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MG MGB Technical - Dropping out of top gear in overrun?

Does anyone have a explanation as to why my gearbox (4 speed all synchro overdrive) drops out of top gear in overrun - with OD disengaged - i.e. going down a long hill with my foot off the throttle? It does have a new rubber gear stick grommet. Are there a worn detent ball or spring or other gearbox component that would cause this?
Mike
Mike Ellsmore

Hey Mike
Did it jump out before you fitted the new boot
Maybee the boot is too stiff- It doesn't take much
I remember when 6 speed boxes cam out in Falcons and 5 speeds were still an option, The 6 speed boot , if fitted to a 5 speed car would flip them out as soon as you lifted off the throttle

Cheers
willy
William Revit

Indeed, remove the boot, try it then, and check the lever is not fouling anything when in 4th.

I can remember from years back very old gearboxes jumping out of gear with internal wear, but I can't recall anyone having that on an MGB.
Paul Hunt

It's usually due to interference with the shift lever boot or sometimes worn transmission mounts that allow the shift lever to hit the floor and knock the transmission out of gear. RAY
rjm RAY

The modern rubber boots are too thick and inflexible. If you compare the old and new you will see. I threw my "new" boot in the bin and put a puncture repair patch on the old one. I't's still going strong!!
Allan Reeling

My 79 4-speed MGB does that in third gear. Have not changed any boots. I was told it is due to a worn synchro. I just hold the gear shift in the third gear position when decelerating. The car has 127k miles on it and the oil has been changed to a 90w gear oil.

cheers

Gary
79MGB
gary hansen

Gary, it could well be a worn synchro ring, but making certain that the tunnel isn't fouling the shift lever, while in third gear, is a quick and easy test to rule that out. And it would be a lot cheaper than dropping the transmission and replacing the synchro. RAY
rjm RAY

Gary, as the gearbox mounts decompose and the box drops the "throw" gets more restricted by the transmission tunnel, check the condition of the mounts.
Allan Reeling

Ray & Allan Good thought on the transmission mounts. Recently the car was on a lift and a tech pointed out the transmission mounts were worn and soft. They looked original to the car.

I did notice also, when the transmission did drop out of 3rd gear, I could go to 4th but NOT downshift back into third unless I shifted through 1st and 2nd THEN to 3rd.

Cheers

Gary
79MGB
gary hansen

The 4 synchro transmission is known for being near bullet proof internally. The transmission mounts, however, are known for deteriorating when exposed to oil for any length of time. This allows the transmission to move about and lets the shift lever come into contact with the floor, often causing the transmission to come out of gear. RAY
rjm RAY

Jumping out of gear is often a product of them not being fully engaged when selected. Worn linkage or selector forks maybe, or, as mentioned previously restricted lever throw.
Allan Reeling

Thanks all for your comments. For the record the problem is with my MGA Coupe that is fitted with a 4 synchro LH overdrive gearbox - see photo of gear stick and grommet in 4th gear position (I don't expect my setup to put anymore force on the gear stick than a regular MGB installation).

I will disconnect the boot and try down a local hill where it usually disengages and report back. (Gear box mounts are new so should be able to dismiss that cause).

Is there any detent ball or other arrangement when the gear shift is in top gear, if so can someone let me know the item and page no in the Moss Catalogue or workshop manual of the pieces?

Regards
Mike

Mike Ellsmore

I realise it is an MGA but a pal had a similar problem with a 4-synch gearbox in a 3-synch MGB tunnel. The cover has to be moved back somewhat for that, and he had the lever hitting the chrome ring in 2nd, 4th and reverse whereas there was an inch or more clearance in 1st and 3rd. I don't have a note as to how he resolved it, and he is no longer with us.
Paul Hunt

When I did the conversion, on my '67 B, I cut a part of the tunnel top cover off and moved it aft about 2" then remounted the chrome ring with no fouling problems. RAY
rjm RAY

Mike Check that rear mainshaft companion flange nut is tight.I have had this problem and simply retightening the nut to correct torque cured the problem.. Jim
jim soutar

"I cut a part of the tunnel top cover off and moved it aft about 2" then remounted the chrome ring with no fouling problems."

That had already been done on my pal's car.
Paul Hunt

Paul, the only thing, that I can think of then, is that he didn't move the chrome retaining ring and tunnel cover back far enough. I've never had a problem with my 4 synchro/OD transmission, mounted in my '67 B, hitting the tunnel or ring. The fact that your friend still had 1" in front of the shift lever, in first and third gears, would suggest that the tunnel cover hadn't been extended far enough back. RAY
rjm RAY

gary

In the past i had a B jumping out of 2nd gear and it ended up being the bush inside 2nd gear being worn and letting the gear wobble around on the mainshaft enough to make it walk out of gear
Others here have had the same happen to them
I guess it would be the same for 3rd gear, but it would have to be a very high mileage car- it's usually second that cops the most wear

hope it's your mountings----------

willy
William Revit

Mike
Does your car have the gearbox strut to stop the engine/gearbox moving forward-
If not that gearlever looks close enough to the main part of the boot to maybee give it a push back out of gear on the overrun/downhill sort of situation

It would be interesting to give it a run without the boot and see what happens there-then

willy
William Revit

"he didn't move the chrome retaining ring and tunnel cover back far enough"

He could have been excused that, as he was blind :o) However the car came to him already modified, but dismantled, so he didn't know if it had ever worked properly.
Paul Hunt

Willy... I too hope it is just transmission mounts. I will have them replaced as I am sure it is long overdue.

The first time it came out of 3rd was on a down side of a mountain road. It would not go back into 3rd. It could go to 4th, but not return to 3rd. I pulled over and stopped the car, put it into 1st, 2nd then 3rd and finally 4th. My obvious concern was that something came loose inside the transmission so I just held the clutch to the floor when I went "through the gears" while stopped. Carried on after everything shifted smoothly It has happened a number of times since. So now I just hold it in 3rd when I decelerate. There has never been any grinding or gnashing while either upshifting or downshifting - always smooth.

Will post back after I get the mounts replaced.

Meanwhile, Happy New Year to all. And as always thanks to all for posting

Ro & I intend to be in the UK later this year to celebrate our 20th year together. We hope to tour a bit through the EU then return home

Cheers

Gary
79 MGB
gary hansen

Perhaps check the run of the OD wire loom beneath the stick cover and the tunnel. It is possible that it has moved at some point and may be opposing the third selection position.
R Taylor

R Taylor... The car is a straight 4-speed, with NO O/D unit.

cheers
Gary
79 MGB
gary hansen

Sorry Gary, my confusion between Mike's original post and his 4th gear issue and your separate 3rd gear problem. My suggestion to Mike was based on the photo he provided.
R Taylor

This thread was discussed between 24/12/2015 and 04/01/2016

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