Mic'ing rods

4sd911 · 08-05-2009, 06:58 PM

What's a proper method of checking the inside diameter of the rod's big end. Needing accuracy within 0.0001. I bought some outside Mitutoyo micrometers to for checking the crank. And I have the plunger type to try and get a inside diameter then using the outside mic's to measure the plunger, problem is getting a consistent measurement with that method. I don't mind paying a few hundred for a inside mic but it would need to be able to use them on different applications, not just this one type of rod, i.e. case bore and rods from different types of engines.

In other words what type of mic would be using to get a proper accurate inside reading for the case and rods.

TimT · 08-05-2009, 08:54 PM

You can use a bore gauge

Some bore gauges

can be used for cylinder taper also etc..

304065 · 08-06-2009, 05:57 AM

First of all, do you mean imperial or metric? Measuring to "ten-thou" is not as precise as measuring to 0.001mm like the factory.

Cheapest method: Bore gage with high precision dial indicator. Set this with an outside micrometer- calibrate the mic with a standard, then set the mic to the diameter, then set the bore gage up inside the mic jaws. You will probably need to mount the mic on a clamp stand to do this.

Better method: get a ring gage in the exact size and use that to set the bore gage.

Even better method: pistol-grip type bore gage, there are Chinese ones available that are a LOT cheaper than the Japanese ones.

Better: Mitutoyo Holtest

Ultimate: Brown & Sharpe Intrimik

Super-Ultimate-- Digital CMM with ruby tip-- get a three dimensional map while your'e at it.

I love dimensional metrology!

But seriously, try the bore gage and mic method first, it's what most of us use. It's an aircooled engine for heavens sake, once it gets up to temp the cylinders go all banana-shaped anyway.

cgarr · cgarr's Garage

.001mm = 0.000039370078740157485 I hope you are not holding the rod or breathe on it when you take this measurement!

304065 · 08-06-2009, 07:04 AM

Quote:

Originally Posted by cgarr

.001mm = 0.000039370078740157485 I hope you are not holding the rod or breathe on it when you take this measurement!

Craig, I usually like to make these measurements in a specially constructed metrology room that is anchored directly to the bedrock for no vibration.

304065 · 08-06-2009, 07:10 AM

Actually, measurement to the 1 micron level (0.001mm) is not that difficult or uncommon and was done by the factory in the old days, as illustrated in the FWM:

You can buy these today:

4sd911 · 07:23 AM

0.0001 inches, although the tenth of a thousandth would be estimated. This one? http://www.use-enco.com/CGI/INSRIT?PARTPG=INLMIP&PMPXNO=953256&PMAKA=612-5930

cgarr · cgarr's Garage

I use bore gauges but really don't like them, there is an art to using them, this is how we used to measure rods:

4sd911 · 08-06-2009, 07:56 AM

One question I missed. Do I need new rod bolts when checking the rods, and any problem torquing the new rod bolts to test and then using them in the final build, these would be factory or OEM bolts and nuts.

304065 · 08-06-2009, 08:12 AM

Yes, the ractory rod bolts are a problem.

Factory rod bolts are a "torque to yield" type-- the fastener is actually tightened BEYOND the yield point when installed. Once that's done, if it's removed, the fastener is junk. Factory rod bolts cannot be re-used for this reason-- if you were to torque it again, you would elongate it even further past the yield point and into the territory where it can snap under load.

This isn't the case with good aftermarket rod bolts like ARP-- these can be used in a rod vise for measurement of the big end. Resizing should take place with the new bolts installed and preloaded (I used a stretch gage) to the manufacturer's spec. Then the bolts are removed, the rod installed and the preload set to spec again.

4sd911 · 08-07-2009, 07:55 AM

So, if I have the rods resized with new factory bolts, then replace them after resizing with new factory bolts, will I be half way defeating the the resizing?

304065 · 08-07-2009, 04:47 PM

Well, in addition to wasting a perfectly good set of factory bolts, you would be relying on the second set being similar to the first to ensure the geometry of the big end.

I don't know anyone who uses factory bolts on a rebuild, the aftermarket ones are just too good and very competitively priced with the factory ones.

4sd911 · 08-07-2009, 05:34 PM

I don't mind the cost of the ARP bolts, but then I need a $150 stretch gage to use them, which I may end up doing. I don't suppose a standard outside diameter mic will work?