To further explain the reasoning behind my decision to upgrade the big end bearings and rod bolts....
As we look to take Monty's little 2.0 four banger past 200hp, the loads seen by those standard Ford silicon aluminum big end bearings will be higher than they were ever designed for! Ford designed the Duratec well, actually its really a Mazda engine and I strongly suspect Yamaha designed the cylinder head, as such it has huge potential to make more power than Ford intended.
The hard silicon aluminum bearings they chose are actually excellent for long service life, but they're extremely unforgiving if you take the rod loads transferred to the big ends above the design limits of silicon aluminum bearings. The highest horsepower figure Ford ever quoted for the Duratec was 170hp, Ford always test components to ensure they survive 20% more than their highest output number, so I strongly suspect those silicon aluminum bearings will be fine up to a maximum output of 204hp.
However, as you know we're now looking to make more than 204hp, in the new stage 3 state of tune Monty will be living on the wrong side of the edge of it being necessary to upgrade the big end bearings from silicon aluminum to tri-metal. The ARP rod bolts are probably completely unnecessary, but they do become a no-brainer because as we're now changing bearings, the rod bolts will need to be replaced too, so we may as well fit ARP!
If we don't change the big ends now, it's highly likely we will end up running those silicone aluminum bearings at 5% above their absolute maximum design limit, and if I do decide to fit ITBs in the future Monty could easily make as much as 245hp! That's an estimated 20% over the design limit of the standard silicon aluminum bearings, so for peace of mind and to cover future power increases I think paying the extra £350 for ARP rod bolts and the softer more forgiving Mahle Motorsport tri-metal bearings, makes a lot of sense.
They don't have the 200,000 mile durability of the standard hard silicon aluminum bearings, but they should last at least 100,000 miles, at my current 5,000 a year Monty use that's 20 years, at which point I'll be 76 years young and probably looking to drive something a little more refined than a fast road tuned Trad chassis Morgan!
