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Bore guage readings

Engine Building Fundamentals

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So after measuring the dial.indicator on a piston im installing into a block, I zeroed out my bore guage. Taking several measurements in the same location and comparing what I read it appears the clearance is off and cylinder is out of round. This is after the block was re machined and the pistons were supposedly fitted. So I'm trying to determine if my guage is in accurate or if I'm just not reading it correctly. I measured piston number 1 (81.5mm piston) on imperial unit guage. Came out to 3.2064 inches. Bore clearance stopped at 17 lines before reaching 0 indicating 0.0175 inch of clearance. I measured this in about the same spot 4 times each reading the same. On the next location I got a reading of 0.0210 inches at about the same depth. Factory spec says clearance is supposed to be 0.0039 inches to .0043 inches. Pistons are factory and were in motor prior to refresh, sent to machine shop with block. Jad simiular readings further down the bores and Vince pistons were not labeled by as to what cylinder they were supposed to be matched to, trying to help a friend figure it out.


This would be my normal response...

What is the zero measurement of your bore-gauge? Many times those have various bits that are assembled to give different "zero" values. Use the micrometer you measured the piston with to measure the zero of the bore gauge... Then you can do the math to determine the clearance.

However, it sounds like your tools may be different. Can you post pictures or provide model numbers so I could understand how they work.

Oh, often the factory workshop manual will have a specific height from the top or bottom of the piston to make the measurement. Be sure you follow those guidelines as that's what the clearance specs are referencing.


Original pistons - mileage?

Block was 'machined'/'refreshed' -how? If the bore has been honed, or otherwise worked on, it will further increase the diameter over the normal wear. If the engine has an appreciable mileage on it, has been worked hard, has been run without a filter and/or has been poorly maintained there may be appreciable wear, especially on the more heavily loaded major thrust side (the left looking from the pulley end of the engine).

While there may be measuring errors, as David said, I think the real issue is simple wear and material removal during the machine work.

While it may be normal practice, when clearancing an engine, to mark the pistons, I suspect that's the least of your worries.

The micrometer is the same as the one Andrea used in his enginbuilding fundamentals course, the bore guage was zeroed on this and then I measured the machine work. Piston was measured via fsm spec and the was measured 4 times per piston...in order to ensure accuracy...the bore guage is the exact same style as in the corse except in inches. Each line is = 0.025 of inch, and every 10 lines is numbered. I first locked the micrometer and then fitted the closest stem and washer to fit. When centered in the micrometer, the dial spins slightly over 2 full rotations. I adjusted the outer dial so that it reads 0 in the micrometer then locked it in place. When incerting it into the cylinder bore, i watch the needle and its spins past 0 once, then stops between the 8 and the 9 just before the 0 reading where it would sit if inside the piston measurement showing a slightly larger opening. As stated the differences do not measure as it should and as in some areas such as bore #3 i had 1.4 thou of clearance at approximately halfway down the dore massuring front to back, and then measuring side to side across the crank journal for the rod i had .8 thou. Fsm states I should have 3.9 to 4.3 thou clearance. All pistons were checked, exactly the same way and give a slightly less than the 81.5 mm as is normal with all pistons being slightly smaller than advertised. As for the question is am I reading this correctly, or is my way of reading this incorrect. If I'm reading it incorrectly, I would like clarification on how to accurately read the guage. If on the other hand I am reading the guage correctly, then I need to take the engine back to the shop for correction.

Thanks for the help.

I'm 99.9% confident it is as I suggested - if you would answer my questions, it will help confirm that.

You cannot 'correct' excessive clearance except by going to the next oversize piston and machining the bore to suit or, if you really(!) want to keep those pistons, having the block sleeved and machined to suit.

I may be missing something, but from what you're describing, I'm not sure what you expected?

Found a calculator to check - nominal 81.5mm bore is 3.20866 inches.

The piston is apparently measured at 3.2064 inches.

Assuming the measurement was correctly done, that would give a nominal 0.0023"clearance, which sounds about right for a lightly worn factory piston, depending on alloy and design. Exactly what engine is it?

You refer to the bore 'clearance', is that the measured difference between the nominal and actual? I suspect you are actually measuring the wear, which makes sense as it will wear more across the engine than along it.

I don't understand where you got the 0.0175" and 0.0210" 'clearances' from? I would suspect that's bore wear and the 'machining' having removed material, but it doesn't tally with your other statements.

It 'may' help if you set out each measurement in a paragraph, with the measurement technique used?

Not sure, engine is not mine.

I have no engine history other than it was machined. The Clarence in on direction of measurement is not the same going the other. Across the bore at the same depth . When installed, engine is turned by hand and is easily turned until poston gets close to tdc, then the piston gets stuck and requires a bit of force to to turn.

It should not get harder as (#1?) comes to the top of the bore, and it suggests a problem somewhere. Normally, the engine will be rotated with the bare crank fitted and mains torqued, then as each piston is fitted, to pick up any tight spots that need investigation.

At this point, as we don't know what is actually happening, I would suggest you start the process in reverse, removing one piston-rod assembly at a time and checking if it's freed up the engine, as that will give you some idea where to look and investigate further. NOTE, it may be due to other pistons approaching BDC - for example, if the bore was more worn at TDC, as is normal, and you gapped the rings for that, as the bore closes up towards the less worn lower area, the rings will close up and may even start to bind. When you take it apart, carefully check all the parts as you may see signs of scraping, or uneven markings on, say, the bore or bearing shells if a 'rod is slightly bent or twisted.

There are many things it could be, if it is actually a problem, and as we have no idea exactly what was done, or how it was put together, it is VERY hard to try and analyse.

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