Big end shells..

[sumpcracker]

How often do new shells work on an old crank without regrind?

I fitted many to rebuilt engines and all ok but whenever its a quick "in situ"change due to a slight rumble, the rumble has always returned in my expierience...

Is it worth doing or not?

(just bought another 944 with a bit of bottom end noise circa 3000 rpm.)

[jrevillug]

I fitted new big end shells to an old crank - a quick in-situ job to get rid of a bit of a big end rattle - just recently. 20 miles later the rattle was back.

I suspect it depends entirely on whether the wear is in the crank or the shells. In most cases they seem to wear evenly.

I'd be tempted to suggest that a crank regrind would be worthwhile - just doing the shells is a risk, and if it fails then everything that you've spent on the quick change (time, cost of shells, sump gasket, gakset goo, 2x oil and filter change, etc.) is wasted. Having the crank ground straight off would have saved me £50-odd and 5 or 6 hours.

Given that a set of shells for the 944 is ~£90 (just from a quick search - I'm sure there are better prices) I think it would be worth splashing out and doing it properly (or at least semi-properly: reground crank, all new shells, inspected oil pump, cleaned sump - depends whether you think it's worth stripping the engine right down to clean out the oilways etc).

[welshpug]

if you check the tolerances before sticking the sump etc all back up you'll tell pretty easily if it's going to work or not.

If its a rumble not a rattle/knock then you may get away with it, but anything noisier will usually knock the journal and cap out of round

[MrSpeedy]

A rumble is usually main bearings, big ends tend to rattle/knock

If the crank measures within factory tolerances for roundness, size and surface finish, then there is nothing to be gained by grinding

[workinprogress]

it depends...you need to accurately measure the the journal diameters if they are worn under spec you'll need a re-grind...if the journals are scored or marked badly you will need a re-grind...if there are any signs the shells have spun in their caps you'll need a re-grind.

if none of the above apply then you should be fine fitting new shells,,,just make sure everything is perfectly clean,,,never fit the shells dry,,,always lube the contact surfaces and as you do up each big end cap spin the engine over to ensure theres no lock-up through dirt or bad tolerances.

always tighten the caps as per the correct torque settings

[jonallsorts]

I've found as long as you polish the white metal off the crank, new shells work fine. But it was a forged crank so probably puts up a little more resistance to wear in the first place.

[sumpcracker]

Thanks guys, yep forged crank and as you say probably takes a bit more to wear it.

I finished the job yesterday and it seems ok so far (80 miles) i got the shells from promax for £70 and used the original sump gasket (rubber) - fingers crossed..