Measuring bore question
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Melting Slicks
Joined: Jul 2000
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From: Beacon NY
Measuring bore question
First of all, I have never attempted to do any sort of engine rebuilding before, so I have no idea what I'm doing.
I have a digital micrometer and I'm attempting to measure the bore of my engine block. I measured all 8 cylinders and they all are right around 4.037. When I measured my pistons (without the rings on), they all came in right around 4.034.
Is this normal? I was expecting everything to come in at 4.03 since I know it was rebuilt before I bought the car.
Would I be alright buying .03 oversize pistons?
I could understand my measurements being that far off because of cylinder wear, but there isn't a noticible bore wear on any of the cylinders.
I have a digital micrometer and I'm attempting to measure the bore of my engine block. I measured all 8 cylinders and they all are right around 4.037. When I measured my pistons (without the rings on), they all came in right around 4.034.
Is this normal? I was expecting everything to come in at 4.03 since I know it was rebuilt before I bought the car.
Would I be alright buying .03 oversize pistons?
I could understand my measurements being that far off because of cylinder wear, but there isn't a noticible bore wear on any of the cylinders.
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Is this a true bore gauge or a digital caliper?
First I'd go through the calibration procedure on micrometer. Just guessing it seems to be a little high.
I say that because almost without exception, the clearance is built into the pistons. The bores are usually right on the 4.000 or 4.030 or whatever, with the pistons being slightly smaller. Yours are that way showing them to be .003 smaller than the bore, but since they are larger than 4.030 I would suspect the gauge is reading a little high.
Make sure you measure in the correct spot on pistons. Usually down by the pin at the upper end of skirt below ring grooves. The area of piston in the ring groove section will be smaller than the skirt area.
Cast and hypereutectic cast pistons are usually .0005-.002 smaller than bore. Forged ones are in the .002-.007 or more depending on piston material and intended use.
Keep measuring until you can get repeatable results. You are doing exactly what few people take the time to master..working with precision measurements. Keep at it and have fun.....it will save you lots of grief in this hobby!
JIM
First I'd go through the calibration procedure on micrometer. Just guessing it seems to be a little high.
I say that because almost without exception, the clearance is built into the pistons. The bores are usually right on the 4.000 or 4.030 or whatever, with the pistons being slightly smaller. Yours are that way showing them to be .003 smaller than the bore, but since they are larger than 4.030 I would suspect the gauge is reading a little high.
Make sure you measure in the correct spot on pistons. Usually down by the pin at the upper end of skirt below ring grooves. The area of piston in the ring groove section will be smaller than the skirt area.
Cast and hypereutectic cast pistons are usually .0005-.002 smaller than bore. Forged ones are in the .002-.007 or more depending on piston material and intended use.
Keep measuring until you can get repeatable results. You are doing exactly what few people take the time to master..working with precision measurements. Keep at it and have fun.....it will save you lots of grief in this hobby!
JIM
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Joined: Aug 1999
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From: Connecticut, USA
Good for you in checking the bore both now and after it comes back from a shop. I just got back a .030" over bore that was machined beyond usable tolerance, in this case 4.036" I checked the bores before building the engine. A lot of people don't and go by what the shop says or even worse have the shop assemble it! A week or month goes by and the engine blows up. I use a dial bore gauge and set it by using a 4-5" micrometer. I even have pin made up for 4.032" to use like a feeler gauge. The machine work from the shop I used was so bad I have to go .040" now. My lawyer will have a nice report going when I add up all the costs.
