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MG MGA - Knocking sound at idle

I need some ideas. I have a knocking sound coming from the back of the engine/bell housing area.

It does not seem to happen when the engine is cold but appears after the car has been driven a few miles and thoroughly warmed up.

It only occurs at idle with the car in neutral. It disappears as soon as you throw the clutch or rev the engine above 1,000 revs or so.

Some relevant background facts:

The engine is a 1950 5 main bearing unit with a lot of compression and a mid range cam. So it has a fairly rough idle.
The engine holds 70 lbs of pressure at 3000 revs and idles at about 28-30 lbs fully warmed up
The clutch disc, pressure plate, carbon release bearing, slave cylinder, clutch hose and the spigot bearing in the back of the crank are all new and have done less 200 miles.
The only thing that I didn’t replace was the clutch release lever pivot bolt. It has some movement in it but does not appear to vibrate.
The clutch appears to work perfectly and there is no vibration or harshness in the drive line whilst driving.
The gearbox is an MGB Mk 2 overdrive box that has just been rebuilt by a very experienced MG gearbox guy in Melbourne.

I don’t want to have to pull the engine out again as it has been out twice already this year. Can anyone suggest a likely cause??

thanks
Brendan Sullivan

Brendan,
I have the same set up in my coupe. It required a new drive tunnel and removable rear gbox mount. Are you sure you have clearance between the bell housing and tunnel. Have you tried wedging a wooden block in the gap temporarily to see if this removes the knocking.
Mike
Mike Ellsmore

Brendan, My first car was a PA in 1954, it came with receipts for a fully reconditioned engine. It used to make the same noises that you have described and it later turned out to be the gudgeon, instead of the pin turning in the in the small end of the rod the whole thing (including the bush) was turning in the eye of the rod. A new bush (or reaming) cured the problem!
Barry Bahnisch

Mike

Initially I thought it was the tunnel knocking on the gearbox but what bothers me is that the sound only appears when the car is warmed up. Given that a car would if anything vibrate more when cold, this doesn't make sense. I will try the wooden wedges suggestion tomorrow but I am not hopeful.

Barry
My greatest fear is that I have spun a bearing but given that I have good oil pressure and the noise goes away as soon as I throw the clutch I am hoping that the bearings are ok.

Thanks
Brendan Sullivan

Worn thrust bearing?
Art Pearse

Art. A worn thrust bearing evidences itself when the clutch is depressed and the crankshaft is moved forwards because of same. With the Triumphed engined midget 1500, it is possible to see the crankshaft pulley move forwards when the thrust bearing has been destroyed.

Brendan. You either have some portion of the engine/exhaust system hitting the frame or you have some form of internal engine problem. The first thing is to put the car on a hoist (preferred) or on jack stands and start the engine. Then, move under the car and look for areas which might be making contact. Look for dents, scratches, and visible contacting areas. If you do not see anything, assume that the engine might me making the noise and use a mechanic's stethoscope to probe various engine areas to see whether the noise is coming from the front, rear, upper, lower, or mid-level on the engine itself. In this manner, it is possible to refine one's area of search and, hopefully, determine if a full engine removal and tear down is required.

Les
Les Bengtson

Les
When I start the engine cold there is no knock. If I run the engine up to normal operating temperature by letting it idle for 20 minutes or so, there is still no knock.
BUT if you then take it out for a ten minute drive - hey Presto it knocks...only at idle. Blip the throttle slightly and it is gone. Throw the clutch and it is gone.
This makes me think it may be the gearbox rather than the engine.
Are there any circumstances that you are aware of where a recently rebuilt Mk2 full synchro gearbox would make a knocking sound at idle in neutral and only when warm.

Thanks
Brendan Sullivan

Brendan,
Why not take the car back to the guy who overhauled the gbox and get his opinion. Do you live in Melb?
Mike
Mike Ellsmore

Mike
I am in Geelong. But you are right my next step is a call to Mr Needham.






Brendan Sullivan

Brendan,

A thought only, but might the gearbox rear flange locking nut not be fully tightened? Heated, however slightly 'lengthened', components just might produce a fraction of gap and allow 'slap' at the flange except when loaded by driving or gear changing.

Regards
Roger
R Taylor

Brendan
Fairly normal to have gearbox rattle when hot with your combo of high compression and a lumpy cam
Easy fix is slightly thicker oil in the gearbox
Having said that, a standard clutch plate usually is soft enough in it's dampening springs to soak it up
But I would guess that you have a heavier clutch in there preventing this so thicker oil is probably your only option
You mention Needham
Has the g/box been fitted with a straight cut gearset
If so it is going to rattle at idle
willy
William Revit

This thread was discussed between 05/09/2014 and 11/09/2014

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