6 replies to this topic

[minimotor_JD]

Morning,

Just wanted to get everyones view on this problem as its puzzling both me and a couple of guys at the mini club in coventry. Currently putting the my mini engine together, stepping up the engine timing and noticed that there wasn't any movement in the crankshaft.

Loosing the 2 bolts at either end and the main centre strap bolts still no movement in the crank when turning the flywheel end bolt.

Loosing the torques on each big end/shell bearing the crankshaft rotated freely. Everything seems okay in all the bores and only problem I could see was the in bore number 2. When torquing the bolts up in 1,3 and 4 the crankshaft would still rotate.

At this point bolted down the main bolts and centre strap with the crank and no movment.

One thing I need to double check is the flow on each piston to gundgeon pin. Surface is a bit rough but after showing the guys at work its nothing major, If I can't sort it out this weekend I'll be taking the engine to work on monday to get the guys in the garage to have a look.
(Thanks BMW )

It has been bored out to 1293 using new pistons, shells, bearings etc.

See pictures below.





Cheers,

JD.

[pikey7]

check the bearing clearances with plastigauge. How's the endfloat? Too tight on the thrust bearings? Did you clean up the back of the thrusts? sometimes they can have a bit of a burr from the stamping process.

[Cooperman]

Was the crank re-ground? If so, if the journal slightly oversize? It needs to be measured very accurately to check.
Is the offending big-end cap on the correct way round? The two locating lugs on the shells should be on the same side of the rod/cap faces when re-assembled.
Are the inner faces of the rod and of the cap completly clean? Rub them with a piece of very fine wet & dry to make sure.
Are the shells the correct size? Check the size stamped on the back, then measure the actual thickness with a micrometer and compare it to the other shells which are fitted to big-ends that run freely when tightened up.
I once had a standard Rover MPI block in which the centre main housing was too small. It had 'graunched' up the original bearings after 20000 miles and was very stiff after a crank grind with the new shells fitted. I linished the back faces of the bearing shells until the crank spun freely by hand. It's been fine ever since. The alternative in such a case is to use a flapwheel or a hone to open up the housing, but I chose the former solution as the safest and easiest.

[MRA]

The crank should be able to turn freely by hand with the main caps torqued down correctly. once you have got to this stage then and only then introduce the conrod / piston assemblies one at a time checking as you go....

There is no way to accurately do this by hand either with or without hand tools...... the only way to correct a tight bearing is the correct way which is to use a cylinder hone or if really bad a line bore and hone. Any other way is simply lucky

If you want to measure the thickness of bearing shells then use a dti and stand, note this is simply a comparator, but will reduce any risk to the shells by the edges of the mike or vernier,, the shells are not flat and this means that the measuring anvils could damage the specially finished surface of the shell bearing.

If that is a freshly ground crank, look for signs on the stiff bearing shells of any out of place wear patterns, this could be caused by the oil hole burr that is nearly always left by regrinding, they will need to be dressed off using a fine lapping stone or a crank polisher.

[minimotor_JD]

Thanks for your help guys.

Conclusion:

Cam bearings are heavily worn and camshaft will need replacing.
The shell bearings are too tight against the journals
Thrust washers on the crankshaft have worn down to the brass.
No endfloat in the crankshaft and can’t even move it freely.


So I'm buggered.....

Edited by minimotor_JD, 10 August 2010 - 06:53 AM.

[MRA]

Cam bearings are usually machined in situ to fit the camshaft if not done this will destroy them, however if they are too tight there may still be a chance thay can be rectified. Journals need accurately measuring, ie, 4 decimal places with a micrometer (tenths or ten thousandths of an inch). plastigauge will be useless if they are this tight and could damge the shells if used

Isn't this a new engine kit ?? if so how can your cam bearings be shot ?

If the thrusts are too tight sort this out prior to torqueing the crankshaft in to postision.

Thrust bearings are like that they look ok

And it looks like the bolts used to hold your centre strap are the incorrect grade have they got 3 or 6 lines on the top of the heads ? 3 is bad 6 is good

Edited by mra-minis.co.uk, 10 August 2010 - 07:29 AM.

[Sprocket]

I'll bet that center main strap is at fault.

Bolt it down with out the strap with some standard bolts and see if things improve. if they do, refit the strap, if it nips up, turn the strap upside down, if it gets any better, the strap is not flat. If it still nips up, the cap is not flat.

You can always tell if there is a tight big end by moving it side to side on the crank, it should move freely in its own side clearance. 0.003" crank end float is almost impossible to feel when there is oil on the surfaces. Thrust bearings are a copper or bronze material, that is the colour they are, wear on those can normaly be identified by a ridge at the locating tab.

ALL bearing caps should have the bearing notches on the same side as the bearing notches on the block or rods, its a pretty standard rule.

Note the rod cap in the last picture above. That is either the wrong cap, or its on back to front

Edited by Sprocket, 10 August 2010 - 07:08 PM.