All of the Bosch Glow Plugs I have bought were made in France and were less then $10 each.

I had previously had Monaks and one of those died with in one Year and I live in Southren California where glowing a long time is not needed.

Previous to the Monarks I had Autolites and after only 6 months of use the tips on 2 of those swelled making them difficult to get out.

How long Glow Plugs last is largely dependent on if everything is happening within specs during combustion. In particular Injectors that dribble or spray Fuel onto or too close to the Glow Plugs shorten their life.

There is also the previously mentioned carbon build up that was reamed out.

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84 300D, 82 Volvo 244Gl Diesel