Cylinder Bore measurements??

[fast400]

So I tore down my engine for a rebuild with new cam, pistons, etc. Before ordering pistons, I decided to measure all bores, and I'm a little stumped because I wound up with 3.773" on the bores +/- 0.001
I thought the bores were 3.800" from the factory...anyone else experience this?
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[TurboDave]

Might want to double check the calibration of your tools. When I tore mine down a few months ago, the cylider bores varied from 3.8002 to 3.8012 depending on measurement depth and orientation.

3.773 sounds more like a piston measurement than a bore measurment.

 

[fast400]

I double checked with and outside micrometer 3x. The measurements are good. The pistons measured 3.771 at the widest spot....

 

[TurboDave]

Wait a minute. YOu said the pistons measured 3.771, and you said you measured the bores at 3.773 +/- .001 ?? That should be your first hint that there's something wrong with the way you're measuring cylinder bore size.
1. What are you using to measure bore size?
2. How far down the bore?
3. How many clock positions are you measuring at your measurement depth?

 

[fast400]

Dave,
I measured and the point of least clearance which is the skirt bottom. The cylinders I measured at several points, vertical and horizontal. According the the manual skirt bottom clearance up to 0.003 is ok, which means my clearances are good. Trust me, I measured correctly...spent over two hours measuring and re-measuring pistons/bores and re-calibrating micrometers, which all were within the correct limits...just 0.007 less than the normal 3.800" I was expecting to see.

 

[turbofabricator]

Just deglaze the cylinder and slap new rings on it. No need for pistons unless you plan on running faster than 10.90 regularly. The stock pistons work quite well. Only thing wrong with 'em is the weight.

 

[Waco84GN]

Hello Fast 400: I am not sure if there is a problem the way your are measuring the bores or the instruments you are using to measure the bores; but something is wrong. Are you 100% sure you have a 3.8 litre v/6 and not some earlier block ; which had a smaller bore. For certain it seems something is not right.What is the casting number on the engine block? Let us know what you find. Jerry Berger Sr.

 

[87GN_70GS]

You realy need to measure with a dial bore gauge and check well below the ridge to get an accurate measurement.

 

[fast400]

You realy need to measure with a dial bore gauge and check well below the ridge to get an accurate measurement.

That's what I'm using.

 

[fast400]

That's what I'm using.

Here's a couple pics:

The bore gauge has a 95mm attachment plus 1.5mm (96.5mm total). I'm going halfway down the bore horizontal and then vertical. Each time the dial indicator winds up somewhere around .65 which I then subtract from the original 96.5mm which comes out to 95.85 (=3.773"). I double/triple checked by measuring the bore gauge with an outside micrometer and my crude green slide ruler, both were right at 96.5mm. It's the original 109 block with 65k miles. I don't understand my reading either...but they seem accurate.

 

[fast400]

Does temperature make a huge difference? My garage is around 40F.

 

[Mike E]

Dave,
I measured and the point of least clearance which is the skirt bottom. The cylinders I measured at several points, vertical and horizontal. According the the manual skirt bottom clearance up to 0.003 is ok, which means my clearances are good. Trust me, I measured correctly...spent over two hours measuring and re-measuring pistons/bores and re-calibrating micrometers, which all were within the correct limits...just 0.007 less than the normal 3.800" I was expecting to see.

.007 less than 3.800 is 3.793. I think turbodave was leading to the fact that .001" clearance is not ever going to happen. .003 is probably the tight side of the clearance spec when new. .005-.010 is probably more realistic with a used piston that has had the skirt collapsed some. The fact that you only measure .001" clearance between the two should be a sign that you have measurement issues. .001" clearance is on the verge of still being a press fit at this diameter.

