Measuring Cylinder Bore Diameter

[Vintage Engine Repairs]

Right, I have been scratching my head over and over about this .12mm diameter difference between TDC and BDC. I decided to get the cylinder back out and do it again. I went to clean it and looked inside the cylinder properly... to those who said it’s not possible to get the difference of .12mm between TDC and BDC with Nakisil lined bored without it wearing through. I just noticed this, this makes so much sense now - It’s lost plating at TDC.




Edit to update, I did it again away from any damaged plating. The difference between top and bottom is 0.03mm

 

[drf255]

Tom,

I measure taper on the lathe with a coaxial indicator. The most I have seen is .001 from the top near the band to the base. But it’s important to remember the the rings usually end up above or slightly into the intake port roof at BDC. So it’s really the area of cylinder wall that the rings ride against that matters.

You could also roughly measure with a piston and feeler gauge between the skirt and the cylinder wall. Or with a ring, checking the difference in end gap at different locations.

I believe the plating would be worn through before there would be enough variation in bore to make any difference.

 

[Vintage Engine Repairs]

Tom,

I measure taper on the lathe with a coaxial indicator. The most I have seen is .001 from the top near the band to the base. But it’s important to remember the the rings usually end up above or slightly into the intake port roof at BDC. So it’s really the area of cylinder wall that the rings ride against that matters.

You could also roughly measure with a piston and feeler gauge between the skirt and the cylinder wall. Or with a ring, checking the difference in end gap at different locations.

I believe the plating would be worn through before there would be enough variation in bore to make any difference.

Thanks for sharing that info mate, that makes total sense, see the post above the one you just shared haha

 

[trains]

I think what I’ll do is continue to practise more with the hateful telescoping gauges on known sizes and if I need measurements which I can’t accuractly achieve with snap gauges I’ll invest into bore gauges. Right now I’m walking a tightrope and my little office / workshop in the 2 bedroom apartment we’re in is slowly spilling out to around the rest of the house, much to the frustration of my partner. I have started to put things in the roof space which she doesn’t know about yet. My single tool box is full and I can only just about get in the office

Careful Tom, if you put too much in the attic, it may well end up in the cellar.

 

[L34]

BTW Tom
Did u contact Bert about your coil rewind.Just interested to know.

 

[Vintage Engine Repairs]

BTW Tom
Did u contact Bert about your coil rewind.Just interested to know.

Hey mate I totally forgot to, but actually put the coil back in. It’s on its last legs and can only jump the gap of the plug, but for now that it’s a shelf queen I think it will suffice. I’ll keep his contacts handy thanks again.

 

[Stihl99]

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

The point where your rotate the snap gauges back and forth inside your calipers jaws, that is exactly what you do when you have the snap gauge in your piston head.
Then you lift the snap gauge straight out after locking them.

 

[Vintage Engine Repairs]

The point where your rotate the snap gauges back and forth inside your calipers jaws, that is exactly what you do when you have the snap gauge in your piston head.
Then you lift the snap gauge straight out after locking them.

Hey. thanks mate, yep that’s what I do. Put the gauge in, tilt it 10 degrees down or so, gently snug the gauge, pull the lower side up through and out. I’m getting less than .010mm variance now which is great. I have ordered some Mitutoyo digital micrometers to work on further consistency and Mitutoyo telescopic gauges as the smaller ones I had weren’t very good. Eventually I’ll get a bore dial gauge, but not yet. None of this is necessary for chainsaws, but I’m wanting to move into a different direction within mechanics and thus want to better improve my knowledge and ability.

 

[flatsfisher]

I know this is a year-old thread but the mechanics of using a bore gauge and using snap gauges is the same. I would rather rely on measurements from a high-quality snap gauge measured with a high-quality micrometer or even a high-quality vernier caliper than using an inexpensive bore gauge.

 

[Mad Professor]

I know this is a year-old thread but the mechanics of using a bore gauge and using snap gauges is the same. I would rather rely on measurements from a high-quality snap gauge measured with a high-quality micrometer or even a high-quality vernier caliper than using an inexpensive bore gauge.

In a "snap" , feeler gauges can be used as go/no go, at the piston skirt (0.002-0.003").

Crude but works.

My uncle was THE master machinist for a company (GE) who did guidance parts for USA's ballistic missiles. They had a temp controlled room for everything sensitive, it was huge.......balances, mics,......

He let me "borrow" tools to build my SBCs, the tools went back home.....to GE

 

[Vintage Engine Repairs]

I know this is a year-old thread but the mechanics of using a bore gauge and using snap gauges is the same. I would rather rely on measurements from a high-quality snap gauge measured with a high-quality micrometer or even a high-quality vernier caliper than using an inexpensive bore gauge.

I certainly agree, a cheap dial bore gauge’s indicator needle often times won’t even return to zero consistently.

 

[Husky77]

I certainly agree, a cheap dial bore gauge’s indicator needle often times won’t even return to zero consistently.

Cheap and accurate dont go together, I got a cheap pressure gauge from that evilbay site and it's so far out its unbelievable. Just tested compression on a 50cc saw and its showing 40psi if that was an expensive names gauge I would be worried but in reality the saw is running fine.

 

[ChillyB]

Was quality manager at a major name that built engines with coffe-can size pistons. Have performed hundreds of gauge studies. For bore measurements I grew to love Dorsey bore gauges with Mitutoyo digital indicators. Those indicators were made for bore gauges, having a minimum-hold function. Press a button, insert the gauge, slowly rock it through the bore, indicator would hold at the minimum, which is the actual diameter.

One gauge, indicator, and master ring would run somewhere around $2k.

After doing many gauge study one knows what will pass for what tolerances. I’d NEVER use even best calipers for anything tighter than 0.010”. Regular mics were good for 0.001” tolerance band. Anything tighter required expensive equipment.

 

[flatsfisher]

my measuring set up. 1" mitutoyo.


this is the one for chainsaws.


I have a set of these 50-75mm, 75-100mm and 100-125mm. some of you will remember that DDR was an area inside Soviet occupied East Germany.

 

[ChillyB]

An inherent disadvantage of telescoping bore gauges is that you’re actually taking two measurements, each with their opportunity for error. The amount of error varies with technique and experience. Other gauges are more forgiving in that regard. But one can’t overcome sloppy technique with high $ equipment. Nor can skill overcome inaccurate/imprecise equipment.

 

[flatsfisher]

this is only my 2nd chainsaw but I've done scores of dirt bikes. with snap gauges I learned to place the gauge where I want it, hold the handle plumb, snug up the lock, rock the tool gently and then set the lock firmly. has worked well for me.