Pulled motor, bore size makes no sense

[Trucksareforwork]

I pulled a problem child motor out of my 1980 c10 that I fouled and glazed during a cam break in with a stuck secondary carb needle.

This is weird and I need somebody to help explain it:

Pulled motor clearly has glazed/worn cylinders.

Pistons are .040 see picture.

On the side I pulled the heads, bores measure 4.005 on an Amazon purchased bore gauge. (Technically 103mm or 4.055 minus .050 indicated on the gauge).

Is there any way that the truck had stock bore and somebody put 40 over pistons in it? Is that even possible?

I’m not a machinist and so don’t really do this much, but I can’t make sense of this bore size and the pistons stamped 040.

I was hoping to just do a hone and rearing but the bore is…too small????

Any experience folks able to explain?

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[bucket]

I'm going to say that your bore gauge is inaccurate, or you are using it incorrectly.

How did you get that 0.050 measurement?

 

[legopnuematic]

Agree with Andy. How did you set your bore gauge?

 

[Trucksareforwork]

I'm going to say that your bore gauge is inaccurate, or you are using it incorrectly.

How did you get that 0.050 measurement?

That’s my guess too. It happens when I try new things.

The gauge has interchangeable rods to set the approximate width. I used a combination of a 100mm rod and a 3mm spacer, and then zeroed the gauge. This should have me at 4.055 at zero.

Then I measure the bore and the gauge compresses in the bore to show how much less than 4.055 the actual bore is. I get .050. That leaves me with a 4.005 bore by my calcs.

I did this very quickly and didn’t have time to tinker with the gauge before life demanded I leave my shop. So I figured I’d post here for the typical “boy you are so stupid” feedback.

I will have to try again to see what other operator error I am introducing.

 

[Smtty58]

You can't base your measurements on the rod lengths. You set the rods base on the general size to measure, then zero the gage to a known standard. In your case I would buy a cheap 4"-5" inch micrometer. Or even a set of calipers would get you in the ballpark. This way you can measure a piston also, to answer that question.

 

[Trucksareforwork]

Thanks. I just got back here to post that I learned exactly that after a quick moment in the shop. I used a caliper to measure the rod and gauge as installed and it came to 103.9. When converted to inches thats 4.091 inches. Subtract my prior .050 and I’m at exactly what I expected: 4.041 …ish. I’m back to potentially a hone and re ring if all holes measure ok. Machine shop if not.

Thanks for chiming in. I didn’t even think about needing to zero to a known standard. Not my area of expertise but I guess now I know.

Edit: you forgot to say “boy you are so stupid.”

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[ChuckN]

Thanks. I just got back here to post that I learned exactly that after a quick moment in the shop. I used a caliper to measure the rod and gauge as installed and it came to 103.9. When converted to inches thats 4.091 inches. Subtract my prior .050 and I’m at exactly what I expected: 4.041 …ish. I’m back to potentially a hone and re ring if all holes measure ok. Machine shop if not.

Thanks for chiming in. I didn’t even think about needing to zero to a known standard. Not my area of expertise but I guess now I know.

Edit: you forgot to say “boy you are so stupid.”

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Nope just learning or finding out like the rest of us.

One thing I learned from my machinist is that even though pistons were .060 over, was that he actually measured the pistons that I’d purchased and set the bore to that for good piston to wall clearance. So even though it’s “60 over” if someone came to measure it after me, there could be minor variations with that, too.