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The symptoms of a 3 bearing crank failure?
Last night Burt became very unwell....

The engine has had a light rattle at higher engine speed when warm, but not significant to pull the engine apart.

But yesterday things got pretty noisy pretty quickly! Symptoms now are:-

- Engine will idle but with a light rattle
- As soon as off idle with no load the engine is extremely clattery/knocking.

It's not a big end noise, and obviously the enigine needs to come out for inspection and repair. But until I do that in the next few days my mind is wondering what it could be. Is it possible for a 3 bearing engine to still run with a broken crank? Could be the flywheel has decided to start removing itself from the crank. Could be a piston fallen apart. No holes in the crank case (yet!). Or maybe rear main has disintegrated?

What is the 'normal' failure mode for a 3 bearing engine?

Yes, it's entirely possible to run a 3-bearing engine with a broken crank! Back in the 'seventies I drove my van 140 miles home from Swindon to Chesterfield thinking I had run a big end, but when I dismantled the engine the crank came out in two halves! It is quite common for the break to be across the centre bearing, and with luck the two halves still manage to hold together.

Having said that, you say that the noise increases when the engine is not under load, which suggests a loose flywheel is a possibility. As you say, a strip-down will reveal all.
Ray, Take the flywheel cover off first and you may be able to tell on jiggling the flywheel about using a screwdriver lever against the crankcase if it is a loose flywheel or a broken crank.
Thanks for the info. It sounds pretty terminal, but hopefuly salvageable. I'll do a few checks whilst it's in the car but I can't imagine whatever it is can be fixed in 5mins with the engine in the car!
My one experience consisted of putting a big end through the side of the crankcase at about 50 or 60 mph on the M5. When later inspected the crankshaft showed classic signs of fatigue crack growth (so much so that my university workshop wanted it as an exemplar), implying it had been hanging on by a thread for quite a while. It was many years ago but I'm afraid I didn't notice any sign that it was coming.

It does sound like something is awry and it might be a good time to investigate. Do please let us know the outcome.
Ray, if the noise stops when you depress the clutch then it is almost certainly a loose flywheel
Centre Main bearing may have failed,I trialled a 3BRG engine for a number of years once it got noisy I dropped the Sump etc and fitted a new shell without taking engine out.Usually lasted about 2 years or 15/20 events.
My Opal engine started to make noises on the up hill drag from Dover , immediately new it was the crank and stopped, was recovered home. 
Took engine out , stripped it down and could not find anything, reassembled and run again but on test rig, same noise, stripped again and eventually found cause, one of the round webs adjacent to the centre bearing had a hairline crack 7/8ths of the way across it. Bit the bullet got Vince Leek to cut the centre bearing housing out and converted to a Phoenix two bearing crank.
I am always interested in any information about Rosengart details or current owners.
Except perhaps when racing, cranksahft cracks progress over a long period and the metal which finally lets go is astonishingly thin. If the separated parts do not hammer classiscal fatigue "tide mark" patterns show as the crack progresses in fits and starts. Often stained red where have fretted for ages.
New 3 bearing cranks are available and quite dependable I understand.

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