Crankshaft sheared completely at cheek

Cranks have been known to break.  Its usually caused by stress raiser sections . Sometimes caused by poor crank grinding which leaves a sharp corner .Sometimes its poor manufacture that leaves the same fault inside the crank . I would not assume that any replacement is any better . 

Like this:

I have several that look just like that - cranks were almost a consumable on race bikes. Just twists the end clean out.

Solved by buying a Nourish one-piece crank. Dave made mine stronger by radiusing the main journal to the crank web - requires the bearing inner race to be relieved for clearance. Sadly, Dave is no longer with us. He sold the business and retired some years ago. The new owner moved everything down south but I don't know anything about them.

Les Emery/Fair Spares/Norvil Motor Co built new engines (everything except the head) using Nourish (NRE) cranks in the 90s so there are quite a few about. If you're offered a Nourish crank in good condition, don't hesitate.

Andover are making new one-piece cranks; not cheap, I don't know who makes them and, not having seen one, I have no opinion on quality. But, apart possibly from NRE, it's the only option if you want a new crank.

As for breaking cranks on the road, I'd recommend you avoid lumping along in top gear at low revs, especially with a 22T sprocket. The engine will do it but it hammers the bottom end.

[Edit - Dec 2023]

Just found this old crank at the back of the shed. Break here is in D/S big end. This one had done 100k+ miles in an Interpol followed by 2 Manx GPs, 2.5 S100s and various other races. Nothing lasts forever...

I believe that the Nourish business was purchased by a fellow called Chris Bushell. Who was based somewhere near London.  Unfortunately Chris then fell seriously ill and required numerous visits to hospital for treatment.  I have not been able to contact him since for over a year.

the new Andover cranks are made by a specialist firm, and early reports are positive from visual inspection and talking to staff. ( recent Stafford show) Dave Nourish’s son is active to some extent, but I don’t know any other details.

   Some riders have had their crank ‘undercut’, with a suitable radius,  at the break area, so cutting slightly into the crank web and the shaft with a smooth curve, all to reduce stress raisers.( special cutting tool required!)

Does anyone have dimensions for the undercut mentioned above?

from an Accessnorton thread: use 0.090 inch radius cutter going into cheek of crank ONLY. ( move it slightly to avoid chatter.) but not to full depth, so less than a full semi-circle groove. Bottom of groove blending into output shaft.

    Then use brass rod in die grinder and grinding paste to polish surface ( and/or shot peen) to reduce stress raisers.

   This is a known technique in other engineering areas.

A friend of mine had this happen back in the 80's, coming home on a wet Sunday afternoon from a NOC rally in Cologne. Stopped for the customs at Aachen & it was making a hell of a noise. he rode it to the station & got back home, via the Calais Dover ferry on the train, rode it the last 3 miles to his home. Best of it was, he got home before me.

Jan, thank you for the reply.

Best wishes, Terry.