Is there a "how to.." for crank measurement, and bearing verification? Options

[malcolm2]

I am gonna make a 2 liter from my 1.8. So I have a refreshed 71mm crank that I have been told has been ground to -0.010.

I measured as best as I can with a digital mic and got about 59.66 on the mains and 49.65 on the rod journals.

Where can I find the STD specs and ranges ? were they 60 and 50, respectively?

Since I know nothing about this crank, how can I verify the 71 mm measurement?

[malcolm2]

Both micrometers were checked prior to the last measurements. I believe you are correct on me being a little aggressive on the dial.... I have re-checked both tools and taken new measurements with just a tad of friction....

#1 2.3518 vs. the mid-point of the spec'd range = 0.0095 under
#2 2.3520 " " = 0.0093 under
#3 2.3517 " " = 0.0092 under
#4 1.5662 " " = 0.00815 under
ROD Journals day 2 measurement all equal at 1.9595 = 0.0084 under

I had planned to take the crank to a shop that I had forgotten about and had heard was reputable, but they are closed 'til January 2nd.

I guess I will move on to another project.


2ndly, Why not use plasti-gauge? How do you know what your oil clearance is without it?

[Superhawk996]

Both micrometers were checked prior to the last measurements. I believe you are correct on me being a little aggressive on the dial.... I have re-checked both tools and taken new measurements with just a tad of friction....

#1 2.3518 vs. the mid-point of the spec'd range = 0.0095 under
#2 2.3520 " " = 0.0093 under
#3 2.3517 " " = 0.0092 under
#4 1.5662 " " = 0.00815 under
ROD Journals day 2 measurement all equal at 1.9595 = 0.0084 under

I had planned to take the crank to a shop that I had forgotten about and had heard was reputable, but they are closed 'til January 2nd.

I guess I will move on to another project.


2ndly, Why not use plasti-gauge? How do you know what your oil clearance is without it?

Plastigage is not well liked by some. Like all tools it is a bit subjective and can be used improperly.

If you have a properly measured ID on the bearing as installed into a properly torqued case, and the OD of the crank, you know the bearing clearance as the difference between the two measurements. You also have to KNOW that the case main bore is perfectly circular for this to be true

What isn't well recognized is the error in measurements that you're now running into just checking the crank OD. Now factor in the error in measurement when checking bearing ID and you get a sense of what error might be creeping into the bearing clearance calculation.

The some of the pro's don't like Plastigauge because:

1) It is time consuming. If you have a highly trusted shop doing your crank grinding and case line boring, AND you trust thier measurement accuracy then Plastigauge is redundant.

1A) If your machine shop's crank grinder is a modern, calibrated CNC crank grinder I'd be willing to take their measurement at face value. If you're using a lesser machine shop with older manual crank grinding equipment, and who knows who doing the measurements, then you're back to quality control, and manual measurement errors. Likewise if your case is line bored with CNC equipment, sure trust the measurent. But if the case is line bored with an old fashioned, bench top, drill powered line bore that has questionable guide busings you have no control over, we'll you might want to question and/or double check how round the case main bearing bore actually is.

Just to ground ourselves on line boring. Ranges from this to full CNC. Nothing wrong with this depending on application but let's not kid ourselves this is not CNC. Not posting this for any other reason that to demonstrate bench top line boring.


https://www.youtube.com/watch?v=bvKZgJ35DoU


Wow! Here's an interesting CNC line bore with a right angle cutter. Right axis cutter adds some variability to things but at the same time a single cutting point is more accurate and CNC virtually guarantees the ability to locate repeatibly along a singular axis. Cool!


https://www.youtube.com/watch?v=u2j8C-3zJzk


1B) Time is money. If you have proper quality control upstream at the machinist you work with for say 20 builds a year, and someone that will financially stand behind their machine work if they screw it up, then why waste your own time with Plastigauge.

2) Plastigage is prone to it's own errors:

2A) it only measures one axis of the crank at a time. If you have an oval case bore, you will need to measure each bearing twice to get a sense of what the ovality is.

2B) Plastigage can "smear" if the part is moved while torqueing it. Especially true of Rod journals since the rods can move easily while being torqued.

2C) Plastigauge like all other measurements has a +/- error and shouldn't be considered absolute.

3) For hobbyists like myself, Plastigage can serve as a sanity check, a redundant backup. I measure both ID, and OD and calculate the clearance. But then I like to use Plastigage as a double check. Remember we are taling about tolerance of 0.0008" between the high end and low end of crank measurement. 0.00025" is a 25% change. Easy to lose sight of how the errors in measurement compound and how the +/- tolerance spread can affect bearing oiling clearances. Plastigage is just cheap insurance.

3A) in the case of an unknown crank like we are taling about here, Plastigauge is a quick way to gague what you're dealing with. I would not suggest that that is all you do but rather it is a double check to what you do.