Gearbox oil - your suggestions please

[palacebear]

My Series 2 has a 948 engine but still has its original gearbox which is now well past its prime. Its reasonably quiet on short journeys but once the oil gets warm, the box sounds painful. It rattles under load like a diesel engine. Finances dictate that I need to try and get another couple of years (or 3000 miles) out of it. I've checked and it has fairly clean looking engine oil in it. Can I use a heavier grade engine oil in the gearbox to try and prolong its life a little further? If so, what grade would anyone suggest?
Thanks. Tom.

[samuria]

ep 90??? just a thought

[palacebear]

I know modern EP oils are safe in theory, but I'd rather not risk the possibility of EP damaging the phosphor-bronze components in the box, especially as I'm trying to prolong its life for a while more. I'd prefer to keep to engine oil and (probably answering my own question now) I'm guessing that 20/50 will be the heaviest I can get.

[liammonty]

Absolutely - EP oil will kill it in no time!! I guess you could try something heavier than 20W50 - perhaps straight 30W or 40W would do it? I suspect that at gearbox temps (i.e. lower than engine oil) the straight 40w would be thicker than 20W50. I think that 30W was recommended for the gearbox before the advent of the multigrades, which were recommended in the later workshop manuals for the gearbox.

[palacebear]

Thanks liammonty. I'm a bit daft when it comes to oil. I assume an SAE40 oil will be thicker (or higher viscosity) across the temperature range than an SAE30, but is it going to remain more viscous than a 20/50?

[liammonty]

Thanks liammonty. I'm a bit daft when it comes to oil. I assume an SAE40 oil will be thicker (or higher viscosity) across the temperature range than an SAE30, but is it going to remain more viscous than a 20/50?

In a multigrade, the first number refers to the viscosity of the oil when cold, and the second number refers to the viscosity when hot (such as in an engine at running temperature). So a 20W50 behaves as a monograde 20W when cold, but then is as viscous as a monograde 50W when hot. That doesn't mean it gets thicker when it's hot, as a 50W will still be less viscous at say 100 C than a 20W at 20 C. It means that it doesn't thin out as much as a straight 20W oil would, if that makes sense? As the gearbox runs a lot cooler than an engine, the point is that I don't think that a monograde oil, such as straight 30W or 40W would thin down that much at operating temperature (unlike if it were in an engine at higher temperature), so would likely still remain more viscous than a multigrade with a lower 'cold' viscosity, such as 20W50. Clear as mud?!

[palacebear]

Bear with me liammonty. If I've got this right, a straight 30w is perhaps my best bet...? (waits for head-slap coz I've misunderstood again )

[philthehill]

If you want to put a thicker oil in the gearbox Castrol XXL which is a 40 grade engine oil is about the thickest oil you should use.

http://www.ebay.co.uk/itm/Castrol-Class ... SwQItT34Rq

Any thicker and you run the risk of the synchro cones not clearing of oil which can lead to difficulty in engaging gears.

[liammonty]

Bear with me liammonty. If I've got this right, a straight 30w is perhaps my best bet...? (waits for head-slap coz I've misunderstood again )

Yup - or 40W as suggested by Phil

[palacebear]

Thanks for the advice folks. I'll go with 30 in the first place and see if its any quieter ... not expecting miracles though!

[pde2000]

I had a noisy series II box, which i tried to fix with a drop of stp. Sadly the flow to the sliding joint at the back, to the prop, dried up and eventually the prop tore the back off. If anything a better modern oil may be better when up to temperature, at least it will flow to all the parts. If its shuddering like a diesel then it may be trying to tell you something.

I've got a spare that pop's out in second, if it's any use to you. It works in every other respect.

[firedrake1942]

Heating oil and sawdust usually keeps things quiet for a while!

[palacebear]

If its shuddering like a diesel then it may be trying to tell you something.

I've got a spare that pop's out in second, if it's any use to you. It works in every other respect.

It drives fairly smoothly to be fair. It just sounds like its got diesel knock under moderate/heavy load. Thanks for the spare box offer but the plan is to build up a better 948 engine and drop it in with a 948 box. The existing 948 engine isn't too bad so I might just chuck a 948 box in later this year.

[pde2000]

Just been reading the operation manual for the series II, and there is big capitals on page 39; Heavy lubricants must not be used in the gearbox. Use one of the engine oils indicated = SAE 30

Running a modern synthetic engine oil in the box could be very rewarding.

[palacebear]

I know there's engine oil in the gearbox at present, but I haven't a clue what sort. I'll try an SAE30 first. If no improvement I'll try a synthetic. At the mo I'm trying to avoid taking the box out if at all possible - I'm slowly getting over injury so a simple oil change is challenging enough!

[pde2000]

Sorry to hear that, hope you recover rapidly. I only ever dropped the box when i was much younger, and it was tough. With the car up on ramps I had to pull the box back and rotate it while pulling it back onto my chest and then roll it down onto the ground. Putting one back was just as unpleasant.

[palacebear]

Thanks for the good wishes. Thats why I'm putting the job off as long as possible ... at least until I can do a full engine/box swap in one hit!

[IslipMinor]

A 20W/50 engine oil is thicker, i.e. more viscous, when hot than either a straight 30 or 40 engine oil - that is what you should use. It is very slightly less viscous than a straight 50 when hot, but a straight 50 is no use cold as it prevents the synchromesh from working!

The only 'thicker' oil that you could use is a 10W/60, which is thinner than a 20W when cold, and thicker than a 50 when hot - they are synthetic oils, so the box must be completely drained first.

A 90 gear oil is slightly less viscous than a straight 50 engine oil, hot and cold, and less viscous than a 20W/50 engine oil when hot. The gear and engine oil numbering systems are not the same.

[liammonty]

Hi Richard,
My understanding is that the issue here is the definition of 'hot' as the gearbox runs very much cooler than the engine. I believe the upper viscosity in the SAE rating is determined at 100 degrees C, which the gearbox oil is never going to reach, hence the suggestion that a monograde oil with a higher weight than the equivalent 'cold' rating for a multigrade (assessed at 40 degrees?) would be more viscous at gearbox operating temp.

[MikeNash]

Mr Bear,
Recently I've started collecting various temperatures on my 1098 Trav using a remote reading IR thermometer measured at the end of runs and so far the gearbox maximum I've seen after a 50 mile run at a steady 60mph (true) was 37C (the engine oil was 88C). My g'box isn't the same as yours, of course, but it's close, and it isn't perfect but showing its age with noise in bottom gear and occasionally jumping out of gear in 2nd on the overrun. So altogether, I'd expect my temps to be relevant to your g'box. In local running out shopping or around the villages I get temps of 27-33C.
So I support Liammonty's point that only the lower temperature viscosity performance of oils is of interest. I note that my BMC Workshop Manual recommends a range of proprietary SAE30 grades and Esso Extra 20W/30 one of the then new multigrades just emerging then into popular use (although in the text in bold type it says "only . . Sae 30 grade . ."). So your use of SAE30 would be in line with BMC's recommendation, and personally I think that SAE40 wouldn't be a step too far; give it a try and please let us know how it goes!
In addition, I strongly urge you to consider fitting a magnet into the g'box drain plug. Most wear in lubrication systems is caused by wear particles themselves - wear begets wear and hence filters. But since most of it in the 'box is magnetic it can be captured and rendered safe. (I've them fitted to my Trav's all 3 drain plugs.) Modern neodymium magnets are cheap and very powerful and available from places like this http://www.first4magnets.com/neodymium-t137 (Alas, I can't say what size you need not having a spare plug to measure, but I expect Phil will know!) Araldite will fix it in.
Regards, MikeN.