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Discussion Starter · #1 ·
New install, converting from a 4l60 to T56 in a SBC. Want to confirm my numbers and math... Tolerance is .005"

Put a dial on the flywheel, measure the inner on the bell. hit 12 o clock, 3 o clock, 6 o clock and 9 o clock. Started at 6 and zeroed it out.

6 = .000
3 = -.007
12 = -.008
9 = -.008

So I calc like this..

positions 6 and 12, .00 - .007 = .007
positions 3 and 9, .007 - .008 = .001

I half the results (right?) so I am .0035 out (6 to 12) and .0005 out (3 to 9)

Within spec right?
 

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I would double check your readings again. Its possible you are getting a little flex in your setup but it appears your opening is out of round. Also, you didn't say + or - on the readings. Sometimes people don't notice that the pointer went to the other side of zero and just look at the number. If you have three readings that are .007/.007/.008, all on the same side of the zero mark, then thats very good concentricity. The sudden drop to .000 at the fourth location seems a little odd. Not saying anything is wrong and it may be ok as is........but I would double check your readings and then I would measure the roundness of the opening with an inside micrometer.
 

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Discussion Starter · #3 ·
I am going to double check it. When I got all neg, I spun it again (indicator mount was same) and the readings were redundant. All of the values were negative so -.008, - .007 etc.

That 3 and 9 o'clock measurement is not what I expected.
 

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If 3:00 and 9:00 really are both negative, that would indicate that the opening is out of round.
If you take a known round hole that is "perfectly" concentric with an axis (crankshaft) and instead of using "zero", you set the indicator to +.007 and rotate it, all 4 locations should read +.007.
That shows that the hole is round and also concentric to the crankshaft. Normally you set it at 0 and hope they are all 0, but if you use +.007, then you hope all are +.007

If you set it at +.007 at the 3 oclock position and it reads +.007 at 3,6,9 but NOT at 12, that indicates that the hole is out of round but basically concentric

If you set it at +007 at the 3 oclock position and the hole is round, then a -.007 (minus) reading at 9 oclock indicates the hole is not concentric.

The thing is that someone checking a bellhousing should first verify that the hole is round. Probably going to find some minor .001/.002 variation, but need to know for sure it is reasonably round before proceeding to check concentricity.

Simply saying that if 3:00 and 9:00 are both negative isn't completely correct. If the hole is round, a -.007 at both locations indicates that it IS concentric. If the hole is not round, it is possible to get a -.007 at both locations and also NOT be concentric. It would just depend on how much the hole is out of round.

When indicating parts in on a milling machine so that they are centered under the chuck, you simply look at the indicator between the 3 and 9 readings and move the part to zero. The indicator will have a plus in one direction and a minus in the other. As you move closer to zero, the plus and minus readings will get smaller. When you are close, you change to the 6 and 9 locations and do the same thing again. Then usually there is a little more adjusting and indicating a second time to get it perfect.......but you have to have a round hole to begin with.
 

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Discussion Starter · #6 ·
I re-mounted the bell and found completely different readings, I am doubting the integrity of my dial mount. my current set up is probably too large for this job. I am picking up a goose neck base and smaller dial.

While re-mounting my bell, it seems pretty sloppy on the locating studs/mounting bolts. Guessing it has .02" of play(?) which leads me to wonder about the tolerance and repeatability in general. I'll check it all again when the new mount arrives.
 

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Its always harder to check an assembled engine .........especially if its still in the car. If you build any engines its much easier if you think ahead and verify the bellhousing with just the crankshaft installed in the block. Corrections are easy to make at that time. Make sure you have an indicator tip that isn't pointed. They tend to catch on minor irregularities. One that is a little larger and less pointed or flater works better.Good luck with it, and glad you double checked it. Now you have learned something that will help you later on.
 
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