Post by crabmaster on Oct 30, 2013 12:15:57 GMT
Have you got an Austin 1800 automatic? If so, then chances are that you have several transmission fluid leaks as well. No, they were not standard equipment – although if you look through the official service sheets, you will find details of the efforts dealers had to go to in order that leaks could be fixed. The exterior joints of the transmissions were painted with 3M EC847 Nitrile rubber sealer, and BMC service recommend that porous castings be painted with a Devcon product – probably a two part hardening epoxy.
I am of the opinion, somewhat optimistically, that oil leaks are potentially fixable and have spent some time in their elimination in my own car. Although “standard” leaks from pan gaskets and speedo drive shaft are easily dealt with, there is one source of leak that is particularly difficult to rectify. That is, droppings from the large-diameter “O” ring on the end sealing plate. Leaks from this “O” ring can be seen as drips coming from the bottom of the transmission casing near the valve body pan where there is a circular shape to the main casting. Unlike a conventional BW35, where the front pump outer “O” ring seals the transmission casing at the front, the BW35TA has the pump mounted separately to the main input shaft. An additional “sealing plate” is therefore required. It is not immediately obvious that this front plate seals to the outside of the transmission because it is somewhat hidden by the converter housing. However, the “O” ring fitted to the outside of this circular plate is indeed an important one since most of it is “under water” all the time. There is no gasket here, just an “O” ring seal to a machined circular bore in the main casing.
The image above shows the seal plate and the outer "O" ring which is the cause of the problem.
It is customary to specify “O” rings by their inside diameter and cross-section. The particular “O” ring in question is 5 5/8 inches, or 142.8 mm ID with a cross section of 3/32” or 2.62 mm. There are suggested requirements for groove dimensions for standard “O” rings and some data is provided below (from Tran Seal Handbook, Western Australia).
Note that the recommended groove depth for a dynamic (sliding piston 2.25 - 2.34 mm) resulting in a somewhat looser fit compared to a static seal (2.03 - 2.08 mm). Now, here’s the problem. The dimensions of the groove in the front plate measure: 2.38 mm. A good match for the standard specifications for a dynamic seal. But, the seal here is not dynamic – it is a static seal. Because of this, with the standard sealing plate, there is insufficient crush on the “O” ring to make a good static seal – the groove is too deep for a 3/32 “O” ring.
The problem is compounded because there appears to be no breather connection between the interior of the transmission case and the interior of the converter housing. When the transmission has the correct oil level, the air space above the fluid in the transmission case cannot vent anywhere. The vent to atmosphere is via the dipstick tube in the converter case. Curiously there is a hole in the sealing plate and its gasket for an air breather to equalise pressure between the transmission and converter housing, but there is no drilling for it in the converter housing casting. To remedy this situation, I decided to drill a breather hole through into the air space in the converter housing. This hole passes quite close to the throttle cable threaded boss so great care has to be taken to put the hole exactly in the right place. Another hole at right angles to this needs to also be drilled inside the converter housing casting to meet up with the horizontal hole from the sealing plate side.
Back now to the “O” ring: Because the groove is too deep, the seal made by this “O” ring is quite weak – the “O” ring is just not given enough crush. One remedy would be to wrap some teflon tape around the groove and then install the “O” ring in an attempt to restore the groove dimensions to the standard recommendations. This could be risky since the teflon tape may leak. I decided therefore to go to the next size up cross-section “O” ring – which happened to be a 140 x 3 mm metric size. According to the recommendations, the desired depth of groove for this cross-section is 2.3 – 2.5 mm which seems to suit the actual depth of 2.38 quite well. However, it is too tight a fit. I had to deepen the groove to the upper limit of 2.5 mm by machining it down in a lathe. No alteration was made to the width. With the 3 mm “O” ring now fitted in this deeper groove, the sealing plate was able to be pushed into the casting with heavy hand pressure whereas before with the standard 3/32 “O” ring, only light hand pressure was required.
Once the gearbox was reassembled, I filled it with oil and let it sit for a few days. No sign of any leaks at all. Let’s hope it stays that way after driving the car.
Tony
PS: Will post an update once the car is drivable again. For the moment, the engine is out having work done on it to repair a burnt piston.