frequently asked questions

What is the proper rod side clearance?

The minimum side clearance is typically .018” on most rods. Always check your service manual. A good practice is to measure and write down the width of the crankshaft before disassembly. When pressing the crank back together simply press the crank to the original assembled width.

Do I Need To Buy A Wristpin Bearing?

No, it is included in the rod kit when required.

Is The Forging Number On The Rod The Same As The Part Number?

No, some rod forgings can be used in multiple applications.

Why Are Most Of Your Four Stroke Rods Called Heavy Duty?

Company four stroke rods are designed with extra strength forgings. Company have increased the material in weak sections of the rod for extra strength. Also, all of Company heavy duty four stroke rods come with a bronze bushing in the wrist pin bore. No more gulled and hard to remove wrist pins.

Why Does Hot Rods Make Such A Big Deal Out Of Their Big End Design?

Keeping the connecting rod bores as round as possible improves the life and reliability of the rod. At high RPM the extreme forces pulling on the connecting rod are changing the shape of the connecting rod bores. The more out of roundness that the bearing encounters the quicker that the bearing wears out.

How Does Hot Rods Reduce The Rod Bore Distortion?

On two strokes by strategically locating the vent slots and side scallops away from high stress zones the amount of distortion the bearing encounters can be reduced. Some OEMs have recognized this problem and design accordingly , however , most do not understand how to correct this problem. Since Hot Rods does not simply copy OEM rods Company can improve upon the OEM design.

On all of Company four stroke Motocross rods Company increase the big end section diameter then use an end mill to "square up" the big end section. This produces the maximum possible strength from the big end diameter.

Why Do You Shot Peen Your Connecting Rods?

Shot peening is an excellent technique for tremendously improving the fatigue life of a connecting rod. By blasting the connecting rod with small steel balls (called shot) the surface is slightly compressed thus making formation of micro cracks nearly impossible. Company remove the copper from Company rods before shot peening so that the copper does not absorb any of the shot blast energy. This is why all “Hot Rods” have a dark gun metal grey appearance compared to other rods.

Why Do You Vibrate Deburr The Rod?

It is very important that the connecting rod’s radiuses blend smoothly together. Vibratory deburring not only removes any leftover forging flash it also gently and consistently blends the complex radiuses of the rod surface together. The smoother the blending the stronger the rod.

Why Are Big End Bearings Silver Coated?

The silver is used as a sacrificial lubricant in situations of low lubrication levels. Silver has a low melting temperature. During dangerous low lubrication moments the silver will actually melt and become a form of lubrication in place of the oil. Obviously this can only occur for short durations but every little bit helps.

Why Are There Two Different Designs Of Crank Pin Bearings?

Bearing manufactures refer to these designs as “FLAT” and “M” profile. The Flat profile bearing is stronger and more expensive to manufacture. It typically has more needle rollers and a cage design better suited for high RPM. The M profile is typically used in lower power and lower RPM application.

Are Bearings With More Needle Rollers Better?

Not necessarily, as the number of needle rollers go up there must be more holes punched into the cage. Thus, the cage usually becomes weaker – more is not always better.

Types Of Connecting Rods (More Than You Ever Wanted To Know)

There are essentially two types of connecting rods: Split end and single piece. Split end rods are sometimes referred to as a two piece rod because of the removable cap. Split end rods are found in almost every automobile application and most of the outboard marine application. For automobile application the split end rod will use an insert bearing. For outboard application (where most engines are two strokes) the bearing will be a needle roller type with a cage that splits into two pieces. A single piece rod is a solid forging that does not have a cap. These rods can be used in both two and four stroke engines with the only unique requirement being that the bearing type must be of a needle roller design. Currently , all Hot Rods are of the single piece design.

There are two main reasons to choose between single and two piece designs. Cost is obviously one concern. A single piece rod does not have the added cost of splitting the cap end and of the high grade bolts necessary to clamp the cap. The second concern is the number of cylinders or more correctly the number of crank throws that have to be trued. Crank shafts come in two styles. The single piece crank is made from a single forging. Whereas, the multi piece crank is made from crank webs and crank pins. To assemble a multi piece crank requires considerable time when truing. The more cylinders the more time (cost) to assemble. There comes a trade off point between cost to manufacture the rods and the cost to assemble the crank shaft. As a general rule it is rare to see a multi piece crank with more than four cylinders. Hence, the more cylinders the more likely that the rod will be a two piece design.

All rods must be guided by either the crankshaft end or the piston end. All crankshaft guided rods require thrust washers to support the rod from excessive side play. Piston guided rods do not use thrust washers. Instead the small end of the rod fits closely into the piston. A piston guided rod usually cools and lubricates the crank pin bearing better because there are no thrust washers in the way. However, in order to adequately guide the rod the piston must have longer skirts. This , of course , increases the weight of the piston and makes for a taller and heavier engine.

Rod Loading (More Than You Ever Wanted To Know)

The highest stress on a connecting rod occurs at TDC. However, on a two stroke engine the highest stress does not occur under full load but when the throttle is closed at high rpm. This removes the gas pressure acting on top of the piston which counteracts the high tensile stress on the rod. When the gas pressure is absent the rod undergoes a severe tensile stress which forces both the small and big end of the rod bores to oval. This always occurs on four stroke engines during overlap and at least explains one of the reasons four stroke rods are beefier. Obviously , the rod bores distort at any RPM or loading , however , the most severe distortion occurs at high rpm and light loading. The ovaling of the rod is why some tolerance is required. If there is not adequate tolerance the needle rollers will be pinched as the bore ovals, ultimately leading to bearing failure. As will be discussed later Hot Rods has developed a rod design that minimizes the rod bore distortion.

