Measuring piston clearance after rebore

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BobUkPipedream
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Measuring piston clearance after rebore

Post by BobUkPipedream » Tue Mar 28, 2023 3:40 pm

Hi All,

Well I am slowly progressing, crank is now good as are the con rods all at 1.5 thou clearance with no binding. They were a pig to balance, I ended up doing twenty measurements and averaging…

So I thought I would check my pistons. These are new cam ground 0.20 oversize. I told the machine shop that bored the block to go with 0.0035 clearance minimum. They protested as the box said 0.002-0.003, I quoted this forum and that really I should run 0.004. They said they would do 0.0035 minimum.

I am not sure they did, but I may be measuring wrong. So I am measuring at 90 degrees to pin as below. Feeler gauges are metric but correspond to very very tight 0.002 at base of piston in first photo and about 0.0035 at mid point in second photo just under wrist pin (90 degrees to wrist pin but at just below height of wrist pin). I am assuming it is too tight, if so I am buying hone and fixing it myself. However what do you all think?
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speedytinc
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Re: Measuring piston clearance after rebore

Post by speedytinc » Tue Mar 28, 2023 4:07 pm

.002 is too tight. I like .0045-.005" Even .006", I would rather hear a little piston slap until the motor warms up than risk seizing.
Modern honing machines can be set for an exact amount of removal & stay perfectly parallel.
Doing the job by hand isnt so easy in both regards. An accurate cylinder bore gauge would be handy to check your work as you go.
If possible, take the block back & have em do it right.

Also check wrist pin fit. They are typically too tight. The pin should just fall out on its own weight when tilted on its side.


Kerry
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Re: Measuring piston clearance after rebore

Post by Kerry » Tue Mar 28, 2023 4:17 pm

Not the bore, you have a bit of variance on the piston skirt, a bit of sand paper will fix that.


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Re: Measuring piston clearance after rebore

Post by speedytinc » Tue Mar 28, 2023 4:23 pm

Kerry wrote:
Tue Mar 28, 2023 4:17 pm
Not the bore, you have a bit of variance on the piston skirt, a bit of sand paper will fix that.
Thats a possibility. Certainly worth checking with a calibrated micrometer.

In my experience, these Chinese pistons are perfect.(at least the ones I have installed)
I have measured them in all ways & weights.(trust but verify) They are real jewels. Of the rare few repop parts available.
The hobby is fortunate to have this fine product. Sad they cant be American made.
A far cry from the old Egge/Jahns pistons.

I realize I claimed "perfect". Well with the exception of the recommended bore clearance & the wrist pin fit.


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Re: Measuring piston clearance after rebore

Post by Kerry » Tue Mar 28, 2023 4:33 pm

Agreed, todays pistons are good, but measuring the clearance at the same spot of the bore as the photo's show, and the only thing you have moved is the piston, then it can only be a variance in the piston wall.


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Re: Measuring piston clearance after rebore

Post by BobUkPipedream » Tue Mar 28, 2023 5:27 pm

Thanks all for replies.

I have just checked all 4 pistons in all 4 holes and they all measure the same. So in-line with the pin about 11-12 thou at base. 90 degrees to pin at base, less than 2 thou. At the level of pin, but 90 degrees to it is 3.5 thou or so.

I had assumed that the taper of the piston continues to the base? So the advice on 4 or 5 thou clearance, is that at the pin level and ignore the very tight base of skirt? Or should it be minimum of 4 thou at base of skirt?

I have been through the pins polishing them and they all now slide out effortlessly, which is really annoying when you take the piston out the box and forget to hold the pin…

If I do hone, I suppose I am starting from a good straight bore and would only be taking a thou or so.


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Re: Measuring piston clearance after rebore

Post by speedytinc » Tue Mar 28, 2023 5:51 pm

The clearance is @ the maximum diameter of the piston skirt. Too tight.

