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MG MGB Technical - COMING OUT OF THIRD

My 1979 MGB is a straight 4-speed with 115k miles. Just developed this "new" item. Going into a curve doing 50, I double clutched, downshifted to third and let the car decelerate into the curve. Just as I came out of the curve, the transmission dropped out of third into neutral. Yesterday, going on a twisting downgrade, I shifted from 4th to 3rd and the tranny shifted into neutral. Then went to shift it back into 3rd (clutch depressed) and it grinded. So then I just put into 4th and braked down the hill.

I suspect I have a worn synchro. Any thoughts on this item.

If the tranny must come out, it will be changed to an O/D unit or a 5-speed.

cheers

Gary
gary hansen

It could be simply a weak synchro retaining spring, allowing the synchro to disengage, but I suspect that you have a worn synchro/hub assembly. A trans teardown, to repair it, would be in order. Try to find an od trans, as this provides you with the ability to use 3rd od as well as 4th od vs the 5 speed. It also keeps that English car feel that these cars are all about. RAY
rjm RAY

Bad engine/gbx mounts can cause it to kick out of gear as the unit thrashes around. A loose nut on the 3rd motion shaft can let things move enough to do it too - have to remove the front UJ flange to check that one.

FRM
FR Millmore

Mine is the same box and does the same thing. It is slowly getting worse. I have also found when it does do it you can't then get it back into third. It graunccches!

I also have a couple of old OD boxes I need to rebuild so when the time comes I can swap it out. I just live with it for now.

I find it happens mainly when I am off the throttle and the engine is developing high vacuum. Usually when coasting downhill off throttle.

Pain in the neck really. Without an OD third seems to be my most used gear!

Something Garth suggested to me is fitting the indent spring form the 3 synchro box. It is stiffer. You need to shorten it a little apparently so it will fit. I haven't been able to try this myself yet.

Simon
Simon Jansen

A good link : www.mgcars.org.uk/mgcc/sf/sf001201.htm

Pierre
P.Y.A. De Rouck

It could be that just the stiffness of the rubber boot round the lever is pushing it out of gear so that's worth trying. It's also worth replacing the selector rod ball and spring to see if that helps but I fear that it is the synchro hub that is worn and will have to be replaced. Live with it for as long as you can or until such time as the engine has to come out for some other reason and then rebuild the box.
Iain MacKintosh

Thanx to all for your input. This seems to be a good winter project. I would like to go with Ray & Simon's adivice and install an O/D unit to the tranny while I'm going through the effort.

Simon, your symptoms are exactly the same as mine.

Ray, I agree with your thought process about the "English Car Feel"

Pierre, Thanx for the website info.

Fletcher, the mounts are new and everything seems solid

Iain, I do have new shifter bushing I will install to see if I'm really lucky to fix it with just that.

Cheers

Gary
gary hansen

Bigger job adding an OD to a non-OD gearbox than swapping the whole thing, you have to change the output shaft. While you are obtaining that, once you have the OD, it would be easier to obtain and swap the whole thing.

Whichever, there were two types of 4-synch OD - one for chrome bumper cars and one for rubber, and they have very different speedo drive gears. Chrome bumper cars had a black label OD, rubber a blue. If you fit the other one you need to change the speedo or get it recalibrated as the former has 1280 turns per mile and the latter 1000.
PaulH Solihull

Paul: So, I really should make the effort to locate a rubber bumper 4-speed O/D unit along with drive shaft and speedo cable including entire sheath. As the car is a 79, should I concentrate on locating a 77-80 unit or does that matter.???

cheers

Gary
gary hansen

Gary - any rubber bumper unit will work just fine, the only difference with the later models was the gear-lever manual switch. As your column switch won't have the OD function you would have to provide an alternative manual switch if you can't get a 77-80 unit with the gear-lever switch.
PaulH Solihull

Gary-
I would suggest taking (or sending) the transmission to John Esposito of Quantumechanics in Monroe, CT and trading it in on one of his rebuilt units with an overdrive. John has been rebuilding these transmissions for many, many years and really knows his stuff. I can personally vouch for his transmissions as I have referred three friends to him to get overdive transmissions and they're happy with both his service and the quality of his work. Just to give you an idea, he both bonds and rivets the friction lining to the conical clutch of his overdrive units! Work like that and there's never a warranty problem. He has a website at http://quantumechanics.com/ You can give him a call at (203) 459-9612.
Stephen Strange

Many thanx to Paul and Steve for the advice. It is good to do the research prior to tearing into these projects.

