big end bearings

[green123]

hello my engine has started to knock and when at high speed revs sound very loud i think this to be the big end bearings ,what should i change when doing this is ? is this a hard job? how long will it take me ? is there any order etc just need to know for if it is the bearings what i should do i had this problem on my old engine and turned out the crank was scored so got a new engine havent been out since started knocking just hoping over the weekend to get it done can somebody help

[chrisryder]

Well, you'll need a sump gasket set at the very least. New big end lock washers might be a good idea, but i've found some new ones are hard to fit and require trimming! Theoretically you should change the big end bolts every time you undo them, as they are designed to stretch when tightened. If you can afford them, i'd go for them just to be on the safe-side! I've seen a block with a 50p-sized hole in where broken big end bolts meant the rod went through the block...

Of course, you'll also need the shells themselves, and there's no telling what size you need. You can't just assume they're standard, especially if you're still on this rebuilt engine, as that may have had a crank regrind in its past.

Unless you've got any paperwork about what work was done to the engine, the only way to tell is the time consuming method of taking the sump off, taking a cap off, and looking for any markings: STD, 010, 020 etc... Only then can you order your shells!

With the cap off, it's also a good opportunity to check the crank for scoring. It's a false economy to put new shells on a scored crank as the new shells will just be shredded by the crank and the problem will be back before you know it. You can get recon cranks surprisingly cheaply from the major suppliers, the only trouble is that you really need to have the engine out to get the crank out.

[MarkyB]

you should change the big end bolts every time you undo them, as they are designed to stretch when tightened

I've only ever seen stretch bolts on much more modern engines and always reused Minor big end bolts and lock tabs without a problem.

[bmcecosse]

I have never heard of the 'change big end bolts' idea - at least not on a road engine.... Full Race engine - perhaps..... And yes if the crank is scored - new big end bearing shells are a waste of time. So it needs the sump off (15 min job) and remove one big-end cap (5 mins max) - and inspect the crank! If it looks perfect (highly unlikely if knocking) then take off the others and inspect them... Also remove the centre main cap - and inspect both shell and crank. DO NOT mix up the caps - and note which way round they are.. Lock tabs are fine for reuse too - at least once - maybe not over and over.........

But frankly the odds are that you need a service exchange crank - and a new oil pump! A day's work to pop the engine out - swap crank and pump - and refit.

[rayofleamington]

I've had a fair success replacing big end bearings on a Minor 1098.
The old bearings had large pockets of missing material, so the crank oil pressure (lack of) would have led to engine death.
New shells and a new rocker shaft (also badly worn and causing loss of pressure) transformed the engine back to 'normal'.
It had been a real clunker which had flickering oil light at idle and oil light took 5 seconds to clear at start up.

[bmcecosse]

I forgot to mention - check also that the oil filter is fitted correctly with underplate and spring - otherwise there is NO filtration - this (after shortage of oil due to neglect) is the by far the most common cause of big-end/crank damage.

[kennatt]

if it has been knocking for some time it is a good idea to change the con rod in the piston that has been causing the knock because there is a chance that it has been damaged and gone oval a new shell will quickly wear out if the con rod is not exactly round, at least I would get it checked by an enginering shop if you haven't got the proper vernier tools to do it yourself.

[bmcecosse]

I have never had to change a con rod............ seriously - that's WAY over the top........ The shell would need to be totally shattered for that to be an issue.....

[kennatt]

it still need looking at just in case it is damaged,Why go to the bother of changing shells and maybe a reground shaft to risk doing it all again for the sake of a check with a vernier.If the bearings have run then it will have damaged the caps .Depends how bad the big end is but everyone to their own,I would change it if it was one of the hundreds of engine I have rebuilt,both proffessional,and privately over the last 50 years.we never had an engine returned but what do I know.

[green123]

hello i have took the sump of today and found the big ends and shell bearings to be at fault alll look good underneath just cant seem to get the main bearing of so anybody help on this many thanks

[bmcecosse]

You can only drop the centre main cap with the engine still in the car. Take CARE to put it back on same way it comes off!

[chrisryder]

you'll soon notice if you put the main cap back on the wrong way around, as you'll struggle to refit the oil strainer.

[MarkyB]

a check with a vernier

Surely this is to check the crank pin?
These can get ovaled, but I've never heard the idea of it happening to a con rod either.

[green123]

so i wont be able to take the to bearings of on the 2 sides with out taking the engine out? i have ordered my bearings. do the main bearings have to be changed? do i have to take engine out to do this many thannks

[Alec]

Hello MarkyB,

yes conrods can go oval as well, not very common I'll admit.

Alec

[bmcecosse]

Con rods going oval would need to be the result of a very serious blow-up..... has never happened to me and I've wrecked a few cranks over the years...... And yes - the two end mains can't be changed without removing the engine - in which case you may as well fit a service exchange crank and the ultra important new oil pump..... . Actually - not strictly true - you could get to the front main if you remove the timing gear and then undo the two screws going through the front plate. If the gasket self destructs then you may then need to pop the front plate off too - to fit a new gasket. Rear main is unfortunately trapped by the backplate.

[linearaudio]

so i wont be able to take the to bearings of on the 2 sides with out taking the engine out? i have ordered my bearings. do the main bearings have to be changed? do i have to take engine out to do this many thannks

Not a definitive answer, but if the centre main looks OK, then I would just replace the bigend shells. The main bearings are a weak point on the 948/1098 engine, but it is a MUCH bigger job to change them than the bigends, also they will go on rumbling for a long while, unlike the ends, where eventually you may destroy the whole engine if a rod goes!
Conrod ovality is something more likely to be associated with modern engines where everything is pared down, and particularly in Diesels, which have a far greater force acting on them (and can also disguise bigend knock with the general diesel knock!!). The A-series conrods, particularly on the "small bore" engines, are over-engineered as standard and can be considerably lightened so that you can exceed the maximum safe revs of the engine more easily.....

[bmcecosse]

Trouble is with worn mains - the oil all squirts out there and starves the big ends, which don't last long. If the crank is pretty much perfect - then new shells all round is the best bet, but replacing what you can will certainly get a few more miles out of the engine - and if annual mileage is only a few hundred, then that could be 5 or 6 years.

[green123]

i got the engine out today and got it in bits turns out crank is scored very lighty and cannot feel it the bearings have been replaced before so i thought i would change them all. its all going well so far so hope the new bearigns will be here tomorow and will let you know how it goes or any opiniinons on the crank i cannot aford a new 1 so i dont know what i should do

[bmcecosse]

We would need to see pictures of the bearing journals. And you really should fit a NEW oilpump. And don't forget to check the filter is all present and correct.