As a side note , the general rule for calculating the tolerance of the rod big end is to take the crank pin diameter and divide by 1000 and multiply by 1.5.

For example: For a 24 mm crank pin

24/1000 X 1.5 = 0.036 mm or 0.0014 inches.

Manufacturing Of A Connecting Rods (More Than You Ever Wanted To Know)

Almost all connecting rods are manufactured from forgings. It is very rare and probably not good design philosophy to manufacture a rod from a casting. The first decision when manufacturing a rod is material requirement and this is dependent on whether the rod will use a caged needle roller design or an insert bearing. The cage needle design (all Hot Rods models) requires a hardened bearing race surface of 58 - 62 Rc. This is accomplished through case hardening of the steel by carbon gas. All carburizing steels are of a low carbon content such as 8620 which only has 0.2 % carbon. Typically, insert bearing rods are made from a higher carbon steel such as a 4140.

After the rough machining process it will be necessary to hardened the bearing surfaces. In order to case hardened steel the rod is heated inside an oven that contains carbon gas. To protect the rod beam and other surfaces from becoming too hard and brittle all non-bearing surfaces are covered with a copper coating that resist the diffusion of carbon gas through its surface. As time passes the carbon diffuses into the unprotected steel which makes the steel surface extremely hard. Once the carbon diffusion depth reaches 1 mm it is time to stop the case hardening process. If the furnace was simply turned off the rod would slowly cool and the carbon would seep right back out of the steel. Instead the rod is dropped into a vat of quenching oil to quickly trap the carbon in the rod surface. With Hot Rods we remove the copper coating so that company shot peening process is able to fully peen the rod surface. After the case hardening process the rod is finish honed at the big and small ends.

Rod Design (More Than You Ever Wanted To Know)

Fundamentally , the most important aspect of rod design is reducing the bore distortion of the wrist pin and crank pin bore under high load. Reduced bore distortion increases bearing life and reduces rotating friction. As stated earlier the high load on a rod is at TDC when the rod is in a tensile load. As the rod approaches TDC the tensile load begins to distort the small and big end of the rod. Earlier the term “ovaling” was used to describe the shape of the distorted rod - this , however , is not correct. The bottom half of the crank pin supports the bore from distorting , thus , keeping it round. Though it may not be obvious, this means that the stress through the bottom half of the rod bore is pure tension. At the top half of the crank pin the rod is trying to pull away from the bearing. Not only is there a tensile load occurring here but also a bending torque as the curved section of the rod is being “pulled” straight. These combined loadings produce the highest stress point on the rod at between the 2 and 3 O’clock position. The exact same stress is mirrored at the 9 to 10 O’clock position on the other side of the rod. These stresses actually produce more of an egg shape distortion than an oval shape. Obviously , the actual distortion is quite small , probably less than 0.001 of inch. However , if the distortion becomes too great then the bore begins pinching the needle rollers against the crank pin which destroys the bearing.

Even though the highest stress occurs at these position many times rods will break slightly below the wrist pin bore on the actual rod beam itself. This occurs due to the fact that the rod beam cycles back and forth between compression and tension (due to fluctuating cylinder pressure) thus producing heavy fatigue stresses. The 2 and 10 O’clock sections of the big end do not really cycle greatly between compression and tension. They only endure a tensile stress. This may not be very obvious but it is to the rod. As a general rule, when the rod big end breaks it is usually due to excessive high rpm loading (called ultimate failure), when the beam breaks it is due to excessive continued abuse (called fatigue failure).

Hot Rods has designed its rods to minimize the distortion of the rod bores by strategically locating the side scallops ( thrust washer mating surface) and vent slots away from the critically high stress areas.

Rod Bearings (More Than You Ever Wanted To Know)

There are essentially two style of cage design for crank pin bearings - the M profile and the flat profile. In general, the flat profile is the better design , however , in many situations it is not necessary. For example, watercraft engines do not typically use the flat profile cage. The reason is that there is not enough power or rpm to warrant there use. All motocross engines use flat profile cages due to the extreme performance of these engines. Snowmobile engines are just beginning to switch to the flat profile cage, however , most of the Rotax and Polaris lines still use the M profile. Another benefit of the flat profile cage is that it allows room for one extra needle roller.

All crankshaft bearing are called OD guided bearings. This simply means that the outside diameter of the bearing surface is supported (guided) by the rod bore (another good reason to keep the rod bore round). Since the flat profile cage has more outer surface area than the M profile cage the flat cage design applies less contact pressure to both the rod bore surface and the cage surface. This of course translates into less heat and less likelihood of lubrication failure at the bearing surface. Incidentally, the silver or copper coating on the cage is used for these lubrication failure events. As the bearing begins heating up the silver will melt away and reduce the chance of complete bearing seizure. Of the two coatings, silver is considered the better.

All wrist pin bearing are ID guided. Thus, the wrist pin surface not the rod bore is used to guide the bearing. A wrist pin bearing rarely fails because of the relatively small movement of the bearing. Crank pin bearings must completely rotate during engine operation , whereas , the connecting rod only swings from side to side at the wrist pin bearing. This considerably reduces the stresses acting on the wrist pin bearing when compared to a crank pin bearing.

Jetting Specifications

Due to the variations that may come into effect when tuning an engine (i.e. pipes, intake, altitude, cams, displacement, standard or high compression pistons, etc.) we CANNOT offer jetting recommendations.