Big end bearings shell doodah - Alfa Romeo Forum
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(Post Link) post #1 of 11 Old 26-09-14 Thread Starter
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Big end bearings shell doodah

Thinking ahead to fixing my water-pump failed then cambelt KIA'd Twin Spark...

Legend has it that a sudden death at the top of the cylinder head damages the big-end bearing shells.

How can I check that the big-end bearing shells are okay and not likely to cause me problems down the line?

If they are damaged, what's the process to replace them?

I imagine I'll take the sump off and peer up at the bottom of the crankshaft... Now what?


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I would say it doesnt matter...

If you have gone that far then I would say just change the shells anyway.. the crank will need to be inspected though to be certain but if it looks lovely and has smooth surfaces.. just measure the crankshaft journals.

but the only way to be sure is to use plastigauge and check the bearings in multiple points including at 90 degrees to the cylinder to check the theory the pistons has flattened the bearing at that point
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(Post Link) post #3 of 11 Old 28-09-14 Thread Starter
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Yes.. I'd fit new crank bearing shells if I got to the point where I've taken the old ones off... but how's it all work?

Are the shells replaceable in situ' (I mean one at a time) or do I need to take the crank out? I guess then that the flywheel and hence gearbox would have to come off (to get the crank out) ... which I'm trying to avoid if I can. The job needs to be DIY'able with just me doing it.

Can I just undo one crank journal at a time and fit a new pair of half-bearings... then do the next one... etc. It doesn't sound like it..

If I have no plastigauge or not much experience how to use it.. can I just fit a bearing the same size/colour-code as was in there before? My old donkey has 154,000 on it, so just a new bearing the same size is bound to be an improvement, I'd have thought..

Talking of big-ends.. are the con-rod bearing shells gonna be okay or do these get KIA as well? My understanding is that just the crank bearing shells (right name?) get mullah'd.


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No bullets for Chaingun..

Last edited by Ralf S.; 28-09-14 at 13:30.
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Crank shaft bearings can be done in situ, this guide might help:

www.imoc.co.uk :: Changing big end and main bearings with the crank in situ
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Ah good.. I did wonder whether the bearing could be rotated out (did a motorbike on a bench about 30 years ago.. )

30 years ago!!


I'm hoping the bearing sizes will be easily identifiable by colour rather than relying on me measuring them.. but I'll get some plastigauge for a second opinion too. As I understand it, the clearance on the bottom bearing is the more critical one (dunno how I'd be able to 'gauge the clearance on the top one anyway, without taking the crank out).

Anyone know the main bearing clearances for the beast? ePER suggests the range of sizes is 1.836mm to 1.909 but I don't yet understand what that is.. .seems too big to the *clearance*.

(I'll need to knoe the torque settings too, if anyone has them... it's a tipo 67106 1.8 T/Spark).

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Pretty sure there are some "how-to's" floating around on here too ...

The TS sump isn't as straight-forward as that MR2 engine .. but it is an Alfa so what do you expect! ;-)

As for big-ends ... they haven't changed much at all in 30 plus years ... the design is similar on my 156 as it is on the A-Series Midget engine I'm building!


As you lost your belt and haven't used the engine again (ie fixed belt and heads) then you will probably find the crank is OK. The problem comes when the top end is rebuilt and the bottom end left. The flattened shells then cause wear and it's then you need to consider a crank regrind .. but you are then taking the engine apart!

cheers, Gary

HIS: ex-1998 156 2.0 TSpark with Sport Pack 2, sunroof and hi-level spoiler in Alfa Rosso. V6 intake mod, clear side repeaters and Zeatek undertray. 10th Sep 1998 to 12th Jan 2017

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Is there a pattern for which main bearing gets flattened? For exampio, if cylinder 1 valves gets mullahed, is it no.3's bearings that get flattened...?

Or could all / any of the bearings be damaged? Just curious.. I'd change the whole lot..


Can anyone decipher which bearings I need? ePer gives a range of sizes from 1.836mm to 1.909 but I've seen new after-market bearings for sale as "standard" or "+0.25" or "+0.5" or "+0.75" etc..

What's the relationship between the ePer size and the sizes that the aftermarket bearings are labelled?


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The last time that I done this was on a Nord engine, but it has been successfully done on a JTS recently:

https://www.alfaowner.com/Forum/alfa-...d-lifters.html
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Quote:
Originally Posted by Ralf S. View Post
Is there a pattern for which main bearing gets flattened? For exampio, if cylinder 1 valves gets mullahed, is it no.3's bearings that get flattened...?

Or could all / any of the bearings be damaged? Just curious.. I'd change the whole lot..

Ralf S.
When our belt went it took out most of the valves ... some were so badly bent that the guides broke too ... and that was literally starting the engine .. not a high speed or high revs ... sitting in a car park!!

The big end that was damaged the worst was no 1. (which I think is water pump end ... some engines are actually fitted the wrong way around to avoid the issues Mr Issigonis had to overcome with the original mini.

But this was a year after the head was rebuilt .. this was before the advise came out to check bearings ... to my (expensive) cost

Last edited by gazza82; 02-10-14 at 14:46.
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(Post Link) post #10 of 11 Old 01-10-14 Thread Starter
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Thanks... that's interesting.

I'll see what happened to them when I take my old shells out... but only out of curiosity. I guess once they're out, I would only re-fit new ones.


Does anyone know the answer to the clearance I need? I found a worskhop guide for the 2.0ltr later engine (CF2) which suggests the shell-journal clearance should be 0.025 - 0.05mm. Is that correct for CF1 1.8 too?

EDIT: CF1 1.8 looks like 0.019 - 0.062mm for side bearings and 0.029 - 0.072mm for centre.

Ralf S.

Last edited by Ralf S.; 01-10-14 at 16:40.
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