Crank preperation.

On the crank there are two sets of journals, mains that sit in the bearings in the block, and big ends which sit inside the bearings in the conrod.

There are only really two things of any real concern to us with regards to the journals on the crank:
Are they the right size?
Are they the right shape?

There is a third of course, which is that there is no visually obvious damage such as scoring or pitting.

You need to check with the manufacturer of your engine (or crank if non standard) to see what the recomended maximum and minimum tolerance for journal size are.

Now with regards to shape, the journals should be perfect cylinders, there should be no taper, and there should be no ovality.

Ovality = deviation from round, as measured around the journal
Taper = deviation from straight, as measured along the journal


The first thing to do is to measure the size of the journal, this is done by using a micrometer, and it needs to be accurate to around a tenth of a thousandth of an inch. If you cant get down to this level of accuracy then just dont bother trying, get someone else to do it for you, even a couple of tenths of power of taper is enough for a very shortneded engine life.

Once we have a size by measuring the journal in the centre, we then check the size either side, and if there is any difference it shows that the journal is tapered, which means that all the force will be acting on one side of the bearing, which leads at best to premature bearing wear, and at worst to a terminal bearing failure such as a spun shell.

To check for ovality we simple measure at different points around the journal, i only ever do this in the centre of each journal but you could repeat at both ends if you really want to, this (ovality) is almost never a problem on cranks (but very common on the bearing tunnel the crank sits in, but i will cover that in the blocks section), but is worth checking while you have the right tool here anyway, as it can lead to shock loads to the bearings in the unlikely event it is ovalled.


You must check EVERY journal, do not just measure one and assume that because its ok the next one will be too.


The other thing we are interested in with regards to the crank is the throw of it, this should be the same on all cylinders, its not actually amazingly important if there is a slight deviation in throw from spec, providing its the same deviation on all cylinders, the key thing we are looking for is the location of the piston at the top of the bore to not alter the CR greatly, its posistion at the bottom has less of a marked effect on CR

The stroke itself is not the easiest thing to measure to be perfectly honest, as its hard to get an accurate distance when the piston is at the bottom of the bore to any usefully small degree of accuracy, well unless you fancy piling up 80mm of slip gauges on the top of your piston


The way that i personally do a quick "sanity check" on the throw of the crank is after dry building to simple check the deckheight in the bores, its perfectly possible that in doing so you end up with one cylinder having slightly more throw on the crank but cancelled out by a slightly shorter piston and rod assembly, but if this happens the effects on CR from a tiny bit of extra flow are negligable outside of things like formula one!
If you do have any variation in deck height you can often correct it by swapping the rods and pistons about, especially if you have spare ones too this process is known as "selective assembly".

However if when you measure the deck heights you find a varation that increases as you make your way from cylinder to cylinder, then its a good indication that the bearing tunnel is out of true with the top of the block, if this is the case, then the block will need machining to correct this fault (if possible witout pushing the CR through the roof), see block section later on in this thread for details of how to ensure the tunnel is true to the block.


NOTE - Deckheight = the height of the piston in the bore when at TDC (just in case that confused anyone!)




Modifications to cranks for performance use.

Not going to go into too much depth here for two main reasons:

1: Nothing here that the home builder can really adjust for themselves
2: I dont know enough about it myself as its always been something ive paid others to do (See reason 1!)

knife edging the crank = Machining the crank so that the leading edge as it roates end as a sharp edge (not razor sharp, just relative to standard) rather than being a a very thick edge, the reason for this is to allow the crank to cut through the oil with less resistance at high RPM, which leads to less of a loss in horsepower from the resistance of the oil

balancing the crank = This is just like doing it on a wheel/tyre! the crank is spun on a machine that measures the oscilation up and down to determine where the crank is out of balance, and its then corrected by removing material to even the weight, this is done either by drlling holes or by machining off a portion of the surface, i actually know one engine builder who uses an angle grinder to do this initially to reomve fairly large chunks of metal, sounds like an horrific bodge, but if you know what you are doing its actually an acceptable way to make quick progress in getting the crank into the right ballpark for final adjustment!

lightening the crank = The idea behind lightening the crank is simply to remove rotational mass so as to allow the engine to rev more freely, this is generally not something you would wish to do on a road car, and IMHO there is never any point doing so until you have first lightened the flywheel as thats weight all at one end which is worse than spread through the crank in terms of loading up the crank and bearings under high rpm usage

cross drilling the journals = on some cranks its beneficial to instroduce a second feed hold from the main to the big end in order to ensure agood supply of oil to the big end, this is vey application specific though and expert advice must be sort before you perform this sort of modification as it can result in weakening the crank or excessive oil flow leading to a drop in pressure