M106 engine is based on M30 block with following exceptions:
- Piston oil jets
- Turbo charger oil feed and return
- Two knock control sensors
- Turbo charger-blower temp sensor
- Pistons with dish
- Turbo charger-air cooler
Block does share common features with other M30 engines including:
- Forged crank, stroke 86mm
- Forged rods, length 135mm
- Cylinder bore, diameter 92mm
- Engine oil cooler
Compression ratio is 8:1 with displacement of 3430cc. Pistons are turbo specific and deliver low compression ratio. There are two oversizes available from BMW (+0.2mm and +0.5mm). You can get pistons with large-bore and forged alloy from aftermarket manufacturers (Ross, CP and JE). With a large-bore (over 93mm) non-factory head gasket may be needed, M90 or S38 gasket should do. Also multilayer gaskets are used for bigger bore. Light forged pistons decrease engine rotating mass and make it more responsive. Forged pistons decrease mass up to 1kg over stock.
Forged pistons have bigger cylinder wall clearance due to thermal expansion, this can be two or three-fold compared to stock. Big clearances introduce piston slap and noise when running a cold engine. On 12000km driven Ross pistons with 0.1mm clearance this can be heard on engine warmup cycle. It should be noted that forged piston are better for serious power than for long-term durability. You should not take granted them holding like stock ones. Durability is dependent on forged alloy used either 4032 or 2618, latter being worse in durability and better in strength.
Beside forging strenght is from much lower silicon content on forged alloys. On cast pistons silicon presents low thermal expansion and tight clearances but significantly reduces strength. Some manufactures offer both 4032 and 2618 alloys for pistons and others just one. Forged pistons are priced closely to originals, but you can freely decide on compression ratio, bore and more. It might be good idea to hold on stock pistons if engine is not aimed for extreme power and oversizes are able to restore bores. Often honing and new piston rings bring life back to block.
Piston oil jets are there for cooling, excess heat is removed by squirting oil directly underside piston. This is somewhat uncommon performance modification on BMW engines at the time. Later on jets can be found on S38B38 Motorsport engine. M30B34 block can be uprated with S38 oil jets, only capable machinist is needed. Using M30B34 block can be easier for replacing original turbo block than searching for a second. There is room for additional bore on block if everything else is fine. Metric Mechanic M30 turbo engine displacement is 3600cc meaning 94 mm bore size.