BN1 Gearbox oil

Have had many BJ8's..first 100..manual says that the filler plug for the gearbox has built in dipstick, mine does not. How can I tell the correct level of gearbox oil on this early model. Also do most of you use castrol 20W-50?

Thank You!

I just fill mine to the bottom of the threads of the plug, there is no dipstick on these. There is sort of a shelf in the aluminum there, add enough oil to cover that shelf. I use regular motor oil, just straight 30 weight -- but I think 20W-50 would be fine. I have heard of people using 90W gear oil and that lubricant might work better if you have a worn OD unit.

Get ready to fill it regularly, I haven't seen many of these trannys that don't leak when parked, usually through a variety of places in the overdrive. I need to top mine up several times a year.

If the oil gets low, the OD will fall out of gear or get very slow to engage.

Bill S

HealeyJag -

There is no dipstick in the BN1 gearbox, you fill it up just until the oild starts dribbling out the hole.

Definitely DO NOT use multiweight oil in the BN1 because

1) Multiweights have friction inhibitors which make the synchros work harder when shifting

2) Multiweights have lots of detergents which keep particles in suspension (to be picked up by the engine oil filter) which can wear out your OD and gears.

The only oils I would use in the BN1 gearbox is either Redline MT90 or Non-detergent 30 weight.

Word of warning, last year, I changed the oil in my BN1 gearbox, likely the first time it had been changed in decades. I simply drained the oil and put in Redline MTL without flushing or any other examination of the box, and within 10 miles the cluster gear's second gear (actually third as the BN1 gearbox has a blocked off first gear) was damaged to the point of requiring replacement of the cluster gear (not an easy find) and an unexpected large cash outlay. I am guessing there had been a lot of sludge in the box which was then freed with the new oil and something, maybe a bearing or synchro chunk, found it's way into the gears. It needed a rebuild already, but I would have save a huge chunk of change if I had it rebuilt before this damage.

(I don't blame the Redline, as there had to have been something solid to have done the damage)

-j

To BN1L 156424,the failure of the gear after the oil change may have been an unfortunate coincidence. The BN1 gearbox is notorious for this failure. Sometimes the cluster (or laygear) fails and other times the mainshaft gear. Austin changed the helical angle on this gear only at some time during production. Could that have been to correct the problem? If so, it didn't work. I heard a theory once that the use of overdrive on second could be a factor in causing this failure. Any comments from senior members welcome. I have had two mainshaft gear failures over the years and the curious thing is, the teeth broken out of the earlier failure went close to fitting into the gaps on the later failed gear! I believe that this indicates an inherant weakness in the design.

Pan -

Yes, second gear (third in the box) is a very weak gear, and one of the main reasons they switched to the 4 speed box with the BN2. It was so bad that very early in the production of the BN1 (I think sometime early in '54) they switched the helical angle of the gears. They had also changed the oil feed hole spacing because for some reason it's the teeth next to the hole that break.

The change had no effect and they still were breaking gears, so then after they started producing the BN2, they had even a third gear design for the BN1 which was used in gold seal rebuilds and sold as a set for dealers to install in broken boxes. I think this third set is a little better, but very very rare these days.

In addition there is a fourth set (or first depending on how you look at it) of which many Australian friends take the gears out of an A90 and put in the BN1. Strangely, the A90 doesn't seem to suffer from broken gears as much despite being a substantially heavier vehicle.

It may have something to do with the fact that the quality of manufacture of the early gears seems to be much much better than the gears they made later on for later BN1s.

This is one of the reasons why I don't have any plans to uprate my BN1 to M spec... I think with the old box its well enough to leave the power band alone.

According to my parts manual and one of my restoration books I most likely had the later gearbox, as they changed it pretty early in the BN1 Production (March 54 iirc, mine was built may/june 54). It may have been a coincidence, but I doubt it, I think flushing the gearbox would have prevented the laygear death, but I still would have required the rebuild.

-j

I've been told that using non-detergent oil is essential. (Don't ask me why), but this was advice I got from an old Healey nechanic.He also recommended SAE 30, so that's what I use.

healeynut, thanks for your reply. My BN1 was built in July 54 but I can't workout if I have the early or late helical angle gears. I have a parts book but it doesn't specify which is which as I can't read any part number on the gears.
I have rescued an A70 gearbox for future emergencies. The Austin Motor Club is still after me for cannibalising their beloved old jalopies!

Pan -

If your 100 was built July '54 then you will more than likely have the later type helical cut.

For my 100 and A90 I have:

2 x A90 spare gearboxes (one of them gold seal)
1 x spare rebuilt BN1 box w/ OD
1 x spare later Laygear in good shape.
1 x original early BN1 box w/ broken leaygear but otherwise perfect!

I think I have all I need to keep these cars going for a few years!! I must be nuts...