When I measure main bearing clearances with a bore dial indicator I am making contact between the indicator and the bearing surface. As I rock the bore dial indicator to get the highest reading it "marks" the bearing surface.

Is this detrimental to the bearing?
Am I doing something wrong?

Is there a technique to avoid this marking of the bearing surface?

Thanks
Bob

 

That is normal, it is more a shadow then anything

 

ok....but it doesn't wipe off. So I must be marring the surface.

Again... normal?

 

you won't feel it with your fingernail though, the bearing is pretty hard...

 

plasti-gauge?

 

Or a T-gauge and micrometer. Its a lot more time consuming but its also a lot more accurate.

 

Keep the dial bore gauge, if it is graduated in .0001"

My Mitutoyos have different size spacers, so if you can, use a thinner spacer which will preload less. Still the small marks are not a problem if you use the tool correctly.

Zero the dial bore to a calibrated mike and off you go... Check at 0, 45, 90 and 270 degrees.

No plastigauge or T gauge is going to show you tenths.

 

It depends. Set a bore gage and have 5 people measure a hole. Then give 5 people a T-gage and the same micrometer used to set the bore gage and have them measure the same hole. I'll bet the range of sizes with the T-Gage is 2-3 times the range of the bore gage measurement. As in many things, "feel" is important.

My $.02

 

That is normal. Don't worry about it.

 

to measure clearence, do you just subtract the bearing bore diameter by the crank journal diameter then divide by two? I though you assembled the crank into the main bearings then used a feeler gauge between the bearing and the journal. Then again, I haven't built a motor before. Thanks!

 

I agree with OldSS 200%, feel is important and a lot of people don't have it.

Here is the easiest way for a novice. IMO

NOTE: I use the word 'mike' as an abbreviation for micrometer

1) Use a mike with a ratcheting or friction thimble in the end of it. NEVER overtighten the mike, always use the thimble.

2) Using a standard (2" or 3" that came with the mike), check that the mike is calibrated. This is assuming that everything is clean. Don't use rags with lint and clean the ends of the standard and mike by lightly wiping against your skin.

Adjust if necessary and re-check that the mike zeros properly, by measuring the standard and wiggling the standard a little as you get close to tight (thimble only). Do this without looking at the mike graduations. Many people try to tighten until it reads what they want it to read. Do this a few times, to get the feel for the mike and check that it is zeroed each time. With a bit of practice, you will get the feel and your measurements will be consistent.

Caution: Before taking a mike to a crank journal, you may want to try the next part on the snout of the crank so that you don't scratch the journals while you get the feel for the mike.

3) Now mike the (clean) main in question. Once again, when you get close to tight, rock the mike back and forth lightly and wiggle lightly. Note: You are rocking to try and find the biggest point or actual diameter. You are wiggling to make sure that the mike is square to the journal. Remember all of this is done with a VERY light touch while slowly tightening the mike with the thimble. If the mike binds at all, gently loosen and or wiggle the mike, or you stand a chance of scratching the journal.

4) Once you have the mike on the actual diameter of the journal, then gently tighten the lock, and then slide the mike off.

5) Check that the measurement is within the specs of the journal. If not, then ither you measured wrong, or the mike is not calibrated or the crank is out of spec.

In any case, what you are really looking for is the clearance, so if the crank is a little on the high or low side, but still within spec, then you should be good to go.

6) With the mike still locked, try to slide it onto the journal. When it gets a bit light, wiggle and slide. If it gets real tight, then stop and go back to step 2. It should slide on with just a small amount of resistance.

7) Put a rag over the C section of the mike and clamp it VERY lightly in a bench vise. Clamp only enough that it doesn't fall out.

8) Assemble your dial bore gauge using the proper tip and spacers.

9) Place the assembled gauge in the mike starting with the plunger end first and then the fixed end. This can be tricky at first. Once both ends are resting against the mike surfaces, center one end with your thumb and forefinger and gently rock the other end slightly up and down and then left and right. You are looking for the smallest reading on the dial. (The furthest reading in a clockwise direction on my Mitutoyos.)

10) Read the dial and make a note of how far off zero is from the needle and in which direction. Remove the gauge from the mike and rotate the dial by the amount noted and in the correct direction. Repeat step 9 until the dial reads zero. Also make a note of where the small dial is (usually marked 1 to 5 with a smaller needle), and then lock down the dial clamp.

11) Now you can assemble a main with bearings and check it at the four postions.

I like to start with the rear main and work forward. This method also gives you the actual clearance, which is what you are really looking for.

Sorry for being long winded, but I thought that some might find it interesting...

