Sudden big end noise?

[NIGELAN]

Any opinion would be useful.

I have a 711m bottom end built on a big valve head less than 5000 miles ago. Started car this morning, only 3 cylinders, not too unusual as damp and cold. Ran at a little above tickover to warm up and suddenly got what I assume is big end knocking noise. Engine still ran but obviously turned off ignition fairly quickly. Any idea what would suddenly cause loud big end noise / failure. Regular oil changes, not low on oil, good oil pressure, never revved above 5500 rpm.
I think I know what I have to do but I do not understand how a big end can suddenly fail. Can anyone educate me?

[StressCraxx]

I am assuming you have a cast iron crank, either the 2737E or 711.

Gearbox in neutral, remove lower dust cover at the flywheel. Use a 5/8" spanner on the front pulley bolt.

Hold the edge of the flywheel with one hand and turn the front pulley bolt with the wrench back and forth. Feel for looseness.

Pull spark plugs, stick a wooden stick or long screwdriver down spark plug holes, turn the crank forward and back and feel the piston top.

The longer stroke tall block cranks are known to fatigue and fail between the #3 and 4 crank pin web. Since the failure occurred at low RPM, hopefully nothing else was damaged.

[NIGELAN]

Thanks StressCraxx,

Taking a bit of a risk, i started up again and slowly drove 100 yds to get it back into garage. No problems other than noise.

Does this alter your response?

Thanks in anticipation.

[Chancer]

Are you familiar with what big end noise sounds like? for those of us of a certain age it was a regular and unforgettable occurrence but apart from at a race circuit I have not heard a vehicle on the road with rattly crankshaft bearings for d?cades.

The failure mode you describe is not at all what I would expect to run a bearing, the running on 3 cylinders points to something else but its a frequent enough occurrence itself on a Lotus.

If it is idling on 4 cylinders then removing one plug lead at a time will silence any big end noise on cylinder which is no longer firing.

[NIGELAN]

Okay, very helpful.

I admit I was perplexed that a big end would suddenly go from no noise to a lot of noise.

I have a clear lclack clack' noise, engine rpm related, louder at no.1 cylinder when listened to with a stethoscope.

I will try the suggestions and see what results I get.

Thanks

Nigel

[gjz30075]

It's possible that one of the cam follower buckets is breaking up and wobbling in its bore.

[NIGELAN]

Thanks Greg, I will add it to the list of diagnostics.

[nmauduit]

I have a clear lclack clack' noise, engine rpm related, louder at no.1 cylinder when listened to with a stethoscope.

I will try the suggestions and see what results I get.

just to rule that out, are you sure it is not an exhaust noise (e.g. leak from gasket at the head)?

[NIGELAN]

Interesting!

Will check, earliest time will be on monday.

I will report back.

Many thanks all.

[Chancer]

There really is nothing else that sounds like a shot big end bearing and unless the rod is about to make a break for freedom you dont hear it at tickover.

A loose flywheel can sound just like multiple main bearing failure at a steady RPM and a really loose one, like about to go part company sounds like like a bucket of bolts at tickover so the cracked crankshaft suggestion is plausible.

[NIGELAN]

Am really hoping for exhaust gasket leak, I don't fancy the cracked crank diagnosis

Will report back, thanks.

[Chancer]

An exhaust gasket leak would manifest itself at wide open throttle/high revs not at tickover and sounds nothing like a big end bearing failure.

[rgh0]

When faced with a noise I don't know the cause of I try to troubleshoot by elimination of easiest quick things first and then progress to the harder things. So I would do something like this.

1. Check the engine externally when running both sides, top and bottom to try to determine the source of the knocking and to exclude things like exhaust leaks. remove 1 plug at a time to determine if the noise stops

if nothing then

2. Take off the cam cover and look for any issues with cams and valves

if nothing then

3. Pull the plugs and look down the plug hole with a borescope for damage inside the combustion chamber. Modern scopes are very cheap and you can get ones that pug into your phone for around $25

if nothing then

4. Then its probably in the bottom end, you can pull the sump in situ if you want and you have a removable lower chassis cross member to investigate further but personally these days I prefer to pull the engine for further work.


You may find something in the earlier step that enable you to jump a few steps if your lucky

cheers
Rohan

[NIGELAN]

Thanks Rohan,

I have great respect for your knowledge.

Hopefully I can get on to some diagnosing on monday

Thanks all, will give feedback when I can.

[StressCraxx]

Thanks StressCraxx,

Taking a bit of a risk, i started up again and slowly drove 100 yds to get it back into garage. No problems other than noise.

Does this alter your response?

Thanks in anticipation.

An exhaust leak at the manifold flange is a very plausible explanation. It can be very loud and easy to detect.

I have had a tall block crank break. Twice. Once at 5800 rpm. There was not much left to salvage. Crank broken in four pieces. Tossed almost every single part under the head. The second one at 3500 rpm only damaged the crank and bearing shells. It's a dramatic noise, even at tickover. Usually an exhaust leak is very dramatic when cold, then as things warm up and expand, may subside. When you said it started up firing on 3 then got very noisy, I didn't suspect exhaust by your description. However, turning the crank by hand, gently back and forth with the plugs out will help detect looseness if there is any.