Big Ends - How Long From First Knock to Death?

[Mj17]

Hi,
If your big ends start to knock how far are you likely to get if you keep driving?

I ask because I think I started getting a noise on the way back from Le Mans (24hrs).  It started at the toll pay booth on the A16, so 40 miles from the tunnel and seemed to kick in at 130kph but be fine at 120, so I did 120.  Back in the UK for the 70 miles to home it seemed to happen at 70mph, so I did 68/69.  The noise seemed to be the same the whole way home, not getting any worse/louder.  The noise is related to RPM and comes in at the same point (between 3000-3500) in both 4th and o/d 4th.

If the big ends were going would the noise have just stayed the same?  Can you usually avoid the noise if you're just under a certain RPM?  Would you be able to drive over 100 miles without things getting any worse?

The reason I ask is because it's also 100% possible nothing's changed and I just had that sudden noise paranoia.  You know, the one where you suddenly hear a noise that's always been there but talk yourself in to it being something new, usually terminal and expensive - and often goes away when you switch the stereo off and realise it's just part of the song that's playing.

Does it sound like paranoia, or a problem.  And if it's a big-end problem can you change them with the engine in on a Spitfire?

[piman]

Hello MJ17,

if it's been knocking for that distance you may get away with just shells but be prepared for a crank grind. What is the oil pressure like, does the warning light come on at idle when hot?

Alec

[sparky_spit]

This sounds very similar to what happened to us in our Mk3 Spitfire on the 2011 10CR.  I knew oil pressure was marginal but stocked up on some tins of STP to help it along and it seemed fine.  On the way down through France it was okay until we started climbing in the Vosges. By next morning the oil pressure was down and the engine was making a growling noise as the revs dropped on each upward gear change.  This turned into a loud knock as were on our 4th or 5th Alpine pass of the day and got so that you could not rev it enough in 2nd gear to snatch 3rd without it knocking, so we couldn't really continue without really damaging the crank.

The car was recovered back to the UK 3 weeks later (we continued on the 10CR in hire cars) and and when I looked at the shells and crank we had just saved it in time.  On number 3 big end there was 4 thou clearance and others were out of spec. All shells were right through to the backing. The crank was still in spec with minimal marks.  New big end shells and new mains were fitted in situ, although I didn't do the front main as it is quite tricky to do in situ.  It's been fine since.  Famous last words.....

The only way you are going to know for sure and have peace of mind is to drop the sump and have a look.  Changing big end shells is easy in situ, but a bit messy.  Mains are nearly as easy except for the front one.  You can measure the big end journals with a micrometer in situ, but not the mains, although you can use Plastigauge on both.

[junkuser]

Big end rattle is usually more related to load than RPM.
Quietens when accelerating  or on over-run and noise at float. (Engine neither applying drive nor braking load)

[Mj17]

Base on your replies I'm starting to think it's NOT the bottom end then.  The only time I see the oil pressure warning light is when starting up, never while the engine's running (though idle is at 1000RPM so never goes THAT low).  Noise is definitely RPM and not load-related and no noticeable loss of power.  Was happy to pull up/coast down the hills on the M20 in o/d 4th provided I kept it even just under 70mph.

My imagination/its always been like that is an option but I guess something gearbox is also there.  It does sound a bit like gears running together - not in a bad, metal on metal way, just a more a Victorian steam-powered vehicle way than a Triumph way.  Oh and while the gearbox does leak a little oil from the overdrive sump plate it was topped up before Le Mans and the overdrive was still switching in/out at a flick of the switch (it's been like that for years and I'm used to when the oil's low engagement gets slower and it's time to top-up).

Travelling for work at the moment but should be able to get the small jobs fixed, oil pressure gauge fitted and some test driving in this weekend.

[Kev W]

Ha Ha! Noise Paranoia! I suffer terribly with it! (clap)

[Mj17]

2396 wrote:

Ha Ha! Noise Paranoia! I suffer terribly with it! (clap)

My worst attack was when this deep, rythmic thudding noise suddenly started.  Turned out my passenger was tapping his foot against the firewall in time to the music...