Connecting Rod Honing Basics

Sunnen's Complete Cylinder Head and Engine Rebuilding HandbookOver the past few weeks we’ve been talking about Connecting Rod Reconditioning in Tech Notes. Thanks to our friends at Sunnen® Products Company, we’ve been able to share material from their book, Sunnen’s Complete Cylinder Head and Engine Rebuilding Handbook. Unfortunately, this book is now out of print. If you’re looking for a copy of the book, they’re pretty scarce but you might be able to find one on Amazon or eBay. We also did a short search and found that it is available as an e-book, but a membership is required in order to read or download the text.

As we move into our last section on Connecting Rod Reconditioning there’s a lot to cover. If you’ve been working on engines for any length of time, you already know most of it. With that in mind, we’re just going to cover a few basics here and give you some links to additional resources.

Cap and Rod Cutting

Face and side view of connecting rodAs we’ve talked about in previous articles, all connecting rods have a parting edge. Most are flat surfaces machined into the cap and rod sections. These surfaces must be straight and perpendicular to the rod sides. If they aren’t, cap misalignment can reduce clearance between the rod and the crankshaft journal.

In most rebuilding cases, you will remove .003” (0.08mm) from each mating surface for a total of .006” (0.15mm). This small of an amount of reduced center-to-center distance won’t interfere with the compression ratio significantly and general doesn’t compromise the deck-to-piston clearance; even when the deck is resurfaced.

You will, of course, need to remove rod bolts when getting ready to work on the mating surfaces. To do this, you will probably need a press and disassembly fixture. Be sure the surfaces are clean as well before doing any machining.

A Few Important Tips

  1. Be sure to identify the type of parting edge you’re dealing with as each is handled differently. More on this later.
  2. When machining your rod(s), be sure to clamp them into the machine firmly. If they are at all loose, the grinding wheel may push the rod up so that you are removing less material than you planned.
  3. Always machine the full set of rods and caps the same.
  4. When finished grinding, clean bolt holes to remove any chips or debris that may have accumulated then install new bolts.

Parting Edges

Fractured Parting illustrationAs stated before, most of the connecting rods you will deal with have a straight parting edge. You may also come across rods with Tongue & Groove (T&G), Serrated or Fractured parting edges. Due to the many irregular edges common in serrated and fractured parting edges, there isn’t much you can do to machine these.

Tongue & Groove Parting EdgeTongue & Groove (T&G) parting edges can be machined but there is a very specific way in which to work. First off, it’s essential to note that most manufacturers incorporate a slight clearance between the tongue and bottom of the groove. When grinding these rods, be sure the amount of material removed does not exceed the amount clearance or you will need to grind the tongue to restore proper clearance.

Tongue & Groove Parting Edge Grinding“To grind tongue, place parting edge of gauge rod so tongue surface DOES NOT rest in any of the grooves of the gauge rod and place a shim under the groove surface approximately equal to the thickness of the tongue. Clamp tightly, remove shim and grind as normal.” – page 301-302

Rod Honing

The most common way to recondition connecting rod housing bores is through honing. Machines can be set up quickly and produce a round straight bore that is often equal to or better than the OE manufacturer’s. Interchangeable mandrels of various sizes reduce set-up time so you can produce more in less time. Connecting rod mandrels use a double-wide stone arrangement designed to increase the stone surface area to better alignment and faster material removal.

Rod Honing OilAs with most honing operations, be sure to use enough honing oil. Always use a honing oil that is specifically formulated for this type of honing such as Goodson’s Rod Honing Oil (RHO-10 or RHO-50) or Sunnen’s Mineral Based Honing Oil (MAN-845). Also, keep your supply of honing oil clean by filtering it and changing it regularly. This will enhance its performance and improve your finished product.

Most rod honing is performed with a horizontal honing machine. These are available in manual, power-stroked or CNC options.

“Let’s examine some of the capabilities of manual horizontal honing machines. They can be used to size connecting rod housing and pin bores, small engines and motorcycle cylinder assemblies, and to fit steering king-pins, just to name a few. Any bored hole used for a bearing surface or alignment purposes can benefit greatly from honing. Closer tolerances can be maintained with greater ease and productivity.” – page 303

Common Bore Errors

There are ten common bore errors associated with machining, heat treating or holding the part. These include:

  • 10 Common Bore Errorsout of round
  • bellmouth
  • waviness
  • undersize
  • barrel
  • taper
  • boring marks
  • reamer chatter
  • rainbow
  • misalignment

Honing can correct all ten of these errors. Honing is characterized by “large areas of abrasive contact; low cutting pressure, low velocity, floating tool or part and automatic centering of the tool by expansion inside the bore.” – page 303.

Key considerations when Rod Honing:


Select the proper stone composition. There are generally four stone compositions from which to choose; roughing, general purpose, finishing and for steel. Both Goodson and Sunnen stones use the same numbering system. Roughing stones are 5s (for example: Sunnen’s KL-5 or Goodson’s HK-5), General Purpose stones are 7s, Finishing are 13s and Steel at 14s).

Select the correct housing unit. Honing mandrels are available in a wide range of bores. Depending on the part you’re honing, select the most suitable mandrel size. Follow the manufacturer’s directions for assembly and set-up.

One final thought

A final step when reconditioning connecting rods that is often overlooked is demagnetizing the parts before you start putting the engine back together. This is critical to prevent premature engine failure. Heat and friction from engine operation and the machining process can induce magnetism which must be removed. Check out this previous post on the importance of demagnetizing.

This has been a brief overview of connecting rod reconditioning. For more information, check out Engine Builder Magazine’s “Back to Basics: Reconditioning Connecting Rods” and “Connecting Rod Reconditioning: More To It Than You Might Think”. You can also read the entire section from the Sunnen Engine Rebuilding Handbook here.

As usual, if you have any additional questions about this topic, contact the Goodson Techxperts by email or phone at 1-800-533-8010.

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