Series reliability is a mater of recognizing where the weaknesses are, of putting the vehicle into top shape and keeping it there by religiously following the factory recommended maintenance procedures using top quality parts.
Series Land Rovers require frequent maintenance to stay in top shape.
There is a link to a copy of the factory maintenance schedule near the top of this page:
Series tune up specs
The schedule is in Adobe Acrobat format. If you are going to have a reliable Series Land Rover you need to get religious about the maintenance.
Now about the parts:
Frame - ladder box frame that is very strong but can be killed by rust. Salted roads are their downfall. Most of the time they rust from the inside out so if you galvanize a used frame there may be substantial rust damage already within. Replacement frames are readily available & by far the best are new galvanized ones. Best to own a pressure washer and pressurize the underside after each run in the mud or salt. Best to pay particular attention to the tops of the frame where mud can collect and sit.
Body Panels - Aluminum alloy with steel door frame parts, steel bonnet frame, steel bulkhead & steel radiator bulkhead. Genuine factory panels are thicker and stronger than aftermarket panels. Avoid taking the aluminum panels down to bare aluminum if you can when painting, You will never find a modern primer that adheres as well as the original factory primer. Protect the steel body parts as best as possible.
Engine - Heavy, low power (gutless) but very robust. Petrol and diesel versions share the same engine block design so the petrol engines are very robust and will usually keep running long after normal engines have worn out & expired.
Gearbox - The design was considered innovative then it was first introduced in 1934, but even though there has been several revisions to strengthen it the box is not all that robust for moving a 109. Parts are being discontinued for the SIIA gearboxes, and the NLA lay shaft sometimes break. You can still get all the parts for a SIII gearbox. Figure you should have less than 160 lbft of torque going through the boxes. The Series engine will bolt to a LT77S five speed gearbox but the length is too great for an 88. Ashcroft sells an adapter to fit a Series transfercase to the LT77S.
Transfercase - Really strong and very hard to kill if you keep oil in them.
Prop shafts - reasonably strong but very short slip joints. If you decide to play any suspension games have the propshafts rebuilt with long slip joints and high angle U joints. If you play with the front end for increased articulation best to take a divot out of the top front of the bulkhead transfercase directly below the front propshaft.
Front axle - about as strong as a Dana 44. Seldom gives trouble as long as you maintain them properly.
Rear axle - The 10 spline axles are too weak for serious off road travel, especially for a 109. One ton Land Rovers used ENV diffs & Series III 109's and Defender110s through 1993 used Salisbury rear diffs. The Salisbury is a Dana 60 built under license in the UK.
If have a 109 strongly consider a Salisbury upgrade. If you have an 88 you can buy 24 spine of bigger axles to fit inside the Rover housing. A must in my book. Salisbury drive shaft flanges don't break but they do wear & cause a drive train clunk when they are worn.
Diffs - Not very robust. An ARB or Trutrac is a much stronger replacement carrier and aftermarket 4.75:1 R&Ps are much more robust than the stock 4.7:1 R&P.
Steering - An important part of anyone's exercise programme. Robust if maintained. When you first get a Series rig pull the steering rod ends and inspect the inside of the tubes. They tend to rust from the inside so it never hurts to verify how much metal is there to start with.
Springs - the leaves tend to rust together and friction builds up. When in doubt replace them with new ones. Some people switch over to Parabolics. There are some good ones & some bad ones. Hystee automotive and Rocky Mountain are 2 brands with good reputations. Be sure to keep the U bolts torqued per the maintenance schedule.
Shocks - too short and the limiting factor for articulation when good springs are installed. The only real way to fix this is to relocate shock mounts so you can install longer shocks.
Electrical system - Points manufactured these days are poor quality and a constant source of problems. Consider a Pertronix system. The biggest problem with most Lucas electrical systems is oxidation of the connectors or switch contacts. Always carry electrical contact cleaner for switches & 400 grit sand paper for cleaning up bullet connectors. It never hurts to keep a collection of barrel connectors & just swap out the oxidized one when sanding down the bullet connector. A positive earth combination ignition and light switch is designed to handle the current from stock wattage lamps & only about 13 amps max on the switched electrics. You get maximum reliability if you add relays for the headlights and add a relay to run all the switched electrics.
Use Lucas fuses. UK fuses are rted differently than US fuses. A 30 AMP US fuse will handle much more currant than a 35 AMP Lucas fuse.
The stock generator can not handle the power draw of 2 wiper motors, a Kodiak heater on high and headlamps. Think Alternator conversion. A Delco with built in regulator usually works best.
Brakes - They work OK and are reliable if you follow the factory maintenance schedule. Both front shoes on a 109 are leading shoes which gives your three pairs of shoes stopping your 109 in the forward direction but only a single pair stopping you in the rearwards direction. A 109 can be real hard to hold stationary in a steep nose up attitude. I consider front disc brakes to be a safety upgrade for 109s used in the mountains. Never Mix US and UK brake fittings. They fit together but are incompatible.
Thats mostly it. Take care of your equipment and it will take care of you. Series Land Rovers are the tinker toys of the 4X4 world and can easily be modified to fit your needs. They are very simple to work on. Most experienced mechanics call them seriously overbuilt when they first look at a Series Land Rover. Except for the rear axles, Rover diffs and gearbox they are right. They will take a lot of punishment and will keep going long past when most other vehicles die from lack of maintenance. You just have to remember to maintain them if you want them to be in top dependable condition.
They are primitive, noisy, drafty and leak. If you can't overlook these minor attributes and be blinded instead by the Charisma, a Series rig is not for you.
Car and Driver magazine best summed up a Series and Rover in a 1964 new vehicle performance review "A Land Rover is less a car than a state of mind"
) with a motor that is perfectly fine, but some computer says it isn't so you're not going anywhere (although I drive a 22RE Toyota so that never really happens either 
