The Ford/Yamaha Zetec se engine a.k.a sigma engine

The sigma engine was a Ford/Yamaha venture that debuted with the mk5 fiesta, mk1 focus and the mk1 ford puma.

It's a brilliant piece of engineering built by Yamaha.

The production technology behind these engines is as such, that they are said to be 'not serviceable' on the bottom end, side of things.

In fact, ford sells entire armed blocks, and the typical part supplier has no parts for the bottom end in stock.

This, of course, is just a quality sealing way of doing things.

There is engineering to the engine that makes them very tough on the bottom end, also very technical in terms of service procedure. So being inexpensive, it's easy to opt for a armed block replacement, and being so demanding in terms of service, makes sense that Ford claims it not to be serviceable and just replaceable.

But I'm not much to stick with rules made for majorities that are not tech savvy.

 

Truth is, you have SEVERAL engine builders that use normal production engines, disassemble them and rebuild them to new specs. 

The fact alone that they are assembled at a point in their initial production life means that  you really can assemble these yourself, as long as you know the secrets and have the gadgets needed to do so.

 

These is and excellent paper by William PERRY, Malcolm MCDONALD, Peter C. CHARLTON called "Quantitative Analysis of Big End Journal Form and Bolt Installation using Ultrasonic Time of Flight Measurements" that actualy take the SIGMA engine rod assembly.

the Why:

The Zetec SE is a very interesting design. For starters the bottom end has a ladder design (much like the Honda's High Revving engines).

The Crank is forged much like the rods, but the way the rods are built is in it's self different. Ford used a Metal Powder Pressure mold forging process. I essence, the metal is in powder state and pressed liquid into a mold that is then further pressed into shape. This results in a close to Forged strength and durability, but in a less inexpensive way to produce.

But then, the rods are forged into one piece, so they are then thermally stressed on the division point and broken by force, The face of these now 2 parts is rough so they will not be able to slide away under stress. It's like a fingerprint.

As such, each rod has only one way to slot in correctly. And this method also requires your torquing to be equal across the rod surface contact between the 2 previously separated parts.

 

Taken from the referred paper:

 

 

They are, as such, torqued, both screws at the same time, and then after the max torque is reached, the bolts have been slightly stretched for increased strength... reusing those is, not recommended... go for fresh new bolts.

So, by looking at the graph, it's easy to say that the proper torque is between 45 and 48nm.

You can torque further into the 52nm range, but you'll be fully relying on the bolt fabrication quality to be that close to the limit. I wouldn't!

Stick with 35nm + 90 Degree (should get you close to 40-42nm)... there are yamaha engines with 25nm + 75 degrees torque specs.

You can go up to:

Initial  42nm (BOTH screws at the same time)

Secondary to 47nm (again, both screws simultaneously).

If not possible to screw the 2 bolts at the same time, make a max 2nm stepping per screw till you reach the figure you want.

This is a lot trickier to do by steps, so I really recommend you to have 2 torque wrenches.

So there is no real dark magic as to why these engines are considered non serviceable... but in truth, they are and have been serviced by a lot of tuners over the years. 

Remember that the Fiesta and the Puma used to compete in the s1400 and s1600 classes. So these engines ARE serviceable as they are also tunable.

It is, however, difficult to find the parts you need to rebuild. Most Tune-shops really only focus on big engines and forced induction, VAG or JDM engines.

However, you still have the Good old brand specialists, particularly in the UK. 

 

For Fords, I STRONGLY recommend BurtonPower. 

These are images of the parts I've brought to rebuild the bottom end of my old 1.4 zetec-se... and as you can see, IT IS POSSIBLE AND THE PARTS DO EXIST, regardless of what people sell you.

The factory rods handles A LOT more than what the car has standard, and the bolts are good for turbo applications, so they too can handle a lot more.

Sure you can go all out and forge the thing, but unless you are force-induction"ing" the car, beyond 250 bhp, I see no reason for that.