The symptoms of a 3 bearing crank failure?

[Bob Culver]

The broken crank photo from Ruairidh seems to illustrate grossly negligent grinding of the crankpins! Some years ago I read a book on the development of the BMC B engine (A50, A60 MGA etc) Increasing the radius from .060 to .090 had a huge beneficial effect on fatigue life. Jowetts discovered the same, although their need was more desperate! Reconditoners have undone nearly all
At 7000 rpm forces are 4 times 3500. A 10% increase has a huge effect on fatigue. Cracks progress in laps instead of decades and the appearance is different..

[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.

Has there been a meaningful investigation of WHY it failed Tony? I ask because both my Ulster rep and Nippy have been running on Reliant 3- brg cranks - these are pressure-fed so also drilled - since about 1984, without a murmur. I had the Ulster engine apart for overhaul recently and took the opportunity to crack test it but nothing found. The Nippy doesn't get out much but the Ulster has taken me all over Europe.

[Stuart Giles]

I take the Australian club 3 bearing cranks are an exact replica of the Austin crank?

If I built another 3 bearing motor for myself, I would get Phoenix to make one from billet but with the side valve Reliant big end offset. This allows for much thicker centre webs. This crank would work with the SV Reliant rods and a compression plate to accomodate their extra length, or get Phoenix to make a set of 6" rods with the increased offset.

[Chris KC]

I take the Australian club 3 bearing cranks are an exact replica of the Austin crank?

If I built another 3 bearing motor for myself, I would get Phoenix to make one from billet but with the side valve Reliant big end offset. This allows for  much thicker centre webs.  This crank would work with the SV Reliant rods and a compression plate to accomodate their extra length, or get Phoenix to make a set of 6" rods with the increased offset.

I don't know if they're an exact replica Stuart. Despite the thicker webs on the Reliant version, my expectation would be a satisfactory crank life IF the alignment of the C/M was also properly addressed, so that the centre journal is supported (as opposed to either stressed or whipping around in thin air).

[Dennis Nicholas]

The broken crank photo from Ruairidh seems to illustrate grossly negligent grinding of the crankpins! Some years ago I read a book on the development of the BMC B engine (A50, A60 MGA etc) Increasing the radius from .060 to .090 had a huge beneficial effect on fatigue life. Jowetts discovered the same, although their need was more desperate! Reconditoners have undone nearly all

Further development on the A+ 1275 (1300) crank gave it a rolled fillet radius between big end and web.  That is instead of the big end surface and the web surface at 90 deg meeting with a radius, there was a rounded depression put in...rolled fillet radii.  However the drawings had to be strictly adhered to, as otherwise, as Leyland found out, reliability was compromised so production varied and some had rolled and some not until the full problem was sorted.
(mind we are talking about using the A+ at 6,000 to 8,000 rpm for racing)

(I found luckily I have one of the properly done cranks and as I will not be using such revs in the Nippy I should be safe for a very long time) 

Dennis

[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.

Has there been a meaningful investigation of WHY it failed Tony? I ask because both my Ulster rep and Nippy have been running on Reliant 3- brg cranks - these are pressure-fed so also drilled - since about 1984, without a murmur. I had the Ulster engine apart for overhaul recently and took the opportunity to crack test it but nothing found. The Nippy doesn't get out much but the Ulster has taken me all over Europe.

Careful examination showed the crack spread from the journal corner radius through the web- possibly from an inclusion in the casting.
It was a standard splash fed three bearing crank made locally using the ADI process.

[Bob Culver]

The three bearing had a reputation for failure in its day. When I was running my car as everyday transport through the 60s several strangers volunteered that encouraging info. No one ever suggested that the 2 bearing were somewhat prone and my father who ran the car previously never heard mention of. (The later big bang came as no mean surprise!)
 
For any that may be interested, for multi main bearing engines rigidity of support of the crank is vital for extended life. The large bore Model T had a bent wire crank but supported in a rigid cast iron crankcase. I have been involved with Jowett Javelin cars. These have a short robust looking crank but were notorious for failure (partly due poor heat treatment process),  It is evident from bearing wear patterns (and engine thump!) that the split alloy crankcase deflects markedly.
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. The Big Seven went to the other extreme with bearing housings checked into the rigid deep iron crankcase.  Hi performance engines, including the Merlin, have bearing caps bolted both vertically and horizontally to ensure rigidity.
Rolled fillets were later common on modern  cars. As with most compressive processes considerable gains were obtained. Nitriding, presumably what gives Phoenic cranks their life, develops a compressed layer.

[Ritchie Wilson]

Colin Chapman in the 1951 750MC Goodacre Trophy races used a 3-bearing engine in his Lotus III (as did the "Australian Single Seater" racers). Presumably the crank was changed frequently. I have seen somewhere that Lotus III's crankcase was strengthened and used, in true Chapman fashion, to stiffen the front of the chassis. In "design for competition" Jack Cotton describes a method of doing this. Has this been widely tried and found to be useful?

[Chris KC]

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.

[fatcatvera]

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