Tuesday, January 12, 2016

Crankshaft Running Clearance

So far, my endeavors to build my own engine have suffered a few setbacks, ranging from fairly minor through to rather devastating!! The previous post is a very good case in point, thankfully towards the fairly minor end of the spectrum though as I'd installed the crankshaft without first checking the running clearance.

Although the crank felt fairly good after I'd installed it, it's always better to be safe than sorry. So before I progress any further, I wan to know for certain that the running clearance is within acceptable specifications.

So the obvious stuff I need to get my head around before I can get started is firstly to understand what exactly the running clearance is, secondly how am I going to measure it and thirdly what the upper and lower limits are. Thankfully chapter 2B.17 in the Haynes manual covers the whole topic very comprehensively. So after reading it over a few times, I knew everything I needed to get started.

What is it?
The running clearance is the tiny but necessary gap between the crank journals and their respective bearings in which they sit. If it's too tight, it will inhibit the oil from circulating properly, too loose and the crank would slap around in the bearings resulting in the dreaded 'bottom end knock' noise.

How do you measure it?
As it's not possible to measure this clearance with feeler gauges, Plastigage must be used. As I'd never heard of it before, I didn't even know where to get it from, but of course eBay sells everything so I was able to find some on there in about 2 minutes. This is what arrived.

In the envelope there were some instructions, a bit of card which is used as the gauge and the actual Plastigauge stuff itself. Gently rolling a small piece between my fingers, it feels like a softish material that I'd describe as cross between plasticine and wax.

After reading and digesting the enclosed leaflet, I started work on the journal closest to the front of the engine. The process is pretty straight forward. Remove the cap and clean away any excess oil then cut a length of Plastigauge and lay it on the journal.

Carefully replace the cap and torque it down to the necessary 63 lbf ft (85 Nm) making sure not to disturb the Plastiguage or rotate the crank. Now simple undo the bolts and remove the cap and what you are left with is a squashed blob of Plastiguage stuck to the crank journal.

Now simply compare the width of the squashed Plastiguage with the supplied gauging card to determine the running clearance. The card has 'mm' printed on one side and 'thou' on the other. I chose to use the mm side of the card which read 0.038mm.

What's the specification?
As the running clearance specification for my engine is between 0.025mm and 0.068mm, I'm very happy with a reading of 0.038mm it's nicely within spec.

When I measured the other two journals they also displayed more or less the same level of squashed plastiguage that equated to the same running clearance of 0.038mm.

Centre journal
Rear Journal
Cleaning up.
Afterwards the Plastigauge needs to be cleaned off the journal. The leaflet advises using an oily rag for this, but I found it easier to gently scrape away the majority stuff first with an old credit card so as not to damage the journal surface.

After scraping and a quick wipe with an oily cloth, I couldn't see any trace of the Plastigauge left on the journal. The leaflet does say that "any Plastigauge left behind is oil-soluble and cannot harm the engine in any way".

How Small!
So there you have it, after a little research and a small purchase, it's actually possible to measure things as small as the running clearance. When you consider that the range between the lower (0.025mm)  and the upper (0.068mm) limit is only 0.043mm, it's pretty impressive it can be measured so accurately with nothing more that a small plastic worm!

Let's look again at that running clearance measurement of 0.038mm and consider for a moment, just how small that actually is. The term 0.038mm can also be be written as 38μm, meaning 38 micrometres and as a micrometre is exactly the same as a micron, you could also refer to it as 38 microns.

Looking at the chart above, you can see that 38 microns is considerably smaller than the diameter of a human hair. In fact, it's just below the threshold that your eyes can actually see and just larger than a white blood cell.

When you look at it like this, it becomes obvious why it's so important to make sure that the journals are spotless when the crank is being installed. Any spec of dirt that you are capable of seeing on the journal surface would pose a problem as it's probably greater than 40 microns and would get trapped within the running clearance and probably scratch the bearing surface.

Before I can start to think about installing the crank though, there's still the matter of the crankshaft end float being out of spec to deal with, so guess what's next on the agenda...

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1 comment:

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