The camshaft and valve train layout is identical to that of the spark-ignition engines. On 1985 and early 1986 diesel and turbo diesel engines, the cam lobes operate on hardened steel valve adjusting discs that are located in recesses atop the bucket-type cam followers. Valve clearance is adjusted by changing the thickness of the adjusting discs. Later engines use hydraulic cam followers like those of the spark-ignition engines and do not require periodic clearance adjustment. Dual valve springs and both upper and lower spring seats are used on both diesel and turbo diesel engines.