The symptoms of a 3 bearing crank failure?

[Ruairidh Dunford]

I still find it extraordinary that the last bit stays together!

[Charles Levien]

The centre bearing of the Seven dangles from the thin roof of the alloy crankcase so deflection under load must be significant. The mountings are frequently distorted out of static alignment which makes matters worse.

For sure chances of success are improved by isolating the crankcase from the chassis, one of the rear bolts should be left out or assembled only loosely. More extreme versions include sitting the crankcase of valve springs so it 'floats' above the frame. I can only guess the Lotus was engineered to do what was asked of it!

Another thing which helps is to chuck the Austin long centre bearing studs; a pair of inserts is put into the casing so the C/M can be screwed into the crankcase alone, with long 3/8" bolts, and thus divorced from the desire of the cylinder block to lift itself off.

Have used valve springs under tension to replace fixed bolts for many years — an improved oil feed to the centre main in conjunction with an oil filter is a feature of most engines assembled by Vince Leek and seems to make sense.

Charles

[Chris KC]

Here are some shots of the cracked crank. The crack hasn't propagated all the way across the web.

Caught that just in time! Bit of gorilla glue and you're sorted!

(that was a joke, just in case...)

[Hedd_Jones]

Our Club Secretary had a comprehensive engine failure with a Club three bearing crank when returning from a Club run earlier this year. The faiilure is through the rear web.

He has now has a new engine with a two bearing crank.

Isnt that a 2 bearing crank in a 3 bearing case with the middle bearing hacked out?

[Chris KC]

Our Club Secretary had a comprehensive engine failure with a Club three bearing crank when returning from a Club run earlier this year. The faiilure is through the rear web.

He has now has a new engine with a two bearing crank.

Isnt that a 2 bearing crank in a 3 bearing case with the middle bearing hacked out?

I think you're right Hedd

[Alastair]

   
And here's mine after a failure year a year or so ago. At least when the three-bearing cranks start to go, they usually give notice of their intentions and provided you stop quickly enough the crank-case usually survives.

Alastair

[fatcatvera]

RAt least the failures are consistent - exactly the same failure as mine.

What did you do to repair Alastair?

[Alastair]

'What did you do to repair Alastair? '

I think I personally am beyond repair but as for the crank, it was a case of sourcing another one and crack detecting it before and after it went for regrinding..  I don't know how successful it has been because the rebuilt engine is now sitting in the chassis of my AVK van project, which has traveled less than two feet in the last year!
Good luck with yours
Alastair

[Tony Press]

Our Club Secretary had a comprehensive engine failure with a Club three bearing crank when returning from a Club run earlier this year. The faiilure is through the rear web.

He has now has a new engine with a two bearing crank.

Isnt that a 2 bearing crank in a 3 bearing case with the middle bearing hacked out?

The original three bearing crankcase was a little far gone to 'hack out the middle bearing' so he replaced it with a two bearing crankcase and new crank.

 

[Hedd_Jones]

Our Club Secretary had a comprehensive engine failure with a Club three bearing crank when returning from a Club run earlier this year. The faiilure is through the rear web.

He has now has a new engine with a two bearing crank.

Isnt that a 2 bearing crank in a 3 bearing case with the middle bearing hacked out?

The original three bearing crankcase was a little far gone to 'hack out the middle bearing' so he replaced it with a two bearing crankcase and new crank.

 

He had a two bearing crank before it broke