Article on The Internet Mini Encyclopædia
TOP TIP - BEFORE YOU STRIP THE ENGINE DOWN label up with Tippex each of the main bearing caps and big end caps so you can get them back together on the right rods/blocks, you could carefully centre punch each pair with a number of dots ( *, **, ***, ***) at the same end - the caps are line bored as pairs so you might not get the same clamping alignment if you treat them like pick and mix.
Standard thrusts are copper on steel. The shells are lead-indium
on copper on steel.
Or to put it another way...
Shells : Grey = good, copper = bad
Thrusts: Copper = good, grey = bad
The backing is steel, then there's a layer of copper,
then the lead-indium material that serves as the actual bearing surface.
The supported part should rest on a layer of oil, then pressurized oil, once the engine is started and running. If any particles manage to get past the filter they will try to intrude on the this layer of oil.
Really microscopic ones will pass through into the sump, but some a bit bigger may get laid into the lead-indium layer and stay there, possibly introducing wear to the supported part.
Any loss of oil pressure during application of power or over-run will cause wear to the lead-indium surfaces, eventually baring the copper backing in places.
Consider here the flickering oil pressure light you get when hard cornering a Mini without a centre oil-pickup pipe fitted.
Consider the improved oil pressure at high revs if you have a cross-drilled crank (it offsets the centripetal effect flinging the oil out the big ends and starving the mains.
The copper does not provide as wear-free a surface as the lead-indium does, so further wear will be more rapid, plus the larger clearances due to the erosion of the lead-indium layer will result in less oil-pressure and even more rapid wear. It's definitely all downhill from there, right into the scrap bin.
If it ever gets to the steel while still running, it will
nicely weld things together, occasionally expelling them out the front of the
block, more rarely out the back; often dropping them with great force into the
sump, where the gears will happily join them and jump out the bottom of the
Obviously, this is an undesirable conclusion. If you can see copper, the bearing is toast and should be binned. Keep the sump topped and the oil pressure up. Don't overstress the components until everything is nicely warmed up and the oil pressure has stabilized.
Be aware that high performance bearings are made up of sterner stuff and might not give the same colours with wear.
Bearings, Vandervell part numbers
|998 mains pre '85
|VPM733 (offset locating lug)
|998 post '85
|VPM91885 (central locating lug)
|1275 Mains pre '85
|VPM91120 (offset locating lug)
|1275 Mains post '85
|VPM91886 (central locating lug)
Big end bearings, mixed up
Don't worry it can be an easy mistake if your rebuilding
an engine for the first time.
I Wish to stress...at no time force a rod to turn on a crank if it is stiff/stuck/stubborn... you will destroy your big end bearings!
Remove all rod's pistons etc from the block, remembering to mark ( with white paint/ typists correcting fluid) what came from where! (you don't necessarily have to remove the crankshaft, though it may make things easier)
Replace your big end bearings if you've already tried to insert the pistons and to your dismay couldn't turn the engine (They will be scored and after all the work you put in, you don't want low oil pressure)
Use some patience from here on in... it is important
Take rod/piston no 1
Using your eyes, find a bearing cap that looks as though it may fit squarely onto the rod. (Look for the overall shape of the rounded section etc)
Fit rod/bearing cap to it's usual place and put it in an elevated position, so that if you let go of the rod/piston it will turn on the crank i.e. put it exactly where you would normally attach it (making sure you face the rod ends in the right direction)
Put the two bolts in and tighten to the recommended rating (35 foot pounds?)
Lift crank up (if unattached) and while holding piston in elevated position...let go of the rod/piston. If it rotates very freely on the crank (under it's own weight), then you have found the right cap for that rod. If it does not rotate, then do not force it as you may damage the bearings
If you have found a matching bearing cap/rod then continue the process until you have matched them all (may take some time). Eventually you should be able to tell before you tighten up the bolts, whether a rod/cap is a match.
If you haven't got a match, undo the bearing cap and try that same cap around the other way (turn it upside down - yes it can matter), tighten up the bolts and test it again. If it's correct, go onto the next rod/cap.
If it's still not correct, try another bearing cap,
remembering of course to make sure that you remember which caps you have/haven't
FOR FUTURE REFERENCE: use a punch to mark each bearing and rod so that you don't get them confused in the future, i.e. 1 dot on the rod and cap of number one piston, 2 dots for number 2 etc etc.
Continue the process until you have found the correct cap
for all of the rods. You may find that some of the matches that you thought
you had are not totally correct as you fit more rod/bearing combinations, but
with patience you will soon figure the correct placings, it is nothing but trial
Join the Minilist share your joys and woes with 500+ mini owners worldwide via Minilist at Yahoogroups