Reinforced inlet valves SUPERTECH BMW M20B25

BMW engines covered

The M20B25 is the 2.5 litre, 12-valve, single overhead camshaft inline six-cylinder petrol engine of the BMW 325i and 325iX E30 (saloon, Touring, convertible), of the 525i E34 and of the Z1.

It has 6 inlet valves, BMW part number 11 34 1 706 631 or 11 34 1 708 895: 42 mm head, 7.0 mm stem, 102.5 mm length, held by 3-groove collets and operated by rocker arms with adjusting eccentrics. Designed for the standard engine, they reach their limits with a sharper camshaft, firmer springs, higher revs or forced induction.

SUPERTECH reinforced inlet valves

The inlet valve lets fresh air in on every cycle, strikes its cylinder head seat again and slides constantly in its guide, dozens of times per second at high rpm. It works between 400 and 550 °C, where the original chrome-silicon steel is already losing strength: 500 MPa at 500 °C, 250 MPa at 600 °C, i.e. 50 % less. More lift, stiffer springs, higher revs or forced induction increase the impacts, friction and heat it endures.

The SUPERTECH valves offered are forged from a competition chrome-nickel stainless steel of the EV8 (21-4N) type, which keeps 650 MPa at 500 °C, 30 % more than the original steel, and 550 MPa at 600 °C, 120 % more. Their harder and smoother black nitrided surface limits friction and wear in the guide.

The +1 mm oversize takes the head from 42 to 43 mm, with the stem and length unchanged from original: the passage at the seat widens, the cylinder fills better and the engine gains power, provided the cylinder head seats are cut to this diameter.

Inlet valve technical specifications

Valve fitting

SUPERTECH pairs its 3-groove valves with its KPR-VW7-T collets; new collets are recommended on reassembly.

The +1 mm oversize fits a cylinder head whose seats have been cut to the diameter of the new head, a job to plan with the cylinder head rebuild.

The M20B25 has rocker arms with adjusting eccentrics: after replacing the valves, check the clearance and set it with the eccentrics.

Technical guide

The brand offered

SUPERTECH, in San Jose, California, has been making competition valvetrain parts for more than 30 years. Each valve is designed for its engine, checked by ultrasound, X-ray and laser measurement, then validated by engine builders, from circuit racing to drag racing.

How to choose

On these 3-groove valves, the choice is about the size, that is the head diameter.

When reinforced valves become necessary

• Sharper camshaft or firmer valve springs
• Raised maximum rpm
• Turbocharger or supercharger added
• Cylinder head rebuilt for a circuit, drift or rally engine
• Original valves marked, worn or already at repair size

Vehicle compatibility

OEM part number (inlet valve): 11 34 1 706 631 ; 11 34 1 708 895

Don't forget the exhaust valves

A rebuilt cylinder head also takes new exhaust valves and valve stem seals. The exhaust valves for this engine are available here: