Measuring Cylinder Bore Diameter

[Vintage Engine Repairs]

I have had no success using these bloody telescoping / snap gauges getting repeatable results until recently.

 

[MagraAdam]

Hey mate, silly question but can't you use the other side of the calipers?

 

[Vintage Engine Repairs]

Hey mate, silly question but can't you use the other side of the calipers?

Hey mate, only within the first cm or so of the cylinder. After that you can’t go in further with verniers.

 

[MagraAdam]

Hey mate, only within the first cm or so of the cylinder. After that you can’t go in further with verniers.

Ok, silly question #2 then.. aren't they the same diameter over the entire stroke?

 

[Vintage Engine Repairs]

Ok, silly question #2 then.. aren't they the same diameter over the entire stroke?

No a cylinder wears towards the top more than the bottom. In this case the variance is .12mm between BDC and TDC. It’s the reason you should always set ring end gap at the bottom of the cylinder. Set it towards TDC and you will be too tight when the piston returns.

Excessive variance between the two readings may require a replacement cylinder from the research I have done. Though best to confirm this with Jerry. In fact @pioneerguy600. Can you share your wisdom on this?

From a logical perspective, if the wear is such that the ring end gap is within spec at BDC, but out of spec when measured at TDC the the cylinder should be changed.

If this even happens with nikasil I have no idea. Queue Jerry.

 

[MagraAdam]

Ah! I assumed it was for confirming the piston size or sizing the rings etc, I didn't think about wear being uneven. Thanks mate!

 

[Vintage Engine Repairs]

Ah! I assumed it was for confirming the piston size or sizing the rings etc, I didn't think about wear being uneven. Thanks mate!

All of the above, the rings need to be within spec, so does the cylinder and piston. all wear at different speeds.

 

[Mad Professor]

Uneven wear is more of an issue on cylinders that have cast iron sleeves, or complete iron cylinders.

I'm kind of surprised that nikasil lined cylinder has that much taper, 0.12mm/0.005". I don't think the plating is too much thicker. I've seen 0.15mm claimed for an aftermarket nikasil cylinder plating thickness.

I like the technique used on the snap gauge. You might be able to get more accurate measurements with a good micrometer rather than a vernier. I'm not sure what the +/- tolerance is on the calipers used?

One other thing. Pistons come tapered top to bottom and are largest at the skirt. The top of the piston is held away from the bore by the rings but the skirt just floats on a film of oil. The skirt is where you will see the wear. The new pistons I've measured had ~ 0.005-0.006" taper top to bottom.

Note: taper was for ~70cc saws with 50-52mm bores.

 

[Canyon Angler]

"A" for effort, but good luck getting accuracy to 0.01mm (less than half a ten-thousandth of an inch) with that method (not that you need such precision)...now that I think about it, I wonder whether the difference you mentioned between TDC and BTC (0.12mm which is less than 5 mils) even matters. Are there any rule-of-thumb "go" and "no go" measurements for this difference? What are the typical ring-end gap tolerances?

With my dial caliper, I can generally read it to within about +/- 5 tenths...

 

[Vintage Engine Repairs]

Uneven wear is more of an issue on cylinders that have cast iron sleeves, or complete iron cylinders.

I'm kind of surprised that nikasil lined cylinder has that much taper, 0.12mm/0.005". I don't think the plating is too much thicker. I've seen 0.15mm claimed for an aftermarket nikasil cylinder plating thickness.

I like the technique used on the snap gauge. You might be able to get more accurate measurements with a good micrometer rather than a vernier. I'm not sure what the +/- tolerance is on the calipers used?

I also tried the same with a piston ring and a set of feeler gauges and got .15mm

Re the skirt, yes absolutely that’s my understanding too.

 

[Vintage Engine Repairs]

"A" for effort, but good luck getting accuracy to 0.01mm (less than half a ten-thousandth of an inch) with that method (not that you need such precision)...now that I think about it, I wonder whether the difference you mentioned between TDC and BTC (0.12mm which is less than 5 mils) even matters. Are there any rule-of-thumb "go" and "no go" measurements for this difference? What are the typical ring-end gap tolerances?

With my dial caliper, I can generally read it to within about +/- 5 tenths...

I was taught by Jerry and a few others to replace the rings at around 16 - 20 thou but to keep it scaleable I noted to change rings at 0.25mm end gap per 25mm of bore diameter.

I was getting it pretty much within .01-.03 every time unless I messed up which would change the reading to .10mm or more - quite an obvious difference. Maybe it was just luck, but I did it over and over. I’m really not sure honestly!

 

[Mad Professor]

"A" for effort, but good luck getting accuracy to 0.01mm (less than half a ten-thousandth of an inch) with that method (not that you need such precision)...now that I think about it, I wonder whether the difference you mentioned between TDC and BTC (0.12mm which is less than 5 mils) even matters. Are there any rule-of-thumb "go" and "no go" measurements for this difference? What are the typical ring-end gap tolerances?

With my dial caliper, I can generally read it to within about +/- 5 tenths...

I also tried the same with a piston ring and a set of feeler gauges and got .15mm difference so very close and that could also be because the feeler gauges aren’t .01mm increments, they vary but generally .03 - .05. I’m not sure what else could give me that degree of difference other than the cylinder wear?
As for the vernier I’m not too sure, I have tried it on the feeler gauges and it seems bang on almost every time. There must be a degree of inaccuracy of course though. Pressure applied alone makes the difference.

That's the reason I mentioned using a micrometer. Most mics are +/- 0.001"/0.03mm, better mics will be half or less than that.

 

[Vintage Engine Repairs]

Are there any rule-of-thumb "go" and "no go" measurements for this difference? What are the typical ring-end gap tolerances?

This is a really interesting question, I’m hoping someone can shed some light on it, I really don’t know. I can only guess From a logical perspective, if the wear is such that the ring end gap is within spec at BDC, but out of spec when measured at TDC the the cylinder should be changed?

 

[Vintage Engine Repairs]

That's the reason I mentioned using a micrometer. Most mics are +/- 0.001"/0.03mm, better mics will be half or less than that.

Yes absolutely right and well said mate, I wanted to get one, but really you need a few to get a range of measurements, then you should buy gauge blocks to test their accuracy periodically. Then you get into temp fluctuation. How far into the rabbit warren do you go. Thus my desire to stick with verniers But no, you are absolutely right, micrometers will read to .001 and better.

 

[Canyon Angler]

I also tried the same with a piston ring and a set of feeler gauges and got .15mm difference so very close ...

A 0.15mm difference in the ring-end gaps (circumference)...or in the (calculated) diameter? (Circumference = 3.14 x diameter)

On a 37.7 mm diameter bore, a difference of 0.12 mm in diameter translates to a difference in the ring-end gap (circumference) of 0.37 mm -- not 0.15mm

I think you're getting down to a level where the metrology makes for bigger differences than the actual measurements...like trying to count how many fairies can stand on the head of a pin.

Most mics I've used go to 0.0001" (one ten-thousandth) and every dial caliper I've ever used goes to 0.001" (one mil) and you can split that by about half (with dot markings between the mil lines on the dial)...if all you care about is 0.01" you can measure that with a yardstick or surveyor's hand chain...

 

[Vintage Engine Repairs]

.now that I think about it, I wonder whether the difference you mentioned between TDC and BTC (0.12mm which is less than 5 mils) even matters.

Absolutely, another very valid point, probably not for a chainsaw, but I have an inquisitive mind (OCD) and I love exploring things and learning.

 

[Vintage Engine Repairs]

A 0.15mm difference in the ring-end gaps (circumference)...or in the (calculated) diameter? (Circumference = 3.14 x diameter)

A difference of 0.12 mm in diameter translates to a difference in the ring-end gap (circumference) of 0.37 mm -- not 0.15mm

I think you're getting down to a level where the metrology makes for bigger differences than the actual measurements...

Ooo such a good point! I’m bloody miles out...

Hmm so what I did was I just put the piston ring in the bottom of the cylinder, and slid a feeler gauge that felt a little resistance and then did the same at the top and subtracted the two?

 

[ozziechainsaw]

This is a really interesting question, I’m hoping someone can shed some light on it, I really don’t know. I can only guess From a logical perspective, if the wear is such that the ring end gap is within spec at BDC, but out of spec when measured at TDC the the cylinder should be changed?

Just a question, so I can see why you are trying to get it as accurate as possible when you are rebuilding these saws, but are they every day logging and commerical use, weekend use or restorations ?

Justin

 

[cuinrearview]

Don't go measuring a new cylinder. You'll be disappointed...

 

[Vintage Engine Repairs]

Just a question, so I can see why you are trying to get it as accurate as possible when you are rebuilding these saws, but are they every day logging and commerical use, weekend use or restorations ?

Justin

Hey mate, I’m literally just trying to learn new skills honestly. i like the challenge I guess, or maybe it just satisfies my OCD? These snap gauges have been pissing me off since I got them, until recently since figuring out a few little changes to what I was doing.