Again, many thanx

cheers

Gary
gary hansen

I've just today had an email from someone asking about overdrive repairs, he said Quantummechanics appears to have gone out of business.
PaulH Solihull

Hmmm. I wonder if there will be a sale of inventory. Items like a rebuilt 4-speed tranny with an O/D unit for a 77-80 MGB.

cheers

Gary
gary hansen

Paul-
I contacted John Esposito of Quantum Mechanics. He has not gone out of business. He has over 30 jobs in the shop right now and is going to Lime Rock race track this weekend for the vintage fall racing event. About 20 of his customers will be racing and he will be there to support them. He wants to know who started the rumor that he had gone out of business!
Stephen Strange

OK, I've passed that back together with the contact information you gave earlier.

PaulH.
PaulH Solihull

I got a 3 synchro indent spring (60 NZ cents!) this week so one of my jobs this weekend will be fitting that. Hopefully will help with the popping out of third issue until I can get one of my OD boxes rebuilt. Will let you know whow it goes.

Not sure how I get to the bolt to actually fit it yet. It seems that's on top of the box. Is there enough room to do it from under the car?

Simon
Simon Jansen

Well, fitted the spring. It is possible from the engine bay but it's a bit tricky I found getting a socket on the bolt and getting enough leverage to undo it. The bolt is a 9/16ths on top of the box on the left hand side. I found I could just get a socket on it with the handle pointing to the right and it could turn one click on the ratchet before the handle hit the bellhousing. Once you break the tension on the bolt you can unscrew it by hand.

I used a stack of small magnets to extract the steel pin and the spring. Then I just popped in the 3 synchro one which I ground down to be the same length as the old one.

I went for a test drive around the hills near my house. It has definitely helped. Hasn't eliminated the popping out of third totally but it's not as bad as it was. Probably now as bad as when I first reassembled the car about 1000 miles ago which would indicate to me the problem has gotten worse over that time.

For the low cost of the spring and the relative ease of changing it it's definitely worth a try I think.

Simon

Simon Jansen

If I was fitting o/d to a later car I would avoid the switch on the gear lever. The wires are known to break where they flex at the base of the gear lever and can cause o/d failure, or a catastrophic short of the unfused white wires to ground.
Stan Best

Hi Folks:

Had a nice conversation with John Esposito at Lime Rock Park on Saturday 4 September about replacing "D" type "O" rings. I am planning a trip to Monroe CT in near future; so he is still in business.

Rich Boris 67B
Rich Boris

Simon, I don't think that I would have ground the new spring down to the length of the original one. Kind of defeats the purpose.
Iain MacKintosh

Hi Iain, it's a stronger spring I think? But you could be right. I need to check with Garth but it did occur to me maybe I am only supposed to take a little bit off so you can get the bolt threads to reengage easily.

Oh well, another 60 cents to try again!

Simon
Simon Jansen

"The wires are known to break where they flex at the base of the gear lever and can cause o/d failure, or a catastrophic short of the unfused white wires to ground."

Which is why I have fused my ODs even though they don't have the gear-lever manual switch. Also both my cars and three I have worked on have all had damaged white wires to the fuel pump, which is similarly unfused, and I have those fused as well where the main harness joins the rear harness. Extremely useful to have both fuses when I had a float sink on the V8 and cause a gross fuel leak. I was able to cross-connect the OD and fuel pump circuits and use the OD switch to turn the pump on and off to put some fuel in the carbs but not overflow, to get me home.
PaulH Solihull

You might try to remove the tail shaft housing & check if the mainshaft nut has come loose. I have seen this several times over the years. It is a bit rough but you can do this without removing the gearbox.
w Bacall

This thread was discussed between 25/08/2010 and 19/09/2010

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