Did a few laps around Barbagallo up against a few 70’s V8’s. When I parked up I found the engine idle dropped notably (stalled most of the time) with the foot on the clutch. Basic diagnosis told me it was the crank thrust washers. Nursed the car for a couple of days and the problem persisted, so ended up pulling the engine apart to find the crank thrust washers had started to fail on the flywheel side.

Washers on the front side (timing chain end) were like new (no wear at all), flywheel end evenly worn as per the photo’s. Corresponding wear on the crank thrust face.

The engine has done about 5,000 kms. Centre main bearing looks like new and there is minimal wear in the oil pump. Oil pressure always looked good and no overheating. Block and crank are standard large journal A+, and is also fitted with a central oil pickup.

What is the cause? If it is over revving or insufficient clearance, why are they only worn on one side?

Is it typical for the thrusts to only fail on one side (when they have been fitted correctly).

The thrusts take a load on one side only because the clutch load tries to push the crankshaft in the direction of the radiator. The heavier the clutch, the heavier the load. The load from the other direction is minimal in comparison.

I'm not sure how the damage occurs in your case, other than age and/or excessive clutch load.

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SooperDooperMiniCooperExpertEngineering

All garage work involves equal measures of enthusiasm, ingenuity and a fair degree of irresponsibility.

They wear on the flywheel side due to clutch release pressure. The fan side hardly wears at all.
If they overheat & wear both sides you have insufficient crank end float.

a. What clutch diaphragm do you run? Blue? Orange? Grey? Double Grey?
b. Was the clutch set up with the diaphragm spring compressed flat when assembled?
c. Do you drive with your foot on the clutch pedal?
d. Do you hold the clutch down when waiting at the lights?

All these things will affect the wear rate of the thrusts.

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DrMini- 1970 wasaMatic 1360, Mk1S crank, 86.6HP (ATW) =~125 @ crank, 45 Dellorto (38 chokes), RE282 sprint cam, 1.5 rockers, 11.0:1 C/R.

Mick, The crank thrust face is worn, but the web proper is OK - Probably salvageable with oversized thrusts.

Kev, Flywheel, clutch assembly is MED with orange spring - not abnormally heavy. Diaphragm spring was close to flat when checked it on the bench and the assembly worked well in the car - no slipping and also good disengagement. I did notice the assembly had a lot of travel when I adjusted the end nuts, but I put this down to having no stop for the diaphragm against the flywheel due to the lightning.

Car is driven daily. Don't hold the foot on the clutch when driving, but do sometimes waiting at lights. We have driven Minis for years (as we all have) but it is the first time I have seen this.

When mine did this i put it down to not properly setting up the clutch probaby due to excessive wear in the taper of the lightened flywheel, all my own fault for not taking as much care as i should. This time with a brand new MED flywheel, i can tell the difference in how the clutch operates and i am confident it won't happen again.

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the world is a book, and those who do not travel read only a page
66 Mini Minor sponsored by http://www.lifeonthehedge.com.au/ The Dog Harness Specialists
It was a pleasure ausmini. I'll miss all you misfits and reprobates

basic question - were the throwout stop nuts set properly?

clutch overthrow can really be the only cause.... can't it?

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did I tell you that I won a trophy?

If the diaphragm is correctly set flat when assembled, when you push the pedal the diaphragm over-centres, the further you push, the lower the thrust pressure.
There are 3 brass stop rivets in the diaphragm cover, but they are a LONG way from hitting the flywheel, more so if lightened.

The stop nuts were originally fitted in the early 60s, when the old 6 coil spring clutch was used - with those, the further the pedal was pushed, the higher the thrust pressure. The stop nuts (when correctly adjusted) prevented the springs becoming coil bound.

Many historic racers leave the 2 stop nuts off, or cut the shaft off flush as well. Saves weight, dunnit.
I've just left mine loose for the last 20+ years, without incident.

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DrMini- 1970 wasaMatic 1360, Mk1S crank, 86.6HP (ATW) =~125 @ crank, 45 Dellorto (38 chokes), RE282 sprint cam, 1.5 rockers, 11.0:1 C/R.

The throw out stop nuts were screwed out and thus not doing anything - same effect as leaving the nuts off. Total clutch travel being controlled by the pedal hitting the floor. I doubt the load against the thrust washers would be any greater at full travel than the maximum when initially disengaging the clutch. - Similar to drmini's comment.

When it was running (with the problem) you only had to press the pedal a little and it would slow the engine revs. Very little pressure on the pedal.

not good after only 500km but looks like you caught it in time: the crank thanks you;
more a question really , if a standard build tolerance is 5-6 thous crank endfloat but racers go closer to 10..... would the same heat load (fanging with the toranas etc) necessitate a think about the endfloat?

I now use 7 thou on my 1360 after I `almost' spun the centre main shells at Wakeffield Pk track day.
Smallbores you can get away with a bit less, like 5 or 6.
GR uses .008" or more on race 1275s I think.

IMO the .003" in the books is ridiculous unless it never sees more than 5000 rpm.

I'd say if they were too tight both sides would be marked bad (coloured, & copper spalled off), and yours are not.
Maybe the thrust face on the crank is worn (not flat now), it should not have put those grooves in the washers.

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DrMini- 1970 wasaMatic 1360, Mk1S crank, 86.6HP (ATW) =~125 @ crank, 45 Dellorto (38 chokes), RE282 sprint cam, 1.5 rockers, 11.0:1 C/R.

Was the block line bored through the tunnel ? If so make sure the thrusts are not distorting when the cap is tightened. Did the crank turn nicely with the centre main nipped down? So many questions

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A proper mini redlines at 13 500 rpm

The block and crank were both low milage A+. All std but had it taken out 20 thou. Was not line bored. I have only checked the centre main bearing but what wear it has is very even indicating things were straight.
There is no sign of any heat on the thrusts, just the rough wear on the flywheel side
It is like there was a rough spot on the crank face that gouged into the thrusts. Maybe I will find a crack in the crank thrust face that opens up under load.

I would take the crank to mr crank grinder and have that face touched up, and regrind journals to the next undersize only if needed.
That thrust face needs to be flat & smooth, not grooved or have a mottled surface.
I doubt you will find a crack in the face; when they go BANG, it's usually through the corner radius.

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DrMini- 1970 wasaMatic 1360, Mk1S crank, 86.6HP (ATW) =~125 @ crank, 45 Dellorto (38 chokes), RE282 sprint cam, 1.5 rockers, 11.0:1 C/R.