Hi All. I can't seem to get the search engine to come up with answer to this: What kind of oil should I put in the gearbox of my 1970 BSA Thunderbolt? I have a workshop manual that says SAE 50, but the only fifty weight oil I can find is Valvoline "Racing Oil". Is this what I need?
Also: I don't know what was in it before, but when I drained the old oil it came out all creamy and emulsified. I can't imagine that it was water, because it sleeps in a dry garage. If it's just air, is it the fault of the old oil or what?
Yes, the milky look is from water. Some say that condensation alone can result in that much water in the oil. If not, then water getting in through the filler cap or through the clutch cable stanchion - there's supposed to be a rubber boot on that, which is often broken or missing. The filler cap gasket (fiber washer) may also be worn or missing. The water could have gotten in there a while ago... In any case, water in there for too long can ruin your tranny bearings. Good idea to change the oil at least once a year.
It could have been condensation, but I would tend to doubt it. It does get pretty humid in my garage sometimes, though.
Yes, it has the nylon plug with the dipstick. And the boot around the clutch cable is intact and tight.
I'll try filling it with EP90 for now, but I may end up stripping the gearbox to see if everything's okay because I have a phobia about gearboxes seizing up while I'm dodging tractor-trailers on the interstate.
I don't know if it's necessary to strip the gearbox - you can tell if the bearings are ok by turning the shafts with no load on them. For instance, for the high gear bearing, remove the drive chain and, with the gearbox in neutral, turn the gearbox sprocket with your fingers. For the mainshaft ball bearing, remove the clutch discs and turn the mainshaft. Or, if you have the inner timing cover off, you can turn the mainshaft from that end. The bearings should feel silky-smooth. If they feel "rocky", time to replace. This is of course after changing the oil.
I worked on a T140 with a bad high gear bearing, and you could feel that it was bad just rolling the bike across the floor in neutral.
Also check for any end play (lateral movement) on the main shaft. I just replaced main-shaft bearings because of this. Both 'rolled' just fine, but the center was loose. Made my clutch act funny, hard to adjust properly.
Gotta call you on this one, Mike, not a fair comparison. The rear end in your car is a much simpler and less delicate device than your BSA transmission. And for some reason, car diffs. don't tend to absorb moisture. Ever since I first saw milky oil come out of my gearbox, I change it at least once a year.
Actually, what did it for me was replacing the tranny bearings in a T140, ruined because of water in the gearbox. It was someone else's, so I didn't have to pay for the parts, and I did get a small stipend for my work, but I noted that this was not something I want to go through on my BSA, and it's completely avoidable, for the price of a pint or two of gear oil a year.
How often do you change it in the rear differential of your car?
Every 30,000 miles, got the 17mm hex key to prove it. However, as Mark said, our gearboxes tend to admit water, especially with the vented nylon cap. I think I've solved this problem by drilling a vent hole near the top of my inner cover and using an O ring on the cap. Still, I shall change the oil at least once per season.