Big end bearing shell damage.

[PEW]

I've just stripped out the big-end shells from my 1986 made (but remanufactured in '99), tuned 850 cc trials car engine, to investigate a rattle which suddenly occcurred and rapidly got worse. No. 4 conrod had quite a lot of play in the bearing, the other three less so.

The bearing faces of the shells are lead/indium or aluminium (not sure as I didn't build it). All of the shells in the conrod halves are badly pitted on the bearing faces, as if the metal is corroded, but the actual sliding face is polished and there is no sign of smearing. The bearing cap side shells are not so bad but they are slightly dark and discolored.

The crank journals are unmarked, almost perfect. I can discern no play in the main bearings. The engine has done 24,000 miles over seven years.

I've recently changed oil type, from a semi-synth 15W/40 to a mineral 20W/50. This was advertised for "older, classic" engines with wider manufacturing tolerances than modern engines and it gave improved oil pressure as advertised. This oil has done only about 2,000 miles but now smells slightly burnt.

Although I won't be using this type of oil ever again, I'm not sure if the oil is completely to blame; it might be a coincidence. Over the winter of 2007/8 the engine stood for about three months with, unknown to me, a cracked cylinder head around an inlet valve guide, resulting in coolant leakage into one cylinder and about half a pint went into the sump oil. As part of the fix, I used a flushing oil and carried out two oil and filter changes before putting the car back into service.

Question is this:

Just how corrosive is water / glycol coolant in these circumstances, i.e. mixed with engine oil?

Did bearing material corrosion induce a latent problem or was it just more likely to be poor oil quality?

 

[patprimmer]

Was the engine cranked over with the plugs in and was it started after the water leak occurred.

Oil and water don't mix. The water goes to the bottom, so unless the engine was run with water in the sump, or unless there was a lot of water, the bearings should not have seen any water.

When you run an engine, you always expose the oil to some water in the blow by as exhaust gas is mainly nitrogen, carbon dioxide and water.

Regards
Pat
See FAQ731-376 for tips on use of eng-tips by professional engineers &

http://eng-tips.com/market.cfm

for site rules

 

[PEW]

Thanks, Pat. Unfortunately, I first assumed the head gasket had blown, as these all alloy, wet linered engines are prone to do. I fitted a new gasket.

Having replaced the contaminated oil and the filter, I road tested the car. It went down to three cylinders after about ten minutes, again there was a small amount of coolant in the cylinder after it cooled down.

I removed the head and checked the gasket. I thought I could see slight damage on it so I fitted yet another.

When it happened again on road test later the same day, I changed the head.

So yes, there is a good chance that sump oil and a small amount of water were stirred and shaken to some extent.

There is no sign of any other corrosion inside the sump (steel) or the block (alloy).

 

[patprimmer]

Water in the oil can boil at the journal in an engine running under load. This can reduce lubrication and increase corrosion.

It could have also seen a considerable rise in effective compression if it was approaching hydraulic lock.

A crack probably opened up over time, so the engine probably has some miles on it with excess water in the chamber and the oil



Regards
Pat
See FAQ731-376 for tips on use of eng-tips by professional engineers &

http://eng-tips.com/market.cfm

for site rules

 

[PEW]

I hadn't considered the water boiling off in the bearing; that sounds like a good theory.

I have a new set of bearing shells in front of me, I'll fit them tomorrow and go back to a known quality oil.

Thanks again for your help.

Paul W.

 

[PEW]

I know you don't like links to other sites, but to give you an idea of what we get up to with the car, here's one to a video of the most recent event(MG Car Club Midland Centre Autumn Trial) that we took part in. We came second in class 7; I think we would probably have won if I hadn't stupidly gone the wrong side of a marker post at the bottom of the first section of the day and incurred 12 penalty points. Doh!

http://www.youtube.com/watch?v=77RRLlYXZ7U&feature=related

Please have a look then delete this post if you wish.
Thanks again.

 

[romke]

cooling fluid that has been in contact with exhaust/blowby gas can become highly corrosive. if some of that cooling fluid gets in the oil it easily can attack bearings.

 

[twinturboash]

pew,

if you can , measure the wide of the top of the bearing slipper. Anymore than say .002' wider than at the parting line ,you could say is causes by detination.

also look through a 8 or 10X eye piece. Those rough patterns on the bearing may look like 'sheets' of bearing that have 'lifed' out on the parent bearing material.

ash