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MG MGA - transmission noise

Good evening all.
I have just finished a total, body off resto of my MGA. The car drives very well, with one exception: when the car is stationary, engine running, and I push the clutch in, all is well. However, when I engage a gear (any gear)I sometimes get a loud grinding/bearing noise. When I let the clutch out the noise goes and the car drives as normal. This also occasionally happens when I change gear on the move.

I have rebuilt the gearbox, but didn't replace the input shaft bearing because it seemed fine and there was no noise before the resto.

The clutch is new; including release bearing (carbon), fork pin and bush. I also replaced the spigot bush.(soaked in oil before hand).

The noise is intermittent and my reasoning is this:

If I get it when the car is stationary and the clutch in, then theoretically it wouldn't be a gearbox bearing problem, as the box is not turning.

If I don't get it when I depress the clutch, then it's not the thrust bearing?

I'm thinking spigot bush? What do you think?

However, I don't really see how selecting a gear while the box is not turning would cause the spigot bush to wail.

Any help or advice would be appreciated.

m s randell

Matthew, if there is binding in the spigot bush, when you depress the clutch with the gearbox in neutral the gearbox mainshaft is still able to turn with the crankshaft despite the clutch being dis-engaged. But when you select a gear you force the mainshaft to stop, thus giving you a difference in speed between the crankshaft and the gearbox mainshaft and the spigot bearng goes 'ouch'! That's my theory anyway unless someone can come up with a better explanation. I'd drive it for a bit and see if it goes away.
Lindsay Sampford

See here:

Many "new" replacement clutch discs are actually rebuilt (relined) from a used core part. If the splines are worn and loose there is a good chance you may get this LOUD screaming noise when you depress the clutch pedal.

With gearbox in neutral, engine running, clutch up, the clutch is engaged, and the friction disc (and gearbox input shaft and laygear and 2nd and 3rd gears on the mainshaft) turns with the flywheel. When you depress the clutch pedal, the input shaft and disc have to slow down to dead stop before you can select 1st gear. During the slowing down time the disc is turning at different speed than the flywheel and pressure plate. Once the disc stops you can select 1st gear, then let the clutch up to go.

As the clutch pedal is coming up, the flywheel and pressure plate are gripping the friction disc, to make the car go. There is a short period of time when the disc is slipping before being completely gripped to turn at same speed as flywheel again).

During the periods when the disc is NOT turning at same speed as flywheel you can get the loud screeching noise. This can (usually) be silenced with a new spigot bushing and new clutch disc (new center piece not rebuilt). Meanwhile, the web page linked tells how you can train your feet to banish the noise,so you may not need to R&R the engine to change the clutch parts.
Barney Gaylord

Thanks Lindsay and Barney
I can engage 1st and reverse easily and silently with the car stationary, so the box must be still. The noise comes when I engage the gears, but don't let the clutch out. One thing I forgot: before this, I noticed a quiet but distinct and regular 'click' noise, about every second or so, when the car was stationary. It sounded like a noise that a speedo cable might make if catching; but was coming from the gearbox.
Also, don't ask me how, but I ran the box low on oil for about 10 miles. I was concerned that I may have caused some damage, but as I said, the car drives well with quiet gears and good changes.
I'm still thinking spigot bush.
m s randell

Matthew, I think you are right. And it may be that the spigot bush is spinning inside the crank. When I took my engine out recently the bush just fell out.
N McGurk

This thread was discussed between 26/04/2012 and 27/04/2012

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