"How To" by John Bowring
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This is the 3rd of a series of “How to” articles.

Flywheel mounting

It would seem that no matter how long you have been attending to your elderly Austin’s ailments, fixing the flywheel to the crankshaft still defeats many of us.

In order for the job to be done correctly, you have to start early, the crankshaft must be out of the crankcase and must be devoid of all appendages, especially the rear main bearing in toto. (Although if you have removed the crankshaft from the crankcase with the inner rear main intact you will have seriously damaged the crankcase or performed the miracle of the century.)

Clean the flywheel and crankshaft with a degreasing agent and flush with warm to hot water. Using hot water means they will dry quickly and not rust while you watch.

Clamp the crankshaft in the vice with the tapered/thread section uppermost. Look at where the woodruff key fits and ascertain that it is not bell mouthed. To tell by eyes, fit a key to the keyway and attempt to move the key from side to side. If there is any movement greater than a few thou (0.010”/0.015”) select another crankshaft from stock and repeat the aforesaid.

When you have a key which is snug in its machined keyway, the key must fit all the way by hand, do not drive the key in place not even with a soft faced mallet. If it is necessary to reduce the width of the key do so by laying a length of 240# grit emery on a flat metal surface and rubbing one side or both sides until the key will go all the way by hand.

Turn your attention now to the flywheel. Wrap a piece of 240# grit emery around your index finger. Poke it into the centre of the flywheel and rotate your finger 180° whilst rotating the flywheel 1/3 of a circle each of three times. This is to remove rust, dirt and anything that might impede a snug fit. Look in at the taper and you will see that the middle of the tape is machined so as to make contact only at either end of the taper. Grasp a piece of 240# grit emery ribbon between your thumb and forefinger. Encircle the taper and with a medium pressure clean up the taper.

We are now ready to mount the flywheel to the crankshaft. I am assuming the crankshaft is still mounted in the vice. Mount the flywheel so that the keyways are 180° to each other. Rotate the flywheel 90° each way 2 or 3 times and then check to see if the flywheel rocks on the taper. Do this by using both left and right index fingers at 180° to each other, and push alternately left and right. If no rocking is apparent all is well.

If however movement is detected it must be eliminated. Before removing the flywheel from the crankshaft rotate again 90° each way 2 or 3 times and carefully lift the flywheel off the crankshaft. Look closely at the taper for shiny marks. These are where the taper is binding, if the shiny mark is a ridge around the taper at the small end, it could be the flywheel taper has an abrupt ring at its small end. This is best removed with a three cornered scraper, removing such a ridge with a round file is asking for trouble. Removing this ridge will take some time, but worth the time and trouble. If will probably take several “goes” to get the required fit and it may be necessary to file some other areas with a small flat 2nd cut file to achieve a firm fit. (i.e. no rocking).

You will now require some valve lapping paste, medium to fine will do. Anoint the end of the crankshaft (the taper bit) with a thin layer of medium paste, apply a smear of engine oil to the taper on the flywheel, fit the flywheel to the crankshaft again with the keyways at 180° to each other. Semi rotate the flywheel (i.e. 90° each way) 6 or 10 times. At first you will feel a gritty feeling/noise, this will decrease to zero as you approach the 10th time. At this stage just the weight of the flywheel is sufficient pressure to achieve a result.

Remove the flywheel, clean both the flywheel taper and crankshaft taper and observe for high spots, if any. You will want to see an even grey area where the flywheel makes contact with the taper, high spots will appear as shiny spots, attention with your small flat 2nd cut file will be needed. At this stage, look closely at the edges of the keyway in the flywheel and crankshaft. Do not rotate the flywheel 360° at this stage as you will get a false reading.

Again several “goes” will be required to achieve the best result. Remember for best results clean the crankshaft taper and flywheel taper each “go” and re-apply the medium lapping paste plus a little engine oil each “go”.

Now you say to yourself I’m done, - WRONG!

You are about ½ way!.

You did say,”I want my flywheel to stay fixed to the crankshaft under all conditions”; well read on.

A little known fact regarding the rear main bearing inner is that not only should it be a tight fit on the crankshaft (about 0.0005” interference fit) it should also be clamped between the flywheel and crankshaft. To achieve this “double whammy” much skill and brute strength are required in about equal proportions.

Fit the newly mated flywheel to the crankshaft, fit the nut and turn the nut so as to hold the flywheel firmly on the taper, very gentle pressure is required to achieve this. The flywheel should just rotate on the taper when rotated by hand on the outer diameter of the flywheel.

Now the skill bit – Measure accurately the distance between the flywheel flange and the shoulder against which the bearing will rest on the crankshaft. From this measurement subtract the width of the bearing (the one you will use in the final assembly). The difference needs to be 0.025” to 0.030”. To achieve this you may need to lap the flywheel on to the crankshaft so as to reduce the gap. On the other hand if the gap is too small the flywheel will have to be machined, you may have to outsource this work. The need to be precise with your instructions is essential.

Let us now come to the final assembly. All is in readiness for final fitting of the flywheel to crankshaft, engine crankcase is part assembled, rear seal flange in place, both sets of crankshaft bearings in place, crankshaft rotates freely. Most important, the moment of truth at last, DON’T FORGET to fit the woodruff key to the crankshaft.

In order to tighten the flywheel nut with enough force to drive the flywheel up the taper 25/30 thou, brute force is required.

So three items are needed – a large ring spanner about 20” overall length which fits the flywheel nut and also the rear axle nut , a length of black mild steel 2” x 3/8” about 36” long with 2 holes drilled at one end to match 2 studs on the crankcase to gearbox flange and a 12” length of wood 2” x 1” to fit inside the crankcase to stop the crankshaft rotating whilst the nut is being tightened.

With the crankcase on the workshop floor, the bit of 2” x 1” inside the crankcase, the length of 2” x 3/8” steel fitted to the studs and offset so it protrudes past the outside of the crankcase and the ring spanner fitted to the nut in such a way that by pushing on the ring spanner and pulling on the 2” x 3/8” flat steel the nut is tightened about ½ a turn, maybe a little more, the flywheel will ride up the taper and contact the rear main bearing inner and presto you have your “double whammy”.

Do not under any circumstances use grease, oil or any lubricating product on the taper of either part.

You may however use Loctite 222# to act as a lubricant to assist the flywheel moving up the crankshaft taper. If using Loctite leave for 15 minutes before removing the nut and apply some 222# to nut and thread and retighten as before.

Job done!