4.2 Synchro Gearbox selector rod o-rings leaking (still)

[chrisallen01]

I'm just wondering if I'm missing something, I have replaced the 3 O-rings on the selector rods which were rock hard with new ones but I still have a leak. I can only presume that the rock hard old o-rings have worn the top casing so much that the new o-rings won't seal effectively. I think the quickest solution would be to get hold of some oversize O-rings that have a slightly bigger cross section to take up the slack. The other option is to drill out and sleeve the gear box top cover to restore the original dimension, or just buy a new top cover but at GBP600+ this is the last resort.
Does anyone have any other ideas how I can stop the selector (striker) rods O-rings leaking?

[Series1 Stu]

Hi Chris

Are you totally sure that the replacement o rings are to the correct dimensions? I wouldn't be surprised if the supplier just gave you something that was near enough.

Have you measured the top casing to determine the extent of wear, if any? Check for out of round wear too.

If you know the actual dimensions you are working with then you have more chance of finding the correct size of o ring.

Regards

[chrisallen01]

Hi Stuart

Thanks, the O-rings were supplied by one of the usuals under part number C20060 which should be dimensionally correct. I'll strip the gearbox top tomorrow and measure the rods and casing bores as you suggest. I was kinda hoping that someone else on the forum had already found a solution to this problem.

[PeterCrespin]

Is the gearbox breather totally clear to atmosphere?

[Hugo]

Remember that rubber does not compress. It behaves like a liquid. 'O' rings have to be EXACTLY the right size or else they will either leak or the thing won't fit together. There is a formula for working this out but I have no clue what it is.
Don't suppose you still have the old ones so you can compare measurements?

[chrisallen01]

Thanks Peter, Hugo,

Yep checked the breather blew through,just love the aftertaste of MT 90. Measured the bores at 0.628" so a little worn but still round, striker rods 0.623" so again little wear but still usable. My limited knowledge of seals and O-rings rely on an amount of ester to swell the seal material which is found naturally occurring in mineral oils but not found in "synthetic" oils. Perhaps an oil change to a straight mineral gear oil (not hypoid type) might allow the O-ring to swell up a little more to take up the clearance. I've ordered a number of different o-rings with similar inside diameters but thicker cross sections to evaluate, using the UK based BS (British Standard not Bull S***) sizing system against the specified C20060 O-ring supplied.

[Hugo]

Are saying the 'O' ring has to swell before it will seal? That don't sound right.
Another bit of Left Field thinking coming up; - it might be an idea to check that BS numbers haven't 'gone native' like BA numbers did some years ago (in other words they've switched from Imperial to Napoleonic). I have no idea whether that is the case, but it might be worth checking, since you're trying to get a round section rubber seal to sit neatly in a square section recess, and the measurements are crucial.

[politeperson]

I used MT-90 in my all 69' 4 speed all synchro box.

I noticed this oil is is thinner the the good old 75w90 GL4 that came out.

I immediately lost 2/3 rd sychro and got an oil leak around the rod rings.

Drained out the MT90 and put the old fashioned stuff back in. I also changed the rubber o rings.

Synchro came back immediately and the box has been perfect for the last 18 months. Doesn't leak oil either.

[Hugo]

In the good old days, Hypoid oil and EP oil were two different things. If you put hypoid oil in a synchro box it will just make the synchro rings slide & not bite, giving exactly the symptoms you describe.

But now, you have hypoid/EP oil on the market. No idea how that works. I've always found ATF works best with synchro boxes - gives a better gear change.

[chrisallen01]

Thanks for all the replies which are much appreciated. After measuring the original selector rod C20060 o-ring i/d 0.446" (7/16" or 11.1 mm) and cross section 0.104" (3/32" or 2.6mm) I settled on replacing it with an 11 x 3 mm viton o-ring which fits the gearbox top and is just 0.012" bigger overall diameter than C20060. I'll check for leakage over the next few days before putting the E away for winter.

[chrisallen01]

Well, 2 out of 3 selector rods don't leak but the 1st/2nd rod still is leaking dammit, so off with the top again and investigate further.....but the change action is much better now with the straight mineral oil EP90 no clunks or gnashing.

[chrisallen01]

Well, mystery oil leak solved, there is a crack leading down from the gearstick support bracket which is eating the o-ring. Does anyone know if sleeves are available to restore the selector rod bore?

[david muir]

What about filling the crack with Belzona Molecular or some similar paste?

[E600]

Why not get it into a machine shop have all 3 bored and sleeved, problem solved for ever.

[chrisallen01]

Thanks for all your inputs, but having considered that the cracks run all through the gear lever mounting bracket that replacement is the only option. Dennis Welch have re-manufactured the gearbox cover and is very well engineered with extra stiffening to the gear lever mount, just had to drill and tap the thread (16mmx2.0) for the reverse switch. Fitted it last weekend and went for a run no leaks hurrah case closed (literally).