Here's a couple pics:

The bore gauge has a 95mm attachment plus 1.5mm (96.5mm total). I'm going halfway down the bore horizontal and then vertical. Each time the dial indicator winds up somewhere around .65 which I then subtract from the original 96.5mm which comes out to 95.85 (=3.773"). I double/triple checked by measuring the bore gauge with an outside micrometer and my crude green slide ruler, both were right at 96.5mm. It's the original 109 block with 65k miles. I don't understand my reading either...but they seem accurate.

96.5 = 3.799". If that's true that your gauge is a max of 96.5mm bore, it would never move in a 3.800 bore. There are only a couple possibilites here. You either have an older smaller displacement buick v6. A quick search shows that buick never built a v6 in a bore size around 3.773. Or you've got something going on with your measurments. Those green plastic calipers are no better than a tape measure for what your trying to do here.

The way I would check this is put your micrometer (not caliper) in a vice and set it to 3.800". Setup your dial bore gauge so that 3.800" is in the middle of the measurement range. Then place the bore gauge in the micrometer and set the dial to zero. Try that and report back with your measurment.

Does temperature make a huge difference? My garage is around 40F.

Not enough that you can measure with a bore gauge that measures .001".

 

[TurboDave]

.
The way I would check this is put your micrometer (not caliper) in a vice and set it to 3.800". Setup your dial bore gauge so that 3.800" is in the middle of the measurement range. Then place the bore gauge in the micrometer and set the dial to zero. Try that and report back with your measurment.



Not enough that you can measure with a bore gauge that measures .001".

It's difficult, but that's how I do it. It takes a good bit of patience. Another variation I do is to set the dial bore gauge in a vise (with rubber jaw covers). Set my outside mic to 3.8, and set it over the bore gauge measure points. Rock it around a little to find the optimum point, and set the bore gauge ring to 0. Then when you slide the bore gauge down into the bore, it should end up somewhere on the positive side of 0 by .xxx
Aren't your bore gauge pins and spacers graduated in .xxx" rather than mm??

I'll have to try the method setting the outside mic in the vice instead. Sounds a bit easier.

 

[87GN_70GS]

From what little I know, they advise you to use a calibrated ring to set your bore gauge.

Not having that, I found a piece of plastic tube that would fit over one end of the mics, and also fit the fixed leg of the dial bore gauge. That left just the one measurement end of the dial bore gauge to slide around inside the one mic end to find the optimum point.

 

[TurboDave]

From what little I know, they advise you to use a calibrated ring to set your bore gauge.

Not having that, I found a piece of plastic tube that would fit over one end of the mics, and also fit the fixed leg of the dial bore gauge. That left just the one measurement end of the dial bore gauge to slide around inside the one mic end to find the optimum point.

Yes, a ring gauge is the ideal method, and fortunately I was able to borrow a set the last time I needed to set my bore gauges. But they're hard to find, and too costly to buy.

 

[fast400]

Well I re-checked everything and found that the 96.5mm adapter was correct with the spring loaded ball pressed in. Without being compressed the resting value comes to 97.2mm. I forgot the account for the 0.7mm difference. I double checked with my outside micrometer (which was off too) My values now are all around 3.8003. All my piston clearances are right in the middle of the range values.
How much does honing usually get rid of.....
I just ordered pistons, don't know if they're being made yet, but if a change is needed now would be best.
Thanks for clueing me in...I knew it couldn't be right but the numbers kept coming up the same.
I'm sure I'll have more stupid questions to come

 

[ek02]

How much a block needs to be honed depends on the taper or out of round of the bore. If it all checks out round and true, a ball hone will break the glaze and put the proper finish as specified by the ring manufacturer on the cylinder walls. If the cylinders are not true, then a Sunnen hone could be used to make them round and true again. A Sunnen hone can remove a thousanth or three, or more. If you are going to forged pistons from stock, you may be able to hone the block, true it up, and get the correct clearance for a forged piston since they are usually set up looser than stock. Check the cylinders with your bore guage at the top of the bore and at the bottom to check for taper. Check around the circumference of the cylinders for out of round. That should give you a good idea of how much to hone, and the final clearance, if you know the piston measurements.