Crankshaft wobble

[Rogerfrench]

Bob Culver, if you think that pre-loading A/C front main bearings stresses the lip, you're assembling them backwards.
They should be installed back to back, which means you can preload them outside the crankcase!
As made, in matched pairs, they had around .004" preload when assembled.

[Bob Culver]

It is the general rigidity which concerns but original fittings give good service, at least at normal revs. I guess the housing  accommodates a lot of deflection.

Many textbooks describe welding processes for reclaiming cranks so firms cannot be entirely blamed. For well supported multiple main bearings possibly OK . Curious that Sevens often fail where there should be ino bending. Bridges do not fail at the ends.

I am a bit sceptical of non magnetic dye kits. A retired enthusiast had several cranks. I tested with a magnet and fluid and proved all cracked. The owner was sceptical and a bit miffed and only after a lot of fiddling did his (after use by date) dye kit confirm.

[John Mason]

Hi all, my mk1 Ruby ran for At least 13 years. The time between me purchasing it and doing a major rebuild on it I don't know how long the previous owner had it or how many miles done in it. But on stripping down I found the crank braised around where the rear bearing fitted towards the no 4 big end crank web. I replaced the crank with a phoenix crank. However it would appear that the braized crank did years of service with 1000s of miles with no problem. I changed it because I am chicken.

John Mason

["Slack Alice" Simon]

Hi all, my mk1 Ruby ran for At least 13 years. The time between me purchasing it and doing a major rebuild on it I don't know how long the previous owner had it or how  many miles done in it. But on stripping down I found the crank braised around where the rear bearing fitted towards the no 4 big end  crank web. I replaced the crank with a phoenix crank. However it would appear that the braized crank did years of service with 1000s of miles with no problem. I changed it because I am chicken.

John Mason

Was the brazing mending a break or was it just building up a worn seating for the rear main?

I have seen brazing in this area before, and assumed the latter.   Not used the crank, though.

Simon

Maybe I will try magnetic, Bob.

Thanks.

[Bob Culver]

This is not the ideal heading for a discourse on fatigue but....I have read that in constant load lab tests life is only about 10% remaining after first (microscopic) crack. The real world is very different and from the sheer number of cracked cranks and steering arms these must be in this state for a very large chunk of total life.

As evident from any basic text, a slight increase on load or decrease in the fatigue limit has a huge effect on the cycles to failure. Hence, along with more sustained revs,  so many failures since hotting up with raised c.r since the 1960s. 
If heating reduces the strength from say 50 tsi to 40 that represents a huge reduction in life cycles at the same loadings.

When testing cranks I start at the back and work forward. Apart from one in the car, I never got beyond number 3! Many intact cranks are brazed so have obviously done a fair mileage. Apart from heating, welds usually show a crack at the edge of the weld

[Hedd Jones]

The building up of the rear main journal is because the inner is liable to run loose as it is not clamped in normal circumstances.

The first engine I had in my box was bought as an unknown runner off Mick Kirkland. I partly stripped it to find the crank like a dick in a bucket in the inner race. So i glued it in with some loctite. 

About 7 years later, and god knows how many miles (speedo not connected) the front mains wore so the crank occasionally caught the centre camshaft bearing, stripping it to identify the cause of my 'knock' I found the inner race again loose. In hindsight it had probably been like that not long after It went back in the car.

I hasten to add that whilst the engine smoked like my Mondeo. It went like a rocket.

[Chris KC]

Bob Culver, if you think that pre-loading A/C front main bearings stresses the lip, you're assembling them backwards.
They should be installed back to back, which means you can preload them outside the crankcase!
As made, in matched pairs, they had around .004" preload when assembled.

Quite so Roger; but before tightening down the end plate it is well worth checking where the front face of the bearings has ended up in relation to the crankcase.

[John Mason]

Simon in answer to your question about my braized crankshaft is that I don't know as it was done before I bought the car. I replaced it because I thought perhaps the heat used in braizing altered the temper of the crank metal thus weakening it the argument against this is that it lasted for years without issue.

John Mason

[Ian Dunford]

Roger notes that assembling the front main bearings in the correct orientation is probably the most important piece of information in this discourse,an observation with which I am inclined to agree.

[Tony Press]

This is always critical if the front angular contact bearings are the special narrow outer ring pair which must be mounted back to back.

An ordinary pair can be mounted either back to back or face to face.

The housing depth must be always checked in later crankcases to ensure the correct pair of bearings are fitted.

Cheers, Tony.