View Full Version : TR4/4A Overdrive Leak

03-25-2018, 03:03 AM
I need todismantle my entire interior--again--now to access my Overdrive. Attached,below, is a picture of my leak. This thing is unbelievable. I have a full cupof fluid just from the car sitting, and each time I drive it, it drips rapidlyafter cooling down. I've never had a gasket behave like this, so I must reallyhave something going on here.

I'm not seeing any cracks and I used Hylomar Blue, so I'm a bit surprised.

Any thoughts welcome. thx

03-25-2018, 04:52 AM
Are you sure the leak is coming from the gasket, and not say the operating shaft? Sometimes it can be really tough to see oil running down a vertical surface, and much easier to see it dripping off a low point.

When you get the cover off, check that the O-ring is in place. It's easy to overlook. Also examine the gasket carefully (since you used Hylomar, it should still be intact).

Remove the studs and check that both mating surfaces are perfectly flat. I like to lightly rub a flat, fine tooth file (aka machinist's file) against the surface just enough to polish it up. Any low spots won't get polished, any high spots will show up early. There's almost always a high spot or two around the stud holes. (If you are ambitious, it's actually good practice to put a small countersink in each hole to prevent getting a high spot there.) I think I see some small damage to the edge of the cover in your photo, a tiny groove near the front edge. There might be a high spot at the end of that as well.

Reinstall the studs, and spin a new nut down each stud in turn. It should turn easily all the way down the threads. Double check that the space between the nut & OD body is smaller than the thickness of the cover. Do the same checks with the two bolts. Don't forget new lock washers when reassembling, nor the flat washers on the two bolts.

03-25-2018, 12:25 PM
In my experience, you need a pretty bad gasket failure to get the kind of leakage you describe, so be sure to examine all places where a leak could occur. I second the suggestion that the O-rings may be the problem. When I took my OD apart, I found that the O-rings looked OK but had hardened quite a bit; O-rings need flexibility in order to seal properly.

James Christie
03-25-2018, 12:55 PM
This is my first post on here so here goes. After 44 years of ownership of my 3A I decided to do something about the persistent overdrive gearbox leaks at the end of last year. I changed every ( and Imean every) single gasket and o ring and it seems to have solved the problem. An autopsy on the o rings showed that most of them had gone brittle, so leaks wee not surprising. Getting the 'box out is a pain and getting it back in a more serious pain but it is all worth while. Alas the adaptor plate twixt box and overdrive can warp and that requires a bit more effort.
A visit to the TR Register site will be rewarding - and hopfully encouraging.


03-25-2018, 01:41 PM
OK, I'm on it. I'll report back. BTW, I forgot to mention something I felt was irrelevant, but maybe not so. The leak got way worse after I re-tightened the side OD housing plate about 4 months ago. I'm now wondering if the bolts aren't going in correctly, or if I warped the plate by the way I tightened things--though I could only access two of the bolts (or studs, can't recall which). About that rubber seal. I was very careful about that on my rebuild a year ago. new seal.

03-26-2018, 09:09 AM
Been following in the distance...but the fact that the box was rebuilt last year raised a flag. If you do end up pulling the box to find the leak, I recommend you dress all gaskets, and especially the non-gasketed mating surfaces, with Permatex 51813. This is the goop used to seal crank-case halves together with no gaskets at all on motorcycles and aircraft engines. It never hardens, and if by chance you over-dress and some breaks off internally, it is formulated to dissolve, so will not plug OD passages (for long).


03-26-2018, 10:04 AM
John, do you also recommend using the Permatex 51813 on both sides of the "brake ring" of the overdrive? As I recall, the manuals don't mention putting sealant on the brake ring, but I find that is quite often a source of leaks.

03-26-2018, 08:17 PM
I did use it on the ring with my rebuild. I looked at that ring and decided there was no way it was going to still be true enough after 60 years to seal. So far the only leak I had was the actuator rod o-ring that I put on the wrong side of the solenoid plate. That went away quickly after I put the o-ring where it belongs! The rebuild is now 4 years old, but the tranny has only been on the road for a year.

03-27-2018, 03:55 AM
OK, I'll bite. Please tell us which side that O-ring goes on. I don't want to learn I did that wrong, too.

03-27-2018, 10:33 AM
OK, I'll rub some salt on my wound, LOL. Here is a blow-up picture from the Laycock De Normanville A-type OD manual:


In the pic the o-ring for the actuator shaft is distinctly shown on the outside of the accumulator spring cover! Then, in all the other pictures which are updated for the Triumph OD version, the illustrator was as confused as I was, as the pics show the seal disjointed, but more outside the cover than in.

Well, after the shaft started leaking I posted here and other members set me straight. Sure enough, when I removed the solenoid/accumulator cover, the groove for the o-ring seal was right there on the inside of the cover. No leaks since...so far, as it is a Triumph?!?