The objective of this procedure is to show you how to machine connecting rods to operational condition. Once you have ground the rod ends and installed new rod bolts you must realign the connecting rod big ends back to a perfect circle.
Part 1. Preparation and safety
- Machine connecting rods to operational condition.
Whenever you perform a task in the workshop you must use personal protective clothing and equipment that is appropriate for the task and which conforms to your local safety regulations and policies. Among other items, this may include:
- Work clothing - such as coveralls and steel-capped footwear
- Eye protection - such as safety glasses and face masks
- Ear protection - such as earmuffs and earplugs
- Hand protection - such as rubber gloves and barrier cream
- Respiratory equipment - such as face masks and valved respirators
If you are not certain what is appropriate or required, ask your supervisor.
- Make sure that you understand and observe all legislative and personal safety procedures when carrying out the following tasks. If you are unsure of what these are, ask your supervisor.
Part 2: Step-by-step instruction
- Remove piston pins
On these connecting rods the piston pins are semi-floating, which means they are free to move in the piston, but they are pinched into a permanent position in the small end of the rod.
To remove a pressed-in pin, the piston is set in a hydraulic press which forces the pin out of the grip of the small end of the rod.
- Remove rod caps and bolts
With the rods separated, the caps now need to be removed. It is important that if you need to hit these to separate them, you only do this with a brass hammer.
Next, the old bolts are pushed out of the rod with the hydraulic press, and discarded.
- Grind rod ends and caps
Each cap is kept with its rod and, one by one, their ends are very slightly reground in the same manner the main caps of the block were.
After the caps are done, the ends of the rods themselves are ground down a little as well. The grinding leaves a sharp edge that will have a tendency to scrape the backs of the rod bearings when they are installed, so the edges are smoothed with a file.
- Install new rod bolts
Depending on the type of new rod bolts to be installed, the edge of the bolt hole may need to be chamfered to accept its shape.
Any metal shavings are cleaned out of the rods and caps and the bolt holes are cleared to prepare them for the new bolts.
Connecting rod bolts live deep in the motor and take a lot of abuse, so it is strongly recommended that - here in particular - you fit the highest quality bolts that you can get.
The threads are lubricated and the nuts installed and torqued on each rod. With brand new bolts the nuts are tightened and torqued and then loosened again and tightened and torqued again a total of three times. This will break in the bolt and make sure that the proper stretch is achieved for all of the bolts before the rod big ends are resized.
- Realign connecting rod big ends
One by one each rod is honed to return the big end back to a perfect circle. The size is checked frequently during the process and when they are close to specification, they are left to cool for a while. Just as when the block was honed to realign it, the metal expands from the heat of the friction of the honing stones. Once cooled, the job is finished and the rods will all be within tolerance of the correct specification.
Larger machine shops might have a rod production machine. It can be set up to run off large numbers of rods in a short period of time. So, instead of waiting for your rods to be machined, sometimes it makes more sense to buy a whole set, with high quality new bolts already installed.