the central piece of any engine.... the crank

I haven't posted anything to the blog because I've really not made any progress in the past couple of weeks, other than research stuff.  Summertime is difficult....  hot and muggy and plenty of other family plans and activities to occupy my time.

Had a few minutes today though, so I thought I'd get out the core crankshaft I have safely stored in inventory and have a better look at it.  One of the first items I'll be sending away for prep work, but it needs to be measured to see if it still meets factory specs.

This is the crank I inherited from the inventory I purchased in February.  The previous owner had already completed some of the work to make it airworthy and it's in great shape:

The recommended prep work includes magnaflux testing for internal damage, straitening if required, heat treating for improved strength (ion nitriding) and grinding the piston rod journals to improve the fillet radiuses.  Larger journal radii help prevent stress riser cranks from forming, which has led to broken crankshafts in the past.  The process is quite common in certified aircraft engine cranks, so it's worth doing here.

One of the other processes that normally gets completed is to drill and tapping the centre of the end of the crank for a propeller hub safety shaft.  Because the previous owner was working from the official plans, he had already completed this process with his machinist:

I opened my Corvair shop manual, and found the engine spec reference page.  It contains all the measurements for the major engine components:

Using my digital caliper, I tried to measure as best I could the crankshaft main journals and connecting rod journals to see how close they are to factory.  I really should be using a micrometer for this, but my calipers should at least let me know it's in the ballpark.

The manual says the connecting rod journals (they call them crankpin journals) calls for a diameter from 1.7999 inches to 1.800 inches.  To make it easier to measure, I set and friction-lock my caliper to 1.7990 before placing it on the journal.  This is slightly undersize, but as close to 1.7999 I can get.  This way if the caliper slides over the journal without resistance, I know that it is below tolerance and no good for grinding. Anything larger than that leaves that much more room for the machining (good):

Preset the caliper....

With my lovely assistant Brenda taking an action shot with the camera, I carefully measure each of the bearings:

It's real hard to get a good picture, but the caliper won't span the largest diameter of the journal, so we can deduce that they and the main journals (using the same measuring technique) are clearly above the minimum spec and can be used for conversion.  I know it isn't a perfect measurement and my caliper may not be as accurate as a micrometer, but I think I'm in the ballpark at least.

I'm going to see if I can borrow a good micrometer from someone, or maybe buy one for myself to confirm this.  Once I'm sure, off to Florida the crank will go for prep.

Next Wednesday I'm having my first shop lesson with Ron, another Zenair builder.  He's rebuilding a Zenair 701 and has offered to let me help.  What a great opportunity to learn and prep for my build.

​More to come.....