Here recently we have had customers who buy left over NASCAR parts. They call us when they get a set of rods that don’t fit the crank. We are going to show you the process we go through to make this work for you.
A lot of NASCAR rotating assemblies use a piston guided rod. Although this has been around for a while, its just now starting to surface more frequently do to the auctions cup shops have. The customer sees a set of 2000.00 dollar Carillo rods for sale at a give away price not realizing when they go to get the crankshaft, the big end of the rod is way too narrow for the standard crank journal. The rods may have over .100 thousandths clearance side to side between the big ends.
We fix this buy a performing a few processes.
First, measure the small end of the rod to check for Parallelism. This is important to ensure there will be even side to side pressure during operation.
If this is off, stop and correct the rod before continuing.
Looking at the inside of the piston pin supports on the inside, you will find they are usually at a 1 degree taper. Check with your piston manufacturer to make sure of this.
This taper has to be removed in order to have even pressure applied to the pins and spacers.
As you can see in the picture, we cut just enough to remove the taper to get a square wall to work off of.
After cutting the piston, install the pin and connecting rod to measure the total space and distance for the spacer dimensions.
Now its time to machine the pin spacers. After measuring the small end of the rod diameter and width, and the side to side distance of the inner piston, you should be able to machine the pin spacers. We recommend using 7075 aluminum for its strength, however 6061 will work fine. The pin spacer inside diameter should be to the same tolerance as the piston pin fit which is about .0007 to.001 clearance. Sloppy fit will result in engine failure!
After the spacer machining is complete, hand fit each piston assembly to ensure there is a minimum of .005 clearance side to side with a maximum of .007. VERY IMPORTANT
The clearance is important because if there is not enough, there will be a lack of oil and extreme heat will build up causing failure. If there is too much, it will slap side to side causing rod instability and cause engine failure.
As you can see, our rod was designed with an EDM machined hole that allows oil to be fed via the rod journal through the bearing to the wrist pin.Our piston is double pin oiled machined, as well as a DLC coated wrist pin to absolutely ensure there is adequate oiling of the pin and piston. This is mandatory on high vacuum engines. WARNING, do not EDM an oil hole in a connecting rod that was not designed for it. You will take out as much as 25% of the connecting rod strength by doing so.
The rod bearing must be drilled in order for this type of connecting rod oiling to work, however,it is not necessary in general for a piston guided rod.
It is absolutely critical that the piston design have enough skirt support to guide the rod on the journal. A really short piston skirt will fail under these conditions and is not recommended. Check with your piston manufacturer or professional engine builder to make sure your pistons will work for this application.
Once all this is done, the engine should perform perfectly and make a happy customer. We have performed this task numerous times with great success.
For further info or if you would like us to fix you a set, give us a call.
