Broken crank

[BigCHarg]

The crank has just broken on our 1100. I did not know that the developing and intermittent vibration was serious until I had done ~15 miles at a relatively constant 60 mph. Any advice on what will require inspection/replacement, other than the obvious? Also, reading other threads,is a pulley damper a simple bolt on improvement, or more complicated?

Any advice gratefully received.


Chris H

[chrisryder]

blimey, that sounds fun

was it a catastrophic end? i presume the engine must have siezed surely?

the crank will make for an interesting doorstop/talking point anyway!

i'm not sure what things especially to look for after such an occurence. ofcourse, be extra carefull when fitting the replacement crank and when you're bolting up the main caps make sure the crank is always free to turn. that should help make sure the block is ok.

with regards the damped pulley, they're pretty much a straight swap. if you're changing cranks and keeping your old flywheel, then if you're fitting a damped pulley too it could be a good idea to get the whole lot (pulley, crank, flywheel and clutch cover) balanced as a set. balancing should help with smoothness of running, and crank longevity.

when i've had them balanced it's cost about £45 for getting all of those parts done as a set.

[bmcecosse]

You should probably renew the main bearing bolts - and of course check the mains caps are not cracked. A new oil pump will be an essential - and clean out all the oilways very thoroughly. The crank damper just fits in place of the original front pulley. Note that the heavy steel pulley sold by some firms these days is NOT a crankshaft damper. As you have discovered - the 1098 crank doesn't take too many revs - it should be held below 6000 at ALL times........

[chrisryder]

on that subject bmc, as most of us are not blessed with a rev counter, could you divulge what speeds in each gear correlate to 6000rpm?

[bmcecosse]

No idea really - I don't have rev counter either. It will depend on the final drive gearing - but I know mine does 50 in 2nd and 70 in 3rd. I suspect at rather more than 6000............

[chrisryder]

way to go practising what you preach there...

i think the most i do is 55 in 3rd and 80 in 4th. i recall seeing 25 in 1st once...

having said that i saw IGN in 4th

[chrisd87]

From memory, with the standard wheels and 4.22:1 diff, 6000rpm equates to about 97mph in 4th.

[mike.perry]

I snapped a crank on my Series MM on the M 4 one morning.There was a rather load banging noise and I thought "That's not big ends"
Will post a photo when I dig it out of the box

[BigCHarg]

Thanks all for the advice. The failure was not catastrophic, the engine ran until i turned it off on the hard shoulder of the M3. Shame the engine was professionally re-built only 13 months and 3000 miles ago!

Chris H

[bmcecosse]

Professionally rebuilt? I wonder by whom...... Were you thrashing it at the time ? Had you been thrashing it ? Both times i broke cranks - I drove the car home - carefully admittedly, but not at 10 mph either. Figured there was nothing to lose.....

[IslipMinor]

For a 1098, 4.22 diff and 145/80x14 tyres, the speeds at 6000 rpm are around:

1st 27
2nd 45
3rd 69
4th 98

With a standard, unbalanced 1098 engine the odd excursion up to these speeds in 1st or 2nd will probably be OK, but I would not recommend holding more than 4500-5000 for sustained periods.

[MarkyB]

Crikey! 27 in 1st! It would be all too easy to overshoot that, and I doubt that first gear would last very long at all.

[chickenjohn]

To avoid these problems it is simple!

Don't thrash the car!

Change from 1st to second at 10mph, 2nd to 3rd 20mph, into 4th at about 30mph. A little bit of mechanical sympathy goes a long way!

[bmcecosse]

Yaaaaaaaaaaaaaaawwwwwwwwwwwwwwnnnnnnnnnn
C'mon the eco drivers!

[chickenjohn]

Yaaaaaaaaaaaaaaawwwwwwwwwwwwwwnnnnnnnnnn
C'mon the eco drivers!

That is another reason not to thrash the car- petrol is expensive!

[james.ed]

phew.. 27 in first. my car would be literally squelling in pain long before then

[minor65]

I agree 100% with chickenjohn, don't thrash it !!

[phurn]

Tbf I find 4th to be the universal gear, it'l take me from 20 to about 75, which is when I loose bottle with my ropey box!

[chrisd87]

I also tend to use 4th down to 20-22mph in normal driving. I particularly tend to avoid changing down into 2nd as the synchro is very lazy. That said there are occasions when pulling out onto fast roads, when I will go up to about 20mph in 1st, not changing into 4th until 40-odd. Quick getaways are sometimes needed to stay safe!

[MarkyB]

That is another reason not to thrash the car- petrol is expensive!

Absolutely, Roy seems to want 50 mpg while rally driving.
I think a car where flat on the floor equals 1/4 throttle would work wonders for his fuel consumption.