Crankshaft journal fillet pitting

[paulsco]

Hi,

I am in the process of rebuilding my E-Type 4.2 engine. The crankshaft is on standard size mains; it has just been polished and balanced.
I have only just noticed that the crankshaft bearing fillet on the front main is rust pitted.
I have attempted to clean it up, but it is still there. I am aware that when cranks fail it usually starts with a crack in the fillet and that is why they should have a radius.
I have uploaded a picture of it and would like your opinion on whether or not to use this crank?



Thanks,

Paul

[PeterCrespin]

Not ideal but unless you intend to go racing I'd use it. Did they test the crank for flaws? The radius looks fine and although it's a stress raiser you're talking about a huge seven-main crank after all... Can you dress it out with a fine rat's tail?

[paulsco]

Hi Pete,

I have no intention of racing, but I was trying to build a fast street motor.

I have done a little dressing, but I wasn?t sure how far to go; I didn?t want to undercut the journal and possibly make it worse.
No the crank was not tested as such. This is an XJ6 crank that I bought second hand as my original was deemed beyond recovery.
The machine shop polished the crank, but said nothing about the pitting; I only noticed this myself when assembling. It could be they thought it insignificant.
Do you think I should try to smooth it out more?

Thanks,

Paul

[PeterCrespin]

With a bit of luck an engineer who knows what they are talking about will answer, but if nobody else says anything you'll have to put up with me. I just expanded the pic (still on iPhone) and it looks fractionally worse than I thought.

If that were a nick on a conrod I wouldn't use it. If it were a pit on a crank cheek you wouldn't hesitate. Since it's where it is, there's probably room for debate.

On a four cylinder three bearing crank I'd be wary, same as on a four main six. It is on a high-stress area but the XK is pretty much overbuilt and hails from a time long before they pared everything down to a minimum based on computer analysis etc. If it was the centre main on a 4.2 I might be a tiny bit worried but it's not. You're right that cranks can fail at the transition from journal to cheek but that's if they flex and the transition is sharp. Neither applies to that part in your intended usage IMHO. The torque harmonics are a bit of a crap shoot but a small surface imperfection might be insignificant in terms of crack propagation/breakage.

Bottom line is you're still talking about a road engine and you have polished and balanced the crank so I'd use it, fitted with a good fluid damper (which should damp all frequencies).

What's the worst that can happen? You have to find a new crank and conceivably a block and a valve tangle if the timing drifts. I don't think you're risking a total grenade explosion and the chances are very high nothing at all will go wrong.

Since it is off the bearing area and the total anount of metal removed wiuld be small, I think you could dress it smoother with the right tools and not affect total section or balance meaningfully. However there's more than one pit so you might end up chasing your tail a bit. There's no rubbing, so filling is not needed. Probably best to leave it after you've done what you can to take any edges down.

[David Oslo]

Thought about this for a while. Considered in terms of if it was my engine what would I do with it. I think I would take the sharp edges off the pitting using a suitable Dremel grinding attachment. Just 'kiss" the edges so to speak, and avoiding to make the pitting deeper. And then I would happily bolt it all back together and not loose any sleep over it. Limited mileage, and nothing safety-critical catastrophic if it all goes bad.

FWIW I'm a mechanical engineer, but don't use that against me :D

[Dave K]

For peace of mind take to it a shop and let them do a dye penetration on it.
I had a crack in my original crank when the engine dropped a valve, way before I had it.
Coventry metal and boring found it, ground out the crack and submerged arc welded it then ground it up, just over ?100 it cost but that was about 12 years ago.
Dave

[mooney1el]

That looks to me like what happens when a crankshaft has been submerged arc welded prior to being re-ground for size. That looks like the porosity from either the start of the arc or end of it. or perhaps even not a perfect setup. Wouldn't be surprised if somewhere along the crank's life line, it was reconditioned. Personally, I wouldn't worry about it.


Richard

[paulsco]

Thank you all for your comments.

I have had another go at cleaning it up. it's very hard to get a good photo without a reflection and it is smoother than it looks.



I may try a bit more or possibly take it somewhere for welding.
I can't decide.

I look at it and think it's not too bad, then I think about it and I'm not so sure.':?'

Paul

[abowie]

Replacement crank cost = $$
Fancy fluid damper cost = $$+
Engine rebuild cost = $$$$
Replacing engine after crank failure = $$$$ plus wasted time and effort.

Bin it and buy another = peace of mind.

[jag68]

That's odd looking for rust pits - too deep, and in an unusual place. Was the car laid up for some time? Is it possible that it's a casting flaw and been there forever?

[christopher storey]

It is , I'm afraid, the sort of pic which turns up in Air Accident Investigation Branch reports from time to time, showing where the crack started from ! Admittedly that tends to be on gas turbines which have far higher rotational stresses imposed at blade roots etc. Since 1. this is a very high stress point, hence the filleting in the first place , and 2. engines are readily available from scrapyards, on balance I agree with the view that replacement is best