My crankshaft is at the machinist's getting polished, and when it comes back and it's time to start assembling the engine, I'd like to ensure I've got the proper bearing clearances. I want to learn how to properly measure bearing clearances (and bore diameters) with an inside micrometer. I'm a complete newb when it comes to this, but I'm willing to take the time to learn.

1) Considering the micrometer used to measure these clearances is probably only going to be used a couple times at most, I can't justify spending the $$$ on a professional setup, so I'd like to limit the expense to $175-$200 if possible. What brands/models can I look for that would be accurate and not break the budget?

2) How would you feel about taking a chance on a previously-used mic set from E-bay? As I understand it, most can be recalibrated (if necessary) with a good outside mic.

 

Keith, I calibrate these measuring devices for a living, so I may be able to give you some good advise.

First, I would buy a dial bore gage rather than a I.D. mic. IMHO, a dial bore gage is way more forgiving and you can check many more areas of the bore more quickly. The draw back with a dial bore gage is you will need some type of length standard to set up the dial bore gage (i.e. master ring or O.D. mic.)

Second, I like Mitutoyo brand, as parts are easy to get and the gages seem to hold up well. Alot of people will state to buy Starrett which is a very good USA brand, but is very costly, parts are sometimes hard to get, and they seem to rust alot.

IMHO, I would stay away from the non-brand name gages sold in speed shop catalogs, due too not being able to get missing or broken replacement parts.

If you buy a new or used I.D. mic. you will need some O.D. mic. w/ length standards to check the I.D. mic. accuracy.

If you buy a new or used dial bore gage you will also need some O.D. mic. w/ length standards to check the accuracy of the set-up of the dial bore gage.

PM me or post here if you have any other questions.

 

I appreciate your input, Lance. I'll go ahead and pick up the dial bore gauge and outside mics....but this leads to a couple additional questions.

How accurate are the outside mics at their outer limit? The reason I'm asking is this: A stock 455 crankshaft's main journal O.D. is 2.9993", and I've noticed that the name-brand mics have ranges from 1"-2", 2"-3", etc. So if I get a 2"-3" range mic specifically for measuring the main journal, will it continue to be accurate at the extreme outer range of its scale? According the the Mondello Technical Reference Manual, the main journals are 3.0003 - 2.9993 stock. If I were to get my hands on a crankshaft with a 3.0003 diameter, would a 2"-3" mic accurately measure this or would I need to jump up to the next size (3"-4")?

Secondly, if I was going to limit my mics to working on Oldsmobile V8s, would I be correct in assuming that I'd simply need to get one capable of measuring the main journals (3.0003 - 2.9993 stock) and rod journals (2.4998 - 2.4988 stock), and a second one capable of measuring the bore diameter (4"-5")?

 

how bout 2 bucks worth the plastiguage? your going to get a better dimension with everything assembled and torqued.

 

I was thinking the same thing in my 1969 olds manual it show them using
plastiguage for just what you are doing.

 

And a micrometer won't tell you if the crank is bent - just sizes.
At least with plastiguage, you've gotta assemble it, and a bend would become obvious!

 

I'll definitely be using Plastigage when doing the final assembly checks, but for the initial setup I kinda wanted to know for certain what the exact specs were, and so I wasn't entirely certain I wanted to trust the Plastigage. I know, it's an age-old debate...and in fact using a dial bore gauge is probably unnecessary for my build (since this is mainly a street engine), but I not only want to know for sure what the true specs are, but I want to teach myself how to use the tools.

In this link, Car Craft magazine compared the accuracy of Plastigage to a dial-bore gage and micrometer, and there seemed to be a fairly significant difference in clearance readings between the two.

http://www.carcraft.com/techfaq/116_...ter/index.html

However, I WILL say that I spent a little time this evening doing some reading on various other automotive messageboards where members were comparing the two methods, and it seems that while there are some who say that the dial-bore gauge is best, Plastigage seemed to come in a pretty close second. So...I don't know...I'll have to do some more thinking. I have a few days before my block/crank are ready to be picked up, I'll have to decide pretty quickly.

 

I'm no engine-building expert, but I believe the difference is in how much information you can get through each method.
Plastigage will give you a general clearance, while using a mic in three places along each journal, at two or three different angular locations, will tell you about the shape of the journal.
The key question is, is that extra information necessary in your particular case?

- Eric

 

O.D. mic. will come with a fixed length standard. 2-3 mic. will have a 2.0000" fixed length standard for seting mic. zero. 3-4 mic. will have a 3.0000" fixed length standard for seting mic. zero. 4-5 mic. will have a 4.0000" fixed length standard for seting mic. zero.

So to check calibration of the 2-3 mic. first use the supplied 2.0000" fixed length standard, then take the 3.0000" fixed length standard supplied with the 3-4 mic. to check the outer limits of the 2-3 mic.

I would buy a 4-5 O.D. mic. to properly set up the dial bore gage for the
cylinder bores.

So to do what you are wanting to do, I would get the following:

2-3 O.D. mic. with 2.0000" length standard

3-4 O.D. mic. with 3.0000" length standard

4-5 O.D. mic. with 4.0000" length standard

1"-6" adjustable dial bore gage

PS. you know engine machine shops will not like it at all you having these precision measuring gages. Trust me alot of them WILL do sloppy work and will not want you to know about it or you being able to prove it with your gages.