Based on our successful and popular YB piston, the Cosworth YB 4WD long-rod piston is designed for use with a longer connecting rod (136 mm between centres). To do this, we’ve moved the gudgeon pin bore up by 7.5 mm. As a result, the piston is lighter and has an improved rod-to-stroke ratio. This change in rod geometry means that there is less thrust loading on the skirt which will give a reduction in frictional losses, plus the piston dwell time at TDC is longer, both of which give an increase in mid-range power. Note that these pistons can also be used in 2WD engines, the only difference being a reduction of compression ratio by around 0.2.
Each set contains four Cosworth pistons, plus high strength gudgeon pins and clips along with high performance compression and oil control rings.
The pistons are forged in-house at Cosworth from 2618 high tensile strength aluminium alloy. Cosworth use a forge tool that is designed specifically for this piston to give the best grain flow in the most highly stressed areas. Cosworth then machine, inspect and pack the pistons, exclusively for Modatek.
Piston Design Features
There are so many features that Cosworth have included with these pistons, its hard to know where to begin!
Starting on the crown, each piston comes with valve pocket cut-outs already machined. There are two compression ratio options to choose from – the 8.5:1 CR pistons have a dished bowl in the crown, whist the 10:1 CR pistons have a flat crown. Once the crown is machined, it is then brightly polished to reduce carbon build up.
Next, we have a couple of features that are designed to keep control of the piston rings. Above the ring grooves are Cosworth’s unique patented anti-detonation bands. These bands protect the top ring by disrupting any high-pressure pulse waves from pre-ignition and detonation. In between the compression rings grooves is an accumulator groove. This groove reduces the pressure on the top ring’s lower face to reduce unwanted ring flutter. It also reduces the pressure on the second ring, helping the rings to seal better.
The oil control ring groove includes an array of oil drain holes. Any oil that is scraped by the rings from the wall of the cylinder bore can drain back through these passageways into the crankcase or sump. These holes help to stop oil from building up behind the ring, which would otherwise find its way up past the rings and into the combustion chamber, creating unacceptable levels of oil burning.
On the skirt below the ring grooves is a xylan anti-friction coating that provides excellent wear resistance, and gives the skirt a very low surface friction to help reduce any frictional losses. The skirt shape is comprised of a smooth blended barrel form with an optimised profile specifically designed by Cosworth.
Finally, the gudgeon pin bore is offset by 1.0 mm to reduce the thrust loads on the skirt, which in turn reduces piston skirt wear and engine noise.
Bore Size Options
These Cosworth YB piston sets are available in three bores sizes and two compression ratios, just choose from the pull-down buttons.
The three bore size options for 8.5:1 compression ratio are:
- Standard bore (90.82 mm) = set no 20088679, piston no 20088635
- +0.50 mm oversize (91.32 mm) = set no 20088680, piston no 20088636
- +1.00 mm oversize (91.82 mm) = set no 20088681, piston no 20088637
The three bore size options for 10:1 compression ratio are:
- Standard bore (90.82 mm) = set no 20088682, piston no 20088638
- +0.50 mm oversize (91.32 mm) = set no 20088683, piston no 20088639
- +1.00 mm oversize (91.82 mm) = set no 20088684, piston no 20088640
- Pin diameter = 24.0 mm
- Ring widths
- Top ring = 1.50 mm
- 2nd ring = 1.75 mm
- Oil control ring = 3.00 mm
- Compression height (pin centreline to top of crown) = 33.30 mm