Harley Davidson Twin Cam Crankshafts Problems

Posted by By at 14 July, at 17 : 22 PM Print

twin cam crankshaft imageSo what is wrong with my Twin Cam Crankshaft?

The new style Twin Cam Crankshafts used by the MoCo have been criticized often for their poor engineering and the loads of trouble they bring to our beloved V Twin engine… And rightfully so, especially in performance application although I had cranks go bad on me with stock engines… so what’s so wrong with them…?

Many things. First let’s start by looking at the construction of the assembly. First, its an assembly. It was not designed to be taken appart once it is pressed together at the factory. This is why you don’t need truing stands anymore when rebuilding an engine… You can’t take them apart (well, many do take them apart with the proper fixtures in presses… but it is best left to someone who knows what the hell they are doing…

The assembly consists of:

  • Left flywheel, which includes the spline shaft.
  • Right flywheel, which includes the pinion shaft
  • The crank pin, that connects the two flywheel
  • The push rods with their bearings. These are assembled before the flywheels are pressed.

Now, let’s look at a diagram of the our problematic twin cam crankshaft:Crankshaf

Problem number 1:

I don’t know how they spline that shaft, but when they do, the alter the shaft’s integrity and its aliment with the left flywheel. This is a common problem. In fact, if you look at the hole in the spline shaft, you can see it was not used to turn the part and will not yield a correct turn center. This is one of the alignments that must be checked and corrected by the machine shop doing the crank.

Problem 2:

The flywheels are pressed onto the crank pin. As can be seen on the diagram, if one of the flywheels actually moves after being pressed, the two wheels and their shafts are no longer running on the same plane (both shaft are no longer true to each other). This is what we call spinning a crankshaft… and it is not a good thing.

Consequence of 1 and 2:

Run out. When the crankshaft does not run true, you have excessive run out. This creates vibrations and in some instances create problems with oil pumps and cam gears not running true on the end of the shaft. Run out should be measured on both shafts as the spline shaft can have problems of its own (remember problem number 1…). So, when dealing with complaints of vibration or when upgrading camshafts (especially when going to gear drive), run out should be checked… I like my motors to keep below .003 of run out on the pinion shaft. This is the maximum allowable for running cam gear drive anyway… But typically, when I get a crank back from machine shop the run out is lower than .001 which is better than a brand new Harley Davidson Crank…

Balancing issues with the Harley Davidson Crankshaft

Another problem with those cranks is that they are way off balance in my book. Opinions vary on this but I think that their casting process sucks and that weights are distributed differently from one flywheel to another… so its the flywheel lottery… you can get lucky or NOT… This is why you hop on a few brand new bikes and some run nice and others will rattle the teeth out of your mouths… You have to take one ride on a bike with a dynamically balanced crank to see an immediate difference…

When having the crank trued, make sure you include the balancing while you are there… you will be glad you did.

Fixing your Harley Davidson Crankshaft

Harley Davidson Crankshafts are serviceable, you don’t need to buy a new one… and why chance getting another bad one from the MoCo anyway… And if you are building any hotrod motor, its a must to have the crank fixed… those cranks are not made to put the kind of power a 113 or 117 build puts on them… (Then you should consider an S&S alternative…).

Many shops now specialize in fixing those cranks. These are specialized machine shops that have developed processes and fixtures to deal with those crankshafts. Here are a list of the things the shop should do for you:

Disassemble the crank, inspect everything…

Now the shop will tell you if the rod bearings are serviceable and will replace them if required. Its also a good time to hone the rod ends if they are not round…

Straighten the spline shaft.

They insure the spline shaft runs true with the flywheel… without this step, how can you true up the crank?

Balance the crank…

they create bob weights and re-assemble the crank for balancing… To me, its foolish not to have this done while you are there…

Re-assemble the crank

This is when the crank is pressed together and trued…

Weld the pin.

The pins are welded in place and crank is re-checked for true…

A word about plugs…

I have not mentioned plugging the pin at this point. There are varying opinions on this and I am of the opinion that plugin is unnecessary when welding the pin. Steel plugs just add weight to the rotating assembly and aluminum plugs just don’t bring much benefit in my book… a good weld and a tight fit is all you need… but have them plugged if that’s how you want them done…

How good are those fixed Harley Davidson Crankshafts?

Well, they just don’t fail anymore… especially once the rotating assembly is balanced. There is something to be said about the reliability of an engine that runs true and is in ballance… nothing is being rattled in anyway… and the welded pins just don’t move anymore. I believe that anytime you go into the case the crank should be done and then you end up with a crank that will last and that you don’t have to worry about… Good insurance.

If you need your crank worked, give us a call…


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