On another note, I do a bit of chamfering on the mains and caps so as to not shave the bearings, but that is a subject for another day.

 

guys, it's a mic (as in micrometer), not a mike

 

Not trying to get to "picky", but mot standards are set up to be used at ~68*F.

The heat from your hands will "expand" the standard. I know this is minimal, but if we are talking precision, lets talk precision.

 

I thought the lint from the towels was picky... I guess I should wait until spring to check my bearing clearances...

 

A valid question nonetheless.You don't divide by 2.

Thanks mic (Just kidding)

Not to mention controlled humidity, etc. Bottom line is that everything should be at the SAME temp, and not recommended that you measure in your garage at -40F. You could be off by 50 millionths or so... not to mention frost bite

I'm impressed that anyone even read my long winded BS...

Seriously tho...

I am sure that there is plenty of good reading material that describes clean rooms, calibration and measuring techniques, etc. Maybe some could mention good books on the subject for those who are just starting out.

One tip for the novice DIY crowd is ...

In selecting a shop to do your machine work (boring, honing, etc) you should show up with your gages, and ask them to check your stuff. If their results are not the same as yours, whip out the gages and compare on site. Either you are them are in error.

If it's them, then MAYBE take your stuff elsewhere. In any case, the shop now knows that you will be checking and they will probably try to do a better job.

I had a dispute with a shop many years ago regarding piston pin clearance. It called for .0008" (or 8 tenths).

After I measured and they measured and we all re-measured and argued (in a somewhat friendly way) , I whipped out a .0015 feeler and slid it in between the parts in question. That ended the argument. They fixed the problem and since that day, I NEVER had a dispute with them.

Bear in mind that diplomacy is the the best route and that MOST machinists (in any field) are a proud bunch and don't like to be proven wrong.

On the other hand, some shops are just NOT capable of producing really good work, due to bad equipment and/or operators.

Am I

 

Most standards come with a plastic (insulating) section for you to hold onto.

Whatever the temp of your engine building room, the measuring tools should be "soaked" to the same temp by being in that room for a while. I you are antsy about a few degrees, put the mics (or Mikes ) in a pan of room temp solvent between times you use them.

 

Someone should put up a video showing how they do all this.

I have the video and editing stuff, but not the time at this moment.

 

You guys have made a bunch of excellent points.

Instructions on how to properly use a micrometer & associated measuring tools are well written, Kudo's to the writer.

Temperature is certainly a potential issue. Measuring the parts at the shop is a good suggestion. If they measured the parts hot (from machining) , then you pick them up 3 days later and measure the parts in your garage at 20 degrees F, you certainly could have a difference of measurement. Measuring them at the time of pickup may be time consuming but is certainly a useful thing if you're building a high precision engine (if you're just slapping together a rebuild of a 350 SBC for Joe's beater, then it probably isn't worth all that effort).

But be prepared for your measurements and their measurements to differ. Even using the same gage mutliple operators are likely to get multiple measurements. Throw 2 sets of gauges in the mix and you'll definitely have differing measurements.

In the manufacturing world (or at least the chunk I work in), many customer's require Gauge R & R studies (which is Gauge Repeatability and Reproducibility). Basically its a statistical study done with 1 gauge and multiple operators (minimum of 2) and multiple parts (typically 10 pieces which are numbered) with each operator measuring the same feature (on all parts) multiple times (often 3 to 5 times). Ends up being a lot of time spend measuring parts. Enter all the data into a computer program to do the statistical analysis (trust me, you don't want to calculate that by hand) and you get an idea of how good your gauging system and your operators are.

Nothing like making a simple act (measuring a part) overly complicated, right?

'JustDreamin'

 

FWIW I edited my first post to get rid of some typos and explain the word 'mike'

I rattled it off late last night, so please forgive my mistakes. Please everyone feel free to add or correct anything as needed.

A lot of machinists including engine builders are reluctant to share stuff.

Also, someone could explain expansion of metals and how say 10F change in temp will affect the actual meaured size of a part. (I'm too lazy to look it up)

 

Speaking of temperature effects, I calculated this three times because it seems surprisingly high, but with a coefficient of thermal expansion of 7 parts per million per degree F., a 3" main journal (OK, maybe no one here's doing Pontiacs but it's a nice round number...), warmed from the previously mentioned minus 40 F. to plus 70 F., would expand 7 x 3 x 110 = 2310/1,000,000 or over two thousandths of an inch! Proportionally, a 2 tenths change would result from a mere 10 degree difference, so the combination of a cool crank and a micrometer clenched in the hot sweaty fist of the builder is of much more than just theoretical concern...