[Neil Harvey]

911 Crankshaft.

This about what to look for when reconditioning a crankshaft, not any sort of modification. What to do when rebuilding your engine.

Once the rods have been removed from the crankshaft, some basic inspections should take place. How do the journals look? If they show signs of wear, any scoring from dirt, is the flywheel mating surface in good condition. Does it look like the flywheel has come lose and any metal transfer happened? The earlier 6 bolt crankshafts came loose, and the flywheel face would show signs of metal transfer. The later crankshafts went to a 9-bolt connection which has proven successful. These inspections give you an idea what repair may be required.

The threaded bores for the flywheel bolts should be checked along with the front crank pulley bolt bore. If any if these need repair, be extremely careful. Make sure you know the thread class and if the flywheel bores need repair, do not force the repair tap past the bottom of the bore as it will push the thrust face inwards where the bearing runs. This part of the crankshaft is “soft”.

The next procedures should be based on a certain sequence.

Nondestructive Magnaflux crack checking is first. If the crank is cracked it could be non-repairable, so no other checking will be needed. They can be welded if the crack is in a certain direction. Your machine shop will advise here. This can get expensive.

Next, the crank should be straight checked. All cranks will have some bending after use. Remember these cranks do twist and often bend around the 2nd main journal. Harmonics is a problem with these crankshafts. I won’t go into this here, but if interested read my paper on the PD web site.

After the crank is straightened, the galley plugs should be removed so the crank can be flushed cleaned on the inside as well as the outside. An easy way to re plug is to thread the galley drillings. This make future cleaning easier. This should be done now.

The journals could need cutting undersize, repairing back to std or polishing. If polishing is all that is required, they will now be measured to make sure they can be held at the finished size within spec, after polishing. If regrinding undersize is required, the machine shop should call you asking if you want to continue, as bearings become expensive.

Once regrinding undersize and all polishing is completed, the crankshaft needs to be balance checked. As these engines have opposed piston positions, no “bob weights’ are needed when balancing. The crank can be balance checked without any weights but should be done with the front pulley or any damper used. As any balance adjustments are done from outside in, the crank pulley or damper does make a difference. If you think you will not change the flywheel, add this now too. The whole assembly can be done together, or the flywheel can be zero balanced on its own. Done this way, the flywheel can be changed at any time without the need to re balance the crankshaft. Do not be tempted to lighten the crankshaft by knife edging. These cranks need all the counterweight they have. If you want to lighten the crank any, do so after re calculating the reciprocating weights verses the counter- weight. You must decide what percentage of counterweight you want to run. The only safe removal of any material is to slightly bull nose the counterweights. There is essentially no oil in the “sump” the counterweights spin in. All the oil left in the case not scavenged, is stacked up against the inside wall due to centrifugal force. Knife edging looks great outside the engine but inside does absolutely nothing.

Do not knife edge these crankshafts and if you will be using 6 bolt crankshafts, make sure you have the harmonics under control. My advice is, whenever possible use a 9-bolt crankshaft. If you are adding stroke, rod length and piston weight, my advice is to change over to a 9-bolt crankshaft. Engine speed and harmonics has caused 6 bolt flywheels to come loose in the past. The early RSR engine had issues with flywheels coming lose and although these engines ran slightly higher RPM’s and only made approx.325BHP, they could never keep their flywheels attached. Anyone considering a “hot rod’ version of these engines should seriously consider the 9-bolt crankshaft conversion.

Once all balancing is completed the crank requires thorough cleaning. Then the galley plugs should be re fitted and the crankshaft oiled and bagged ready for assembly. If storing the crankshaft for long periods of time, it should be hung vertically.

To conclude, all repair work needs to be done in a process, so no repair is done unnecessarily. Any repair work performed needs to be noted and if the journals need cutting undersize, make sure who ever is doing this, asks you first. Make sure they know the finish sizes after polishing. Polishing can help at times to increase bearing clearances but should only be done when you fully understand the bearings available.

If I have missed a step or you have other relevant suggestions, please go ahead and add here.