Main bearing wear..

Author
Discussion

PeterBurgess

775 posts

145 months

Saturday 11th February 2017
quotequote all
Some may say the crank cannot be destroyed by harmonic/vibrational problems, I beg to differ as many engine builders, usually from bitter experience, would attest.

http://www.enginelabs.com/news/understanding-engin...

http://flat6innovations.com/index.php/broken-crank


Peter


Boosted LS1

21,167 posts

259 months

Saturday 11th February 2017
quotequote all
Jhonno said:
Boosted LS1 said:
Valid point but why is there even wear on both pinch surfaces? The shell won't have moved across the cap. I think there is more to this Sherlock ;-) ?
Such as?

Both end mains were showing odd wear on the lower bearing only.
If there was dirt under one pinch surface/corner then the wear would extend up to it as expected. Your shell has wear up to both pinch corners. I doubt there was dirt under both ends of the shell.

The pictures show wear across the whole of one side of the shell. To me that indicates tilt but I don't know the answer/cause. I'm still thunking and learning :-)

If the fault's on both of the end shells then I'd suspect a crank flexing/balance issue.

Boosted LS1

21,167 posts

259 months

Saturday 11th February 2017
quotequote all
Jhonno said:
All block side big end bearings..

https://goo.gl/photos/HDTvhppLCCzSgFtJ7
They look good from what I can see.

Boosted LS1

21,167 posts

259 months

Saturday 11th February 2017
quotequote all
PeterBurgess said:
Some may say the crank cannot be destroyed by harmonic/vibrational problems, I beg to differ as many engine builders, usually from bitter experience, would attest.

http://www.enginelabs.com/news/understanding-engin...

http://flat6innovations.com/index.php/broken-crank


Peter
2nd links interesting. Obviously the author suggests a balancing issue. As an aside he dislikes 'powdered' metal as a poor material but you can clearly see the cranks a forging. No doubt forged from granular material. I see no issues with that. The crank won't have been his issue.

Jhonno

Original Poster:

5,762 posts

140 months

Saturday 11th February 2017
quotequote all
Boosted LS1 said:
Jhonno said:
All block side big end bearings..

https://goo.gl/photos/HDTvhppLCCzSgFtJ7
They look good from what I can see.
Well that is a bonus..

Jhonno

Original Poster:

5,762 posts

140 months

Saturday 11th February 2017
quotequote all
Boosted LS1 said:
Jhonno said:
Boosted LS1 said:
Valid point but why is there even wear on both pinch surfaces? The shell won't have moved across the cap. I think there is more to this Sherlock ;-) ?
Such as?

Both end mains were showing odd wear on the lower bearing only.
If there was dirt under one pinch surface/corner then the wear would extend up to it as expected. Your shell has wear up to both pinch corners. I doubt there was dirt under both ends of the shell.

The pictures show wear across the whole of one side of the shell. To me that indicates tilt but I don't know the answer/cause. I'm still thunking and learning :-)

If the fault's on both of the end shells then I'd suspect a crank flexing/balance issue.
Right.. Whilst I am not sure about the crank flexing, balance could well be an issue. This crank is cast and know for snapping.. Could it be if it was unbalanced it could be a cause? It revs to 7.5/8k..

Mignon

1,018 posts

88 months

Sunday 12th February 2017
quotequote all
Boosted LS1 said:
Valid point but why is there even wear on both pinch surfaces? The shell won't have moved across the cap. I think there is more to this Sherlock ;-) ?
But there's usually a gnat's cock of play possible on the cap when it's fitted and it will try to centralise itself about the shaft.

Anyway it's a very minor wear issue and all that's happened is the top flash coating on the shell has worn through in places which is only a fraction of a thou thick. Nothing to worry about.

HJG

461 posts

106 months

Sunday 12th February 2017
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PeterBurgess said:
Ollie from TN Racing tells me..... main thing with flat plane V8 is the second order vibration that you cannot get rid of without a balance shaft. The key is ensuring that the counterweights of the crank are matched perfectly to the piston and rod weights. Ours is within a couple of grams and we don't get the bolts trying to undo themselves like others experience.

Peter
Flat plane V8s need not have counterweights to balance the piston. Counterweights in a flat plane V8 and an inline 4 serve only to relieve the main bearings from the centrifugal force created by the rotating components, i.e. crank mass above the crank centreline per cylinder and ~2/3 rod mass

Boosted LS1

21,167 posts

259 months

Sunday 12th February 2017
quotequote all
I don't think that shell was ever sitting fully seated in it's housing. Could it have been raised a smidgen at one end? Even if only by microns. I'll try to articulate this in an exaggerated fashion.

Imagine the shell rotating on an axis mounted across the cap. Viewed side on at the parting line one part of the bearing would rise up meaning the other edge could sink down. The rising edge would be in more contact with the crank's length at the widest part of the crank. This would create contact at the pinch area. The sinking edge would be gaining clearance.

Obviously it's a miniscule amount of misalignment but it could happen especially if the upper shell wasn't fully in contact with the lower when all bolted together. I dunno, I'll go and think some more :-)