HPR Tech Tip - Piston Pins

HPR Tech Tip - Piston Pins

Piston Pin Tech



*Photo from Trend Performance   

We all know what they are and if you have built an engine in the past you have more than likely cussed a number of piston pin locks in your day as well. But what are your options?  What can happen to these seemly small parts inside your engine? 

Most of the time we open our new box of forged pistons and we will find pistons, rings, piston pins, and locks all wrapped up for us ready to go, and many will leave it as that and go on.  Today's kits are generally designed with a good pin and "close enough" machine work that the home user can put it together on their street ride and not have any real issues.  What if your build isn't just a normal street car?  What if you see issues when tearing down and engine, what can something like the piston pin tell you? 

Everyone loves to see the shiny "cool" parts in the engine.  The crank, the pistons, the camshaft, the rocker arms, but when you stop to think what the little piston pin sees for loads on it, you will get a better respect for what it has to hold up to under load.  Take a normal LS forged piston that would weigh in around 470 grams add in the pin (108 grams), rings (40 grams), and small end of the rod (180 grams)...at upwards of 6500 RPM you might have a load on the pin well over 8,000 lbs if not more happening over 100 times a second.  So each yank and throw of the crankshaft is putting quite the load on these guys.  

You will find most kits today from companies such as Wiseco, JE, and CP pistons will include a 0.150 to 0.180 wall, quality steel pin with every kit.  For most of us that is perfectly fine and will never run into issues.  What if you are wanting to do something more than a rebuild or slight upgrade?  What is going to happen?  What are my options? 

As we have talked about before, there is a lot of load being placed on the pin itself.  Like anything, if you put a great enough force on it, it will either break or bend.  In this case we typically only see a slight deflection under extreme loads. 

The Pro Series pistons we typically use and offer from Wiseco will include a 0.150 wall, 5115 Chromium alloy steel pin which will be enough for most performance builds and much stronger than a 1018 alloy OE style pin.  For direct injected engines and higher hp boosted applications Wiseco offers a step up into the 9310 low carbon, hardened steel alloy pin with a thicker 0.200 wall for added strength.  After this point we can go into a number of offerings from manufacturers like Trend Performance with their H-13 Tool Steel pin, which has a high enough melting point that they can be DLC coated if the customer chooses to do so.  Going further up the ladder we can get into more exotic tool steels like M2 and their TP-1 pins that are generally found in NASCAR and NHRA pro racing. 

What are the draw backs?  With anything there is always going to be some trade offs when going to a bigger / better / different pin design and material.  Most of your normal street performance builds would not need the thicker pins, so you can save your money and bob weight by not using a thicker pin.  With most LS builds the difference from the std pin to the 9310 pins can be the difference from 104 grams to 132 grams in pin weight, which adds up.  The more exotic pins, like the M2 and TP-1 can run as much as the cost each of a good set of forged connecting rods, which not only eats up weight but your budget too.  So we always try to balance out what is needed weight and cost wise in each of our customer builds.

There are some compromises out there as well, and that can be a double taper walled pin that we sometimes like to use for our road racers.  These use a heavy wall, tool steel material in the center and then it is taper drilled on each end where the mass is not as greatly needed.  here you can have something with a better material but stay close to a stock pin weight.   You can see an example of that below


Lets take a look and see a few examples of pins and what kind of wear they show and what to look for if something bad is going on. 

 Here you can see a brand new, un-used piston pin.  The surface is free of defects and there are no markings across the wear surface.



Below you can see a pin that has been ran without issue.  There are some markings from the pin and also the piston but nothing more than general wear.  Nothing shown here can catch your finger nail, nor can you feel it across the surface of the pin.  The light scratching you see is typically from small particles in the oil.  The two rings are un-used areas that make up the clearance from the rod to the pin boss in the piston. 


In this picture now you can see what can happen when greater than normal forces have been applied.  In this particular piston pin you can see material transfer from not only the rod bushing (gold color), but also from the piston itself (dull silver on the edge).  In this case the engine was running a little to much boost/timing and had started to slightly lift the cylinder head combined with more than likely a little detonation going on.  These factors combine to make a high load on  the top of the piston causing it to bend around the pin transferring material to the pin as well as forcing the pin into the rod doing the same.  Would an even stronger pin cause this from not happening?  Maybe so, but more than likely no.  When extreme errors happen within the chambers of an engine even the strongest of materials can start to fail. 


It should be noted that when damage to the pin shown above happens that damage is also done to the piston and the connecting rods as well.  This can cause you to have to replace not only the pins but in some cases the pistons and re-bush the connecting rods as well to fix the damages. 


We should also take a moment to mention clearances.  Some times damage can come from improper assembly too.  Today parts manufacturers do their best to make sure you can assemble their kits at home with little more than hand tools and assembly lube but that is not always the case.  Each time we assemble an engine here at HPR we take the time to measure each and every rod and piston to their respective pins to make sure we have adequate clearance needed for the oil and to make sure they are going to work without issues. 

Many times it is nothing more than a burr left over from manufacturing, but in some cases they can be to close, not allowing the proper oil flow around them. 


If you have a question about what might be right for your build, give us a call or email and we would be happy to help you on your selection.