Usually when we sell a crankshaft over the counter the customer wants the best! Only the shiniest full circle race crank will do, built in Italy to exacting tolerances and designed for a max rpm of about twice that of which the bike it's destined for is capable!

Yet we know that chances are an hour or so later this piece of Italian craftmanship will be being hammered into submission by a spotty 16 year old keen to get back down to McDonalds for an evening of wheelie-ing... bearings fitted without thought to sideloading stress liable to cause premature failing and crank considerably less true than when it was in the box!

It's not everyday you replace your crankshaft so here's how to fit it properly... this is not the only way, but this is how we do it in the shop.



It's important to realise that the crankshaft is made up of 2 webs (see pic). These webs are joined together by a crankpin which passes through both webs and the big end of the conrod. The only thing holding this pin in and the webs together is that the pin is a very tight friction fit. Other than this friction there is nothing to prevent the webs rotating against each other under loading. Unless they are absolutely perfectly true to each other the crankshaft will have excessive runout and the PTO shaft and flywheel shaft will not be rotating along the exact same plane which causes vibration, premature engine wear (especially bearings) and consequential loss of power.

Also because of the design if the shafts are pushed together with any force (such as forceably bolting the engine casings together, or hitting the end of the crank with a hammer) the webs are likely to bend together resulting again in misalignment.

A crankshafts 'trueness' can be measure with a dial gauge and V-blocks or a lathe and dial gauge (as we use in the shop). This trueness (or runout) for the very best Italian crankshafts is sometimes as low as 0.001 inches.

Out of true cranks provided they are not far out can often be corrected by lightly tapping with a hammer in the appropriate place for rotational misalignment or the webs can be gently forced together or apart (wedging) on a trial and error basis until the crank is measured to be true. This however is a real art!

It only takes movement of thousanths of an inch to put your crankshaft out of true and careless handling can very easily do this. Just tapping one web lightly with a hammer or dropping the crank will make a measurable difference.

On some race engines we weld up the crankpin to make it less likely to move, especially if the engine is high power where high torsional forces within the crank might cause the possibility of twisting, but even so they can still be 'fine tuned' afterwards by tapping with a hammer.

So in summary, handle with care ! A properly fitted crank and bearing can typically last 15,000 miles depending on state of engine tune, a badly fitted crank defeats the object of the premium paid for Italian craftmanship and can cause the main bearings to fail within hours.

In Part 2 we'll be fitting the crank !