Hi Bro,
I love the way your innovate and think out of the box. We need that as we are the ones taking care of the cars not the workshops. Many workshops don't care.
Having said that, I don't think that patching the expansion tank like this is a good idea for the long haul, compared to buying a new expansion tank. The expansion tank cannot be strong just during normal operations,
it must also be strong also during extended hard driving and overheats. There will be times when your engine might overheat for this or that reason (bad thermostat, bad fan clutch, busted water pump, burst coolant hose, etc) and during those times, the pressure in the cooling system is very high. Extended hard driving also generates higher system pressure. Everything in the cooling system must be able to take this higher pressure.
So while your solution is a good one in an emergency, I think its not a safe bet if you intend to keep the car for the next 10 years. If I were you, I would still go and get a new expansion tank with the 2 new O rings and put it in there. rm300 is not cheap, but we're talking a new expansion tank here. And you cannot sell the car with the expansion tank like it is.
An expansion tank will not go bust except after 10-15 years. And basically, the entire cooling system i.e radiator, radiator hose, radiator cap, bleed screw, water pump, thermostat, and main engine cooling hose (looks like an octopus's arms), engine coolant temperature sensor. plus chelating engine cooling passage flush (prestone super flush, cummin's restore plus), and fan clutch, should be changed once every ten years. It will cost money for sure but I think once every ten years is not a big deal at all, if you divide it as a cost per year and for the perfect peace of mind. Nobody ever buys a BM to save money. So its best to invest your money intelligently. And
only buy oem for the radiator and fan clutch, everything else can be bought from
reputed aftermarket wallahs..
By once every 10 years I mean once every 100k miles, or 160k km. And the waterpump's change interval is half of that. Usually a good idea to change the thermostat together with the wp because it is quite cheap. So when you do your cooling system overhaul, you will be on your second wp and thermostat.
Best part is that the whole cooling system can be DIYed. Its easy enough. Will take around 4-5 hours if you are relaxed. No complicated tools except for the fan clutch's 32 mm spanner. [ Good time to change engine belts, timing belts and timing bearings too as alot of the work is already done, but thats only if your mileage requires it, no need to waste money if not yet required.] And if you can make friends here and ask them to come help you, then treat them to a big BBQ or seafood, it can be done very quickly and easily.
The timing belt and bearings are also to be changed at the water pump and thermostat's intervals. The engine belts are at half of the water pump's interval. So when you do the whole cooling system overhaul, you should also take the opportunity to change the timing belt, bearings, confirm timing mark alignments. It should all come together neatly.
This is what I understand about the E34.
-Cooling system overhaul : Once every 100k miles, or 160k kms. This means radiator, upper and lower radiator hoses, main octopusian cooling hose, fan clutch, water pump, thermostat. Engine coolant temp sensor. Radiator flush with chelating agents that actually bind to and remove rust, don't bother with anything else (only two I've found, Prestone's super flush, and Cummin's restore plus). Switch to aluminium thermostat housing if yours is plastic, plastic housing tend to have microcracks which leak coolant and can burst at any time.
- Radiator flush and new coolant concentrate - 1/4 cooling system overhaul interval.
-Water pump, thermostat and timing belt and bearings : 1/2 cooling system interval. Only use water pumps with a composite impeller, this is not the same as metal, cast iron, aluminium, or plastic. If you use the BMW composite impeller pump **and the thermoplastic water pump pulley), change interval doubles to the same as the cooling system overhaul. The lighter pulley and lighter load due to the composite metal impeller allows the water pump's bearings to last much much longer.
-Engine belts (a/c, power steering, alternator belt) : 1/2 water pump interval, or 1/4 of cooling system interval.
-Spark plug cleaning : same as engine belts, or 1/4 cooling system interval. Only applies to iridium/platinum 4 claw plugs with non adjustable gaps. Clean with a clean toothbrush and carburetor cleaner. Don't clean with cloth or a wire brush, bad results.
-Spark plug replacement : same as cooling system. Only applies to iridium/platinum 4 claw plugs with non adjustable gaps. Bosch plugs are rated to last 100k miles. Even oem copper plugs have been known to go for much longer than 100k miles. NO NEED to change every 2 years. That's another workshop myth.
-Distributor cap, rotor and ignition leads (M20 and non ignition coil engines): Same as cooling system overhaul interval.
-Valve clearance adjustment and valve cover gasket change (M20 and non ignition coil engines) : same as belt change interval, or 1/4 cooling system overhaul interval. M50 engines don't have to do this as they have self adjusting valve lifters hee hee.
