Author Topic: M32 Noises off = broken crank - possible fix?  (Read 1324 times)

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Offline Hamwic

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M32 Noises off = broken crank - possible fix?
« on: 16 Jan 2022 at 16:31 »
Hi All,

Investigating the periodic noises from the crankcase of the M32 revealed that the oil pump drive dog was loose again, and the crank had squeezed up and pinched the rear cylinder big end. Upon dismantling the crank for inspection, the threaded boss of the rear cylinder web came away with the nut. The evidence suggests that this had been cracked almost through for some time, and this last episode has finished it off.

I have a fix in mind involving a large HT countersunk bolt inserted from the rear of the crank web. This would save having to match up a different centre web. Has anyone ever tried this?

Meanwhile I have re-machined the drive side web from 1/2 x 20 right hand to 7/16" x 20 left hand, and made a new drive dog to suit. This should stop the drive dog from coming unscrewed in the future. I believe this has been the cause of all this machine's trouble in the past.

I have advertised on the Wants page for a centre web should anyone have such a thing, and the stroke would appear to be 60.5mm or thereabouts. Is that a common stroke in the Douglas world, or will I just go straight to the fix?

Cheers for now
Doug



Attachment converted to linked file - Dave, 17Jan2022
« Last Edit: 16 Jan 2022 at 18:20 by Dave »

Offline Doug

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Re: M32 Noises off = broken crank - possible fix?
« Reply #1 on: 16 Jan 2022 at 17:55 »
The crank was shared in common with the A32, except that the drive side main shaft was not drilled to save weight on the light ohv models like the K/M32. Unfortunately even for teh side valve models the cwenter web was still I reckon undersized. Same crank was introduced with the A31, used in the the C32 Bulldog and through to the 250/350cc Aero models. All suffered from center web problems; either end wear from the conrod rollers (due to flexing) or breakages. Hence spare crank parts have become rather scarce.

-Doug

Offline Hamwic

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Re: M32 Noises off = broken crank - possible fix?
« Reply #2 on: 17 Jan 2022 at 10:39 »
Hi Doug,

Thanks for the reply. Did the 60.5 stroke ring any bells? I'm guessing on this model they used something that was already to hand.

I'm going to attempt my fix and use that to measure the exact stroke. Should the fix not hold up, then I think making a new central web will have to be considered. Luckily I don't need the oil ways!

Cheers for now
Doug

Offline Doug

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Re: M32 Noises off = broken crank - possible fix?
« Reply #3 on: 17 Jan 2022 at 16:59 »
It ought to be 60mm stroke.

-Doug

Offline alan reid

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Re: M32 Noises off = broken crank - possible fix?
« Reply #4 on: 19 Jan 2022 at 15:47 »
Hi, i have a spare crank out of a 1930ish S6 or T6.

It's not got any rods with it, and the 7/16" thread is damaged!

Regards Alan Reid.

Offline Hamwic

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Re: M32 Noises off = broken crank - possible fix?
« Reply #5 on: 19 Jan 2022 at 22:01 »
Hi Alan,

Thanks for the info on your crank, and I will bear it in mind. Presumably this would be a 60mm stroke item? By 7/16 thread I'm guessing you mean the oil pump dog thread on the end, so it would be interesting to know if this is/was Left or Right handed?

 I'm well on with my fix at the moment, so will persevere with it for a while and see how I get on. However, if it doesn't do the trick I may well be back to you!

Thanks again.

Doug

Offline Doug

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Re: M32 Noises off = broken crank - possible fix?
« Reply #6 on: 20 Jan 2022 at 07:35 »
The T6/S5/S6 cranks are 82mm stroke.

-Doug

Offline Hamwic

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Re: M32 Noises off = broken crank - possible fix?
« Reply #7 on: 20 Jan 2022 at 13:23 »
Thanks Doug and Alan, that's not an option then!

cheers for now
Doug

Offline Hamwic

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Re: M32 Noises off = broken crank - possible fix?
« Reply #8 on: 09 Feb 2022 at 14:19 »
Hi All,

Sorry to keep you waiting, but I've been a bit busy! Here's my fix for the snapped off crankpin end. It seems robust enough. This time it's an M10 countersunk HT screw, threaded right through and Loctited in, but if not suitable there is room for an M12, though the head will need shaving down to clear the opposite rod. A new lockwasher was required for the reduction from 1/2" to 10mm. I have also re-machined the oil pump drive dog thread to 7/16 x 20 TPI Left Hand and made a new drive dog. This RH thread here was apparently the cause of the grief all along. You can see the discolouration where the crank got squeezed onto the rod eye. This might also have precipitated the fracture of the crankpin thread.

Let the adventure continue!

Cheers for now
Doug