on unleaded petrol? Evidently, some of these new valves are made in two pieces and friction welded together(just at the point where the stem flares out to the head). From what I understand, if the whole valve is made in the new material, there is a problem with the stems 'picking up' in cast iron valve guides. I have had a similar occurance with Triumph valves that I am using in my DT engined sprinter. The whole head parted from the stem of the valve, and went right through the top of the piston - see picture. I guess I was lucky, considering I was flat out in second at the time - the damage was confined to the head and piston - there was no damage to the crankshaft or rods.
             Going back to the T35, if the piston is standard size, I can help out with a replacement and have also got some NOS valves for the Mk1.
              Good luck with the rebuild,
                                  Regards,
                                            Eddie.



« Last Edit: 24 Oct 2007 at 07:51 by Dave »

Offline tck

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Re: Bang! What do you think?
« Reply #14 on: 24 Oct 2007 at 18:44 »
thanks Eddie
You may well be right, it was I am told a recent rebuild

I think I have sourced a piston (I hope) its the head that I am stuck for
I have one but the fins are broken and its not too good
Tim

Offline tck

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Re: Bang! What do you think?
« Reply #15 on: 14 Nov 2007 at 12:03 »
It was a NOS valve not a new '2 piece' one
the piston has welded up fine

Offline trevorp

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Re: Bang! What do you think?
« Reply #16 on: 15 Nov 2007 at 05:46 »
it is an interesting problem but i would suspect a valve collet has let go and one valve has hit piston then broken and piston has hit other valve and broken that stem. tlooks like the valves didnt break in the same spot and to have 2 valve stems break is rare indeed if the stem did break though it may pay to check valve springs as the springs are not very strong on douglas and something else may have been fitted
just a thought

 

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