Project BMW 318is: Building A Stroker

While we had the engine out of our 318is, we decided to take the opportunity to rebuild it for more power and torque–because more is always better, right? And there’s still no replacement for displacement. Like our earlier engine work, these modifications were done at Metric Mechanic, where owner Jim Rowe and his gang have figured out a way to build a relatively inexpensive, yet rather trick, 2.0-liter engine out of our car’s stock 1.8 liter. By combining elements of our car’s original M42 engine with parts from BMW’s later M44 engine–the same one we stole the lower timing chain cover from–Rowe and crew created a stroker for us. Their formula uses the unmodified crank from the 2.0-liter M44, some custom, short-skirt pistons they have designed, and the 318is’s M42 engine block with some additional boring.

The M44 crank (left) is significantly different from the one from our earlier engine. Most noticeable is the crank sensor wheel on the back of the M44 crank. Although our stroker engine won’t need this crank sensor’s inputs, it will need the balancing provided by this wheel. (Similar six-cylinder engines also used the crank sensor, but with no balancing component.)

The engine builders at Metric Mechanic believe that a lightweight rotating assembly is the answer to all things performance. Keeping weight down will increase the engine’s ability to rev, will reduce the amount of heat it produces, and will reduce wear and tear on the engine. They drop that weight by using their custom I-beam connecting rods (at right) with lightened wrist pins and ARP fasteners to replace the stock pieces (on the left).

More pro tips: Metric Mechanic uses rod bearings (left) that have an oiling groove around the entire bearing, rather than just halfway around the bearing as on the original engine.

The rod end cap needs to be modified slightly to accept the tangs from the new bearings.

Our new, larger 87mm pistons are custom made for Metric Mechanic with shorter skirts to work with the 2.0 liter’s increased stroke. They are also lighter than stock and coated for less wear and more durability. Ours were balanced to even up rotating mass.

Another trick: Metric Mechanic cuts more clearance in the pistons for the valves, making the engine much less prone to damage if the timing chain fails. The indentations in the pistons also provide a bit of room to deck the block.

The stock water outlet piping is pressed into the head and tends to rot away, so Metric Mechanic replaces it with threaded pipe of the same diameter that can easily be serviced.

The block is cleaned, bored, honed, checked for straightness, cleaned again and then painted in traditional BMW black. Here, modifications also have been made to improve the water passages and mount a windage tray.

The third main bearing serves as a thrust washer on the M42 engine, so before going any further, the technicians check end float to make sure the thrust washer is doing its job correctly.

Piston and rod assemblies are tapped into place with two special tools. The ring compressor that Tyler Davis is holding in his left hand is specially made to assemble engines with 87mm pistons, while that is a plastic–not metal–device in his right hand was built to gently tap pistons into place.

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