-Brake fluid : 1/4 cooling system overhaul.
-Brake calliper seals - same as cooling system overhaul, or whenever you experience uneven braking problems.
-P/s fluid and manual transmission fluid : 1/2 cooling system interval.
-Rear differential fluid : 1/2 cooling system interval (mineral oil), same as cooling system interval (semi/fully synthetic BMW, ZF or other brand oils).
Automatic transmission fluid : 1/2 cooling system overhaul for mineral oil, same as cooling system overhaul for semi/fully synthetic oil from ZF or Japan.
-Auto transmission filter : 1/2 cooling system interval, whether you are using mineral or semi/fully synthetic oil.
- Engine oil and filter : Once every 15k km (this is 1/10 of the cooling system overhaul interval). Only for fully synthetic motor oil from a solid company like Mobil1 etc. Don't change earlier, modern oils can actually go for 25k km (Mobil1 fully synthetic extended performance oil, but hard to find locally), so ignore what the mechanics tells you. If using mineral engine oil, change at half this interval. Change 20% earlier if you're driving something like only 5km each day, most of the time.
- Engine flush : Once every 2-4 engine oil change as per your preference, not less than once ever 4 oil changes. Only use solvents to flush, and the best solvent to use is diesel from a pump. 2 litres. Don't waste time with expensive products that only give you 250ml or 500ml of solution and cost much much more. Nothing special there even from Liqui Moly. Add 2 litres to the engine, idle the engine for 20 minutes (NO revving or driving), then drain the whole lot immediately and change oil and filter as per normal. Can also use kerosene, but switch to 50% volume compared to diesel as its more corrosive so you don't need as much. Any trace kerosene and diesel not drained out fully will be vaporised and expelled out during driving by the crankcase ventilation system.
- In-tank fuel injector cleaning : Once every oil change. Add 3 litres of diesel first, then fill up to full with 95/97 immediately, and then drive. Injectors will be cleaned out naturally over the full tank. No problem to injectors, engines, filters, etc. Can substitute with methanol, ethanol, distilled alcohol, rubbing alcohol, or kerosene, but use the same volume. Need not use it on an empty tank that's immediately filled up with normal petrol. But diesel is cheaper and more convenient so that's recommended.
- Shock absorbers, suspension bushings, shock absorber mounts, tie rods, ball joints, dogbones, trailing arm and subframe bushing : Inspect thoroughly once every 2 engine oil change. Replace only if necessary. PRE-LOAD WHENEVER THESE ARE REPLACED. If you need to replace, only use Meyle HD bushings and shock mounts, where possible. If not available, only use Lemforder. Don't bother with FCP, Ocap, or anything else. Only use Boge/Bilstein/Sachs absorbers.
- Springs : Don't need to be changed unless damaged in an accident, or you want to lower the car.
- Tyres : Must not be more than 5 years old. Must be changed whenever threadwear drops to 20%, or hits the wear indicators. Inspect at every oil change. Rotate at every 2 oil changes. Must be rated for 200kmh or above, if not it will overheat and rupture at that speed.
-Tyre Balancing : All 4 tyres need to be balanced, and not just two. The front two can be high-speed balanced, which needs to be repeated whenever you rotate the tyres. High speed balancing is not essential but is nice to have. 
Tyre balancing should be checked and redone at every rotation (since all 4 tyres are coming off anyway).
- Tyre wear : Must be checked at every oil change for uneven wear. Its a 5-10 minute job.
Alignment : Must be checked and done whenever uneven tyre wear is noticed, or whenever the car drifts too much on a straight road (some drifting is inevitable as no road is perfectly flat and perfectly straight).
- Tyre pressure - checked once every month, rated according to the factory specs on your driver's door. No significant benefit to nitrogen gas fillup, so don't bother with the expense.
Fuel filter - 1/3 cooling system interval.
Wheel bearings - Same as cooling system overhaul interval. Only use oem.
- Brake pads - whenever the brake pad light comes on. Don't need to change all 4 at one go, the rear will last twice as much as the front because the braking system is designed to send more braking force to the front wheels and less to the rear.
- Brake rotors - Check at every oil change. Also needs to be checked, skimmed or replaced immediately, whenever there's shuddering while braking, if there's no shudder but braking seems to be poor and your brake fluid is ok (which means the discs are too thin), and whenever scouring lines and grooves become too severe (this is through visual inspection and by running your fingers over the discs when cold). Tyre shop guys are pretty good at checking this for you.
That's about it. Sorry guys, raining here, and I got into a comfy mood...
Roberto
