Reinforced exhaust valves Mazda MX-5 1.8 BP

Mazda engines covered

The BP is Mazda's 1.8 litre, 16-valve, twin overhead camshaft four-cylinder, fitted to the MX-5 NA 1.8 from 1993 and then to the whole NB 1.8 range until 2005, turbo versions included.

Its 8 exhaust valves, two per cylinder, have a 5.94 mm stem; on the turbo versions they receive hotter, pressurised gases.

SUPERTECH reinforced exhaust valves

An exhaust valve head rises to 760 to 870 °C on a naturally aspirated engine and exceeds 1,000 °C when turbocharged (metal temperature). The seat removes about 75 % of that heat, the guide nearly 20 %. An EV8 family steel goes from 550 MPa at 600 °C to 300 MPa at 800 °C: the head overheats, fatigues, distorts and the seal is lost.

SUPERTECH offers two materials. The black nitrided high temperature stainless steel, with a hardened stem, suits tuned naturally aspirated engines. Inconel is a nickel base superalloy: 860 MPa at 732 °C and 554 MPa at 816 °C, about 85 % more than the original steel around 800 °C; it is the version to choose when turbocharged.

The 1206 versions keep the stem and length of the 1204; only the head grows from 28 to 29 mm. The burnt gases leave faster, the cylinder empties better and the engine gains power; the exhaust seats are machined to this size.

Exhaust valve technical specifications

The four part numbers share the 5.94 mm stem and the 102.0 mm length; they differ by head and material.

Valve fitting

Before releasing the springs, check that the original collets engage properly in the groove of the new valves.

Check the guide clearance with the 5.94 mm stem: a worn guide wears the valve and lets oil through.

The exhaust seats are reground during the rebuild; with a 29 mm head, they are machined to that size.

Technical guide

Our brand

SUPERTECH makes valvetrain parts for racing in the United States; for the BP, the brand offers its exhaust valve in black nitrided stainless steel and in Inconel, with standard head and with a head enlarged by 1 mm.

How to choose

First choose the material for your use, then the size:

When reinforced valves become necessary

• Turbo or supercharger (Inconel version recommended)
• Higher lift camshafts
• Track, hill climb or rally
• Cylinder head rebuild with marked exhaust valves

Vehicle compatibility

OEM part number (exhaust valve):

Don't forget the inlet valves

When the BP cylinder head is off, also change the inlet valves and the valve stem seals.