Advice from engine guys, please.

Advice from engine guys, please.

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Higgs boson

Original Poster:

1,091 posts

152 months

Thursday 27th July 2017
quotequote all
What is the likelihood of an engine that has been run extremely low on oil, and presenting with a bottom end knock, being partially rescued by just fitting new bearings, and not changing/regrinding c/shaft?

For completeness, BMW F20 116i. Low oil light did not, at any stage, illuminate.
When level was checked via interior control, it showed as full.

Car was bought from non-BMW source, and was serviced before collection, approx 9k miles ago. Currently with 48k miles.

227bhp

10,203 posts

127 months

Thursday 27th July 2017
quotequote all
If it's knocking it's probably game over, but until you've had a look you'll never know.

stevieturbo

17,229 posts

246 months

Thursday 27th July 2017
quotequote all
As above, until the crank etc is inspected there is no way to know.

Some engines can be pretty robust though, especially if they use fairly soft bearings. Although if a bearing has spun in the rod, those will need either repaired or replaced.

Little Pete

1,513 posts

93 months

Thursday 27th July 2017
quotequote all
More doom and gloom from me I'm afraid. I can't remember having a crankshaft re ground in the last 20 or so years to repair bearing failure. If it's knocking it's probably already worn beyond the available oversize bearings. I would also be concerned at the condition of the camshaft and associated bearings and/or cylinder head.
Once again though, as above you won't know until you have a look.

Higgs boson

Original Poster:

1,091 posts

152 months

Thursday 27th July 2017
quotequote all
Little Pete said:
I would also be concerned at the condition of the camshaft and associated bearings and/or cylinder head.
This was my other, unvoiced, concern. I think that an engine from a breaker might be required!

Thanks to all for the replies.

stevieturbo

17,229 posts

246 months

Thursday 27th July 2017
quotequote all
Thousands of cranks are reground and fixed every week. It'd need to be one hell of a knock or special crank for it to be un-usable.

Boosted LS1

21,167 posts

259 months

Thursday 27th July 2017
quotequote all
OP, why don't you drop the sump and have a look?

Higgs boson

Original Poster:

1,091 posts

152 months

Friday 28th July 2017
quotequote all
Although I've done the vast majority (99%) of maintenance work on my cars/bike for the last 40 years, the last crank that I had in my hands was from a 250 2 stroke twin, in, I would guess, '81/'82!
I'm not sure how much visual scoring I'd looking for. Colouring? Would I need to resort to Plastigauge?
TIA.

Boosted LS1

21,167 posts

259 months

Friday 28th July 2017
quotequote all
If it looks smooth, feels smooth and your finger nail glides across it without catching on any ridges then that would be encouraging.

Little Pete

1,513 posts

93 months

Friday 28th July 2017
quotequote all
stevieturbo said:
Thousands of cranks are reground and fixed every week. It'd need to be one hell of a knock or special crank for it to be un-usable.
You're right stevieturbo but the biggest issue nowadays is that on a lot of engines bearings aren't available beyond 0.25mm.
In my experience if it's knocking, the wear is already more than that. I am prepared to admit I could be wrong though-just don't tell the wife I said that!