Engine Rebuild Questions 172cid Sleeveless

[TheOldHokie]

There’s something wrong with these numbers
A block bore of 3.901 minus piston diameter 3.873 equals 0.028 clearance.
Actual clearance should be 0.0015-0.0035
Generally on a rebuild with a fresh overbite and new pistons we shot for .002-.003 piston to bore clearance using cast aluminum pistons

I am sorrv- you are correct - I dont know what I was thinking.

Lets go back to OPs numbers: 3.928 bore and 3.920 pistons. Thats a (nominal).030 overbore and .008 clearance. Correct?

Dan

 

[Destroked 450]

Sorry, I hit the wrong button and posted prematurely. Should read, "until I get it right" If those numbers are correct, then a new engine has a clearance between 3.900 cylinder and 3.872 piston of 0.028". Right now using my existing 3.900 pistons and 3.928 cylinder, my existing clearance between cylinder and piston is the exact same as a new engine, at 0.028". So, although I am going to replace my pistons, rings, pins and bearing, I think I will replace them with the same 3.900 pistons with correct rings for that oversize piston, unless someone suggests I may be wrong in my math. Thanks all, Larry. You guys saved me a real headache and possibly a number of returned parts during the rebuild. Oh yea, the actual clearances on each side of the piston to cylinder are half the numbers used because we are talking about a cylinder.

3.900 pistons in a 3.928 bore is way to loose and will result in severe piston slap, It’s likely the engine was rebuilt in the past using 0.010 or 0.020 oversized pistons and after years of use both the cylinders and pistons have worn
With the present 3.928 bore You can get 0.030 oversized pistons and hone the cylinders to size or use 0.020 oversized pistons and have a very lose fit
It doesn’t take much honing to remove 0.002 from a cylinder

 

[mhb@ufe]

I am sorrv- you are correct - I dont know what I was thinking.

Lets go back to OPs numbers: 3.928 bore and 3.920 pistons. Thats a (nominal).030 overbore and .008 clearance. Correct?

Dan

That would be .020 overbore and .008 clearance. If he ac

I am sorrv- you are correct - I dont know what I was thinking.

Lets go back to OPs numbers: 3.928 bore and 3.920 pistons. Thats a (nominal).030 overbore and .008 clearance. Correct?

Dan

3.900 pistons in a 3.928 bore is way to loose and will result in severe piston slap, It’s likely the engine was rebuilt in the past using 0.010 or 0.020 oversized pistons and after years of use both the cylinders and pistons have worn
With the present 3.928 bore You can get 0.030 oversized pistons and hone the cylinders to size or use 0.020 oversized pistons and have a very lose fit
It doesn’t take much honing to remove 0.002 from a cylinder

My original thought was it had .020 pistons and the current measurements are from wear. I agree with you about using .030 pistons and hone the bores to fit the pistons.

Mark

 

[Destroked 450]

I am sorrv- you are correct - I dont know what I was thinking.

Lets go back to OPs numbers: 3.928 bore and 3.920 pistons. Thats a (nominal).030 overbore and .008 clearance. Correct?

Dan

Yes using 3.920 pistons would give 0.008 clearance, but normally pistons are 0.001-0.003 undersize so total clearance will be a little more

 

[TheOldHokie]

Yes using 3.920 pistons would give 0.008 clearance, but normally pistons are 0.001-0.003 undersize

Which is why I suspwct they are meant for a .030 overbore.

Of course we are using the OPs measurements so that last digit may be suspect.....

Dan

 

[mhb@ufe]

Sorry, I hit the wrong button and posted prematurely. Should read, "until I get it right" If those numbers are correct, then a new engine has a clearance between 3.900 cylinder and 3.872 piston of 0.028". Right now using my existing 3.900 pistons and 3.928 cylinder, my existing clearance between cylinder and piston is the exact same as a new engine, at 0.028". So, although I am going to replace my pistons, rings, pins and bearing, I think I will replace them with the same 3.900 pistons with correct rings for that oversize piston, unless someone suggests I may be wrong in my math. Thanks all, Larry. You guys saved me a real headache and possibly a number of returned parts during the rebuild. Oh yea, the actual clearances on each side of the piston to cylinder are half the numbers used because we are talking about a cylinder.

Have you checked the top of the pistons you removed to see if they are stamped? They generally are stamped as STD, 0.010, 0.020, or what ever over size they are.

Mark

 

[560Dennis]

I might have miss this ,was in the plans , was The deck of the block and head checked for flatness with precision straight edge since the head gasket failed.

 

[LarryMaine]

YOU ARE MISCALCULATING CLEARANCE.

Apparently the stock bore is 3.901 Accordingly a stock piston is finished undersize to provide the proper clearance. The piston OEM will decide the clearance needed based on their choice of materials.

As the engine builder your reference dimension for selecting and ordering parts s the bore in the block. According to your measurements your block is at 3.928.which is .027 over factory. Accordingly a .030 over piston (readily available) matches your block bore.

Its as simple as that.

Dan

Dan, thanks very much. This is exactly the information and advice that I needed. Everyone has been really helpful, but people come at this from different directions and with different experience and training. My other re-builds were simple as they were all sleeved engines. Because this one had been re-bored once, and already had some oversize pistons, it got kind of confusing figuring out what was original clearance, what had been done to the engine during the rebore, and what I needed now due to any more recent wear after the prior owners rebuild. Of course, it would have been so much easier if Ford had provided actual clearance numbers rather than telling us to use a feeler gauge and pull gauge to determine the correct parts - fit - clearance. I concur, the .030 over pistons and rings are readily available. I probably could have avoided all this back and forth if I just had the machine shop do another re-bore, but the cylinder dimensions were all consistent, and the govt. was not prepared to increase my social security payments to enable me to afford the additional shop work. Again, thanks for all the follow ups, it really is appreciated. Best regards, Larry.

 

[TheOldHokie]

Dan, thanks very much. This is exactly the information and advice that I needed. Everyone has been really helpful, but people come at this from different directions and with different experience and training. My other re-builds were simple as they were all sleeved engines. Because this one had been re-bored once, and already had some oversize pistons, it got kind of confusing figuring out what was original clearance, what had been done to the engine during the rebore, and what I needed now due to any more recent wear after the prior owners rebuild. Of course, it would have been so much easier if Ford had provided actual clearance numbers rather than telling us to use a feeler gauge and pull gauge to determine the correct parts - fit - clearance. I concur, the .030 over pistons and rings are readily available. I probably could have avoided all this back and forth if I just had the machine shop do another re-bore, but the cylinder dimensions were all consistent, and the govt. was not prepared to increase my social security payments to enable me to afford the additional shop work. Again, thanks for all the follow ups, it really is appreciated. Best regards, Larry.

The thickness of the feeler gauge is the clearance. The spring scale is just a half a$$ed way to compensate for the curvature and associiated drag of the piston and bore . You will find it in lots of shop manuals.

Dan