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MG MGB Technical - weak synchromesh?

In my 72GT i have trouble changing down from 4th gear to 3rd gear, there considerable resistance felt on the gear lever. I presume this is a weak/worn synchromesh?,(going up the gears is fine)
Is there anything i can do to improve the situation, other than removing the gear box?
Would a different grade of oil help?
I am currently using 20/50
Would an additive help, or are they too damaging for the Over drive?
Thanks
C.T ODonoghue

Quick fix if you can reach is heel and toeing as you change down. It's very difficult with the pedals in the position they are, though.
dave

In my experience worn synchro does not increase resistance at the lever. It still feels normal but whhen you try to engage the gear it crunches.

Heel and toeing will tell you if the synchro is the problem. While it is difficult to heel and toe with the standard pedals it is not hard to double declutch without braking - I would try that first. If gesar selects normally, synchro is a problem.

From what you have said though sounds more like a selector issue than synchro. Especially since 3rd is a problem. Usually second gear gives up before 3rd.
grant

If you can change from 2nd to 3rd quickly without difficulty then the synchro is probably okay (although it can be misleading as it takes longer to move the stick in a zig-zag than in a straight line). Another possible cause is a dragging clutch, which doesn't affect upshifts but would make downshifts difficult - but then you would notice this on other gears too.

Heel-and-toeing is easy in Bs!
--
Olly
Oliver Stephenson

Thanks guys,
Just to make it clearer, when i am changing down from 4th to 3rd,i currently have to press the clutch peadal home, keep it down and tickle the accelerator,then release the accelerator pedal and she slots into gear.
it is time consuming but i want to avoid crunching gears!!
Adopting an normal driving technique, and the gear lever ,will stop just short of selecting 3rd gear, with resistance felt, and crunching of gears.
Any clearer?

C.T ODonoghue

That would certainly sound like a worn synchro on third. Have you checked fluid levels?
Dave

Dave,
Yes,oil level is bang on , i have recently changed gearbox oil and overdrive filters,( the problem existed before the oil change), i belive the gearbox to be the original one that left the factory, the clocked milage is 93K, but of course it may have done more.
C.T ODonoghue

"...clutch peadal home, keep it down and tickle the accelerator..."

Sounds like you might not be fully disengaging the input shaft from the engine, with the clutch pedal pushed in. Although you might expect to see problems in all gears, I have seen cases where the problem was not significant enough yet to show in all four gears. Think of it this way. If you are not fully disengaging, then the synchros have to work harder to match shaft speeds. Your 3rd gear synchro may just be the first of your synchros to go. I'd suspect your clutch actuating mechanism (like slop in the fork pivot bush & pin), but I have seen an unusual case where the tranny input shaft was locked to the flywheel (due to failure of the pilot bearing). This was not an MGB though.

I would think that you could check for slop, by disconnecting the slave cylinder from the clutch fork, and then waggling.

Terrence Goodell

Can you select reverse silently when the car is stationary. If it grates going into reverse then the clutch is not fully disengaging. That could at least be part of the problem
Iain MacKintosh

C T I'm not saying this is the problem but it could be.
If you hav'nt had the car that long and a PO has been double clutching you can get a glaized syncro.to overcome this problem (you hav'nt got any thing to lose)sit in the car with the motor running,no clutch, both hands on the gear lever and gently push the lever towards third gear, as hard as you can without actually ingauging the gear. Do it for at least 60 sec. As I said if the syncro is stuffed it is too late anyway but what you do is bed in a week syncro and can add years to its usefull life. What a lot of people dont know is that double clutching and other ways of changing gear that take the load of the syncro can make the syncro seem weak for someone who drives normally. But I must admit I double clutch. Denis
DENIS

Yeah, but this tickling the accellerator business with the clutch "disengaged" makes no sense to me. If this makes some sort of difference then it hints that either the clutch is not fully disengaged which is a distinct possibility or the pilot bearing is tight which I very much doubt. I would say that I am not ruling out a synchro hub problem as well but I suspect that at the very least the clutch has a problem.
Iain MacKintosh

Guys,
I cannot say how the PO drove the car, i have owned the car about 7 months, and in that time i have covered around 2000miles.
The car will from time to time , have trouble engaging reverse when stationary, ie grating as suggested by Iain.
So it may be the clutch, How would i go about finding this out for sure? ( idiot guide please)
and how can it be rectified.
Thanks
C.T.ODonoghue

I suggest a couple of tests.

First, like Iain states, the reverse gear change is a strong indicator. Make sure you are dead stopped, in neutral, clutch pedal depressed for a good 5-seconds or more, then slowly pull the lever into reverse. If she grinds, put her back in neutral, wait 10-seconds, then pull the lever into reverse again. If she grinds again, I'd definitely interpret as a "yes".

Second test, which may not be successful depending on severity of problem. Put her in first gear, car rolling very very slowly on the flat, clutch pedal depressed, then give her some gas. If you feel the car wanting to move forward, then I'd interpret that as a definite "yes" as well.

But like I stated earlier, you can disconnect the clutch slave cylinder actuating rod from the clutch fork, and then check for significant movement at the pivot. If memory serves, disconnecting the actuating rod is simply a matter of removing the clevis pin. It'll take you longer to jack up the car and safely support, than it will to conduct this test.

Another test I've done, but it's a bit involved, is to put a stop valve in the clutch piping. This allows you to pressurize the slave cylinder to its maximum extension, and then get another master cylinder stroke's worth of extension. You need to be careful you don't overextend the clutch release mechanism, and it requires a helper. However, it can confirm whether your problem is in the release mechanism or not.

Hope this gives you some useful ideas. Good luck!
Terrence Goodell

C T if its the clutch it will crunch most times when attempting reverse. The advice I gave about syncros is some thing you try when your sure the clutch is ok. It sounds a bit rough but I have even seen it used on new cars with a weak syncro (with the advice from the company) and work well after. Dont attempt to move the clutch any further than its meant to as you can damage the pressure plate diaphram and you can "feel" if you have a good pedal.
Lain When I started driving there were no syncro G/box's and I guess old habbets die hard. Denis
DENIS

Aye well same here I double declutch all the time and sometimes don't even use the clutch but that's another story. Selecting reverse from stationary is the test. If it grates after having the pedal depressed for say 5 secs the clutch has a problem. Now, if you then select neutral and without releasing the pedal select reverse again and it is silent this time the clutch is still knackered. The reason for this is that the friction plate was probably stuck to the flywheel because of a slight oil leak from either the gearbox or the engine. It therefore becomes sticky. The first selection grates but seperates the plate and flywheel and as a result the second is silent. Hope this makes sense.

Sorry but I still think your clutch is the problem.
Iain MacKintosh

Double clutching??
Could someone explain this please.
G. Teal

The correct description is Double Declutching.

There are many definituions which can be found by a Google search including this:-

http://koniaris.com/auto/blip/

Nigel J S Steward

This thread was discussed between 24/10/2004 and 30/10/2004

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