Main bearings New Pics

[PSL184]

If there is excessive crank end float due to worn thrust bearings the action of depressing the clutch pedal will side load the engine and slow it down and also wear he crank unevenly.

[SvMinor]

The thrust bearings? Are these also known as the upper and lower thrust washers at the centre main bearing on the crank

[PSL184]

Yes - same thing..

[bmcecosse]

Why on earth would it 'wear the crank unevenly ' ???

[MarkyB]

Because the big end and the little end are not parallel any more when the clutch is pressed.
Look at the second group of pictures, half polished, half something else.
I'd call that uneven.

[bmcecosse]

Why would they not be parallel? All that can happen is that the crank can move forwards by about 5 thou - it doesn't tilt !

[les]

I tend to agree as the little end would travel with the big end, as there is some lateral movement between the small end eye and the gudgeon pin bosses.

[bmcecosse]

Of course there is! Nothing can tilt! There is always some offset wear - because the con rods are not straight - they are offset! So any slight wear on the big-end allows the rod (and piston) to tilt very slightly.

[MarkyB]

Oh, so what has caused the discolouration of the journals.
And why do engines with worn out thrust washers stall when the clutch is depressed?

[bmcecosse]

The engine has been running with low oil pressure - the bearings have been dragging - some of the big-ends have actually 'spalled' off some lumps of Al/Sn - and the leaded bronze mains are well worn. . Engines don't stall because of 'worn out thrusts' - they get 'worn out thrusts' because the clutch is over travelling and/or being held down far too long. The 'over travelling' puts far more load on the thrusts than they are intended to carry - and increases the friction load on the slow idling engine to the point where it may indeed stall.

[MarkyB]

It was the rods I was thinking of when I said out of parallel.

The pressure exerted on the crank shouldn't vary much as it is what is transmitted via the return springs on the clutch cover.

From my recollection of driving a car in this condition it is very noticeable long before any 'over travelling' is likely to have occurred.

It's true there is a lot of side play along the gudgeon pin when the piston is out of the engine, never thought to see how much there is when it is assembled.

It's going to take quite a lot of friction to slow the engine down, it seems to me there must be more involved than a bearing which has the same area and a spring that has the same rate.

[bmcecosse]

The more the clutch is over travelled - the harder the springs are compressed -and the more force transmitted to the thrust washers! It's the same problem on a Mini - although at least it doesn't have the carbon thrust release bearing. But the problem rears high when heavy duty competition clutches are used - the thrust washers take a hammering. Very important on Minor (and Mini) to NOT sit in gear with foot on clutch. Only dip the clutch and engage gear at the very last moment before moving off.

[MarkyB]

I'm not getting what "over travelled" actually means.
It's like corporate speak, the words are English but what do they actually mean in practice?
The clutch pedal is pushed through the floor or the linkage is adjusted far too tight?

I'm well aware of what causes the wear, the symptoms are what are in question.

[bmcecosse]

Well - sufficient travel is where the clutch pressure assembly is moved just enough to release the grip on the plate - thus freeing the clutch to allow gear selection etc. If it is pushed even further in - compressing springs even more - that would be 'over travel' - and would unnecessarily load up the thrust washers. So - clutches should be adjusted so they move far enough - but only just far enough - and not too far! The thrust washers on 848/948/997/998/1098 engines are very narrow - the 1275 has much wider thrusts, more able to carry heavy end loading.

[MarkyB]

OK, that makes sense.