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MG MGB Technical - Hard to Get into Reverse

Having a hard time getting into reverse when the car is cold with engine running. I hear a gear grinding noise. Clutch is fully depressed. After some fiddling, I can get it - like going into other gears first - but it's not easy.

However, once I've driven the car around for 15 minutes, it works fine.

Thoughts?
cjd DeHaven

If it's grinding when trying to select reverse the clutch is dragging. This could be due to air in the system but that is less likely to go when warm, and if you have access to another manual car (of any type) then you should notice that the biting point is lower on your MGB than that.

The other possibility - which I have had myself - is gunge in the pilot bearing which means that although the clutch is fully disengaged and works normally the pilot bearing is pushing the gearbox first-motion shaft round. As I say I've had that, and that did go when warmed up, and if I used the car every day. It came back if I didn't use it for a few days.

When I had the engine out there was black gunge on the nose of the first-motion shaft and inside the pilot bearing. I removed all that with solvent then lubricated it, and the splines on both the shaft and inside the (new) friction plate, with Sachs clutch grease and didn't have the problem again. Don't use copper grease here as that does go sticky.
paulh4

I agree that this is a clutch issue. Make sure that the clevis pins are not too worn (no doubt they have some wear) and if you replace them, lubricate them generously with grease. The slightest wear in the linkage translates into a shorter release bearing travel.

The carbon release bearing may be worn down to the point where it is not fully disengaging the clutch. As Paul mentions, gook binding the input shaft to the pilot bushing may also be a factor.

Try troubleshooting the linkage wear points first to avoid an engine-out situation if possible.
Glenn Mallory

Only wear in the linkage at the pedal end contributes to a low biting point, but it would have to get practically to the floor before it started grinding in reverse.

Wear anywhere at the slave end does not affect clutch operation, it is self-adjusting, and this includes the push-rod, clevis pin, release arm, release bearing, cover plate release ring and friction plate. The slave piston simply moves further out and the cylinder back-fills with fluid from the master reservoir. The graphite release bearing can wear down a good 1/4" to 1/2" and you will be unaware of it it, until the casting starts making contact with the release ring on the cover plate. A slowly dropping fluid level is a sign the release bearing is wearing, but is completely normal, and is why you may have to remove some fluid from the master when replacing the clutch and release bearing.

It's exactly the same process as brake calipers where the pistons gradually move out as the pads wear and the cylinders back-fill from the master, likewise when replacing pads you have to push the pistons back and watch out for overflow from the reservoir.
paulh4

My first thought is how cold is it when you do this? My second thought is, "I wonder what oil is being used in the transmission?". The stated problem is difficulty getting into reverse gear when first starting out with the problem going away as the engine (and gearbox) warms up. Back in the old days, when multi-grade oils were just being introduced, "straight weight" oils were the most common and, if you used a heavy weight oil in the summer, you might have to change to a lighter weight oil for winter use. (I remember the syncros being slow, allowing minor gear clash, when shifting in the forward gears.) Back then, there was something called 20W/20 which was an oil that maintained its viscosity, at the same weight, summer and winter. We frequently used it as tranny fluid in cold weather, switching back to 30 weight oil in the summer.

So, first thing to check is what tranny fluid you have. Not a bad idea to change it out as then you will know what you have in it. I used to use 20-50 weight oil for the few years I lived in cold weather climates and do not remember having a problem with it. But, the area of Germany I was living in seldom got down below zero degrees F.

It is also easy to inspect the pushrod link on the slave cylinder to see if the hole in the pushrod and the clevis pin are badly worn. Paul is correct in that, most commonly, wear in that area will not cause the clutch to drag. But, in extreme wear cases, I have found that it can be a contributing factor even if it is not the primary cause of the problem.

The clutch master cylinder, its pushrod, the clevis pin, and the hole in the clutch pedal that the clevis pin fits into can, by itself, cause problems if there is significant wear. This area is more difficult to inspect than the slave cylinder parts. As a result, you often find that it has been neglected even if the slave cylinder pushrod and clevis pin have been replaced over the years.

In addition to the external portions of the master cylinder and its linkage, internal problems within the master cylinder may be present. Most people do not understand that the brake and clutch hydraulic fluids were expected to be replaced, and the systems flushed, about every two years. A clear, glass eye dropper can be used to remove some of the hydraulic fluid from the master cylinder reservoir. It should be clear, clean, and a somewhat golden-bluish color. If it is black with specks in it there is a good chance the internal seals and/or rubber hose going to the slave cylinder have broken down or are breaking down.

I would change out the tranny fluid and inspect the slave cylinder area as a first step. It is the easiest to accomplish and you will know what you have for tranny fluid when you are finished. Then, I would expect the exterior components of the master cylinder complex and check the clutch fluid. Based on what you find, you can decide if further investigation is required.

Les
Les Bengtson

Thick gearbox oil is more likely to stop the gears turning when the clutch is disengaged, so preventing a grind when trying to select reverse.

Thick oil in cold weather could make the gear change baulky - but cold weather shouldn't be an issue at the moment. It's been said a number of times in the past that gear oil instead of engine oil causes this but it doesn't - again if the correct grade is used. Just because gear oil uses higher viscosity numbers it doesn't mean it is thicker. The V8 always used gear oil and mine was a daily driver in all weathers including well below freezing and parked outside and didn't have a problem. Gear oil was even specified for the 4-cylinder in North America at one point, but that may have been rescinded.
paulh4

paulh - as in Bee and Vee? Wow, good to see your name go by...

Paul, your first thought was air in the hydraulic line. Being an engineer, when I see an issue suddenly appear, I think, "what changed?" As it turns out, I recently replaced the brake master cylinder, which involved taking that whole assembly (with the clutch master) out. Could air have gotten in there? Yes. But then, as you mentioned, why does it go away when things warm up.

Your second thought... You mentioned having a similar issue, that tended to go away when you drove the car every day. So, I've driving it every day now, for four days. No change so far.

It's very consistent. Start the car, wait 5 minutes, and it just won't go into reverse. It will for forward gears, but it's not smooth... As of recent, I need to shut it off, put it in reverse, then start it... Then, after driving 10 minutes to the other side of town (lots of shifting) everything's fine. Goes into reverse fine.

As far as oil in the gear box, it's been there for several years without issue. This problem is new.

To clarify... When I say cold, I mean relative to a warm engine/gear box. We're in the Philadelphia, PA, US area; where we're still wearing shorts...

Thanks, everyone. Sooo appreciate your thoughts.
cjd DeHaven

Once it's in gear what's the biting point like? Graphite or 'roller'? In fact they use ball bearings not rollers.

Like you forward gears at a stand still could be difficult, going forward I would sometime have to select third then once moving could select 1st or 2nd. Also for reverse I would have to switch off then restart. Biting point was always good and comparable with other cars. That was the V8.

The roadster was different - always a low biting point that I tried to improve by altering the pedal and push-rod holes, to no avail. But never any problems selecting reverse. Then the release bearing collapsed for a second time - graphite first, roller second. Studied things very carefully and realised the release bearing was being held significantly off-centre by the release arm compared to the first-motion shaft - I was told "They are all like that". 'Adjusted' the release to correct that - and the biting point was way too high. Fitted spare unmodified push-rod and pedal and it's still higher than I would like, but better. If your biting point is comparable to other cars I can't really see it being a similar unbalance to mine, which if worse than mine could well result in grinding for reverse. The V8 is like the midget 1500 in having a guide tube for the release bearing so it is always concentric.

paulh4

This thread was discussed between 13/09/2024 and 17/09/2024

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