Dont assume the bores are straight. VERIFY, VERIFY, VERIFY. Your shop already didnt follow your direction.
They should have known that .002 is to tight. Modern piston clearances in this bore size would be .001" per inch. .00375 here. At least they were 40 years ago.
I would want a bore gauge to confirm accuracy.


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Re: Measuring piston clearance after rebore

Post by Jerry VanOoteghem » Tue Mar 28, 2023 7:03 pm

Rather than use a feeler gauge, I would want to measure the bores with a bore gauge, and the pistons with a micrometer.


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Re: Measuring piston clearance after rebore

Post by Kerry » Tue Mar 28, 2023 7:41 pm

Still sounds like a piston problem, no way they are to taper from the oil ring base of the thrust sides of the pistons to the bottom by .0015" I double checked with new pistons I have in stock to be sure, spot on the same top to bottom.


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Re: Measuring piston clearance after rebore

Post by Dan Hatch » Tue Mar 28, 2023 10:21 pm

Check your wrist pins too. Most need a little honing cause they are too tight and will lock up.

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Re: Measuring piston clearance after rebore

Post by BRENT in 10-uh-C » Wed Mar 29, 2023 8:53 am

As an engine rebuilder myself, I guess I understand both sides of this ....somewhat. My initial thought (-from my experiences) is when a customer starts instructing their machinist to do something that counters what the manufacturer has suggested, the potential for liability always looms. And when the results of something are not as successful as the customer had hoped for, the first to be blamed is the machinist. So, when someone tells me "I read on the internet ...!", I generally start seeing little red flags waving and my stomach starts feeling wheezy. :? Surely we would all agree that some of the worst advice ever given on a subject is often originated on internet Forums. :o

I guess my advice differs from others here but the first thing I would do Bob, would be to load your block & pistons and go revisit your machinist to have him measure each piston and cylinder bore with the proper metrology for you. If you were in my shop, I would use the same micrometer (-that I am going to measure the pistons with) to set my bore gauge. That way I can show you accurate bore measurements of each area at 90° intervals and compare clearances to each piston. At that point, you have accurate numbers to assist you with knowing whether the machine work is correct, and which pistons are best suited for which hole. Based on your pictures above, likely your numbers are inaccurate.


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Re: Measuring piston clearance after rebore

Post by BobUkPipedream » Wed Mar 29, 2023 2:41 pm

Hi all,

Well I called another machine shop that had straightened the con rods for me and took the block and pistons to them. They do lots of vintage stuff including stuff for vintage racing and such. Anyway I said I thought I only had two thou based on my crude measurement with feeler gauges. Well they measured everything right there and then with a very fancy new bore gauge and mic and after several minutes said that I have… 2 thou clearance…

So I left the block with them and they will hone it out to a max of 4.5. I settled on that figure based on the fact that they have new accurate gear, understand the need for more clearance and I will have a newly cored rad and I took the core plugs out and pressure washed the water ways in the head. I have also made sure my pins are good, I polished the holes using autosol and the pins slide out under their own weight. Finally my car is a speedster so will not be lugging around loads of weight under load.

I am also getting them to skim the deck as it did have a bit of unevenness and I noticed the threads are pulling a bit on the head. The head bolts look a bit stretched, so when I get it back, I am going to chase threads, soak the holes in brake cleaner, blast them out with compressed air and then use new bolts.

This car is driving me crazy though, everything seems to just turn into a mini nightmare and every time I buy something or get machining done, it goes wrong in some way; the new bent Scat crank was just one small episode in a list of silly problems. I must have really done something bad in a previous life… However I am winning slowly one battle at a time and hopefully by June it will be running.


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Re: Measuring piston clearance after rebore

Post by Scott_Conger » Wed Mar 29, 2023 3:04 pm

If holes for headbolts look "iffy" now, then now is the time to remedy them properly. Since things are not already clean, it must be assumed that the block has not been cleaned because you're saving the old babbit? With a new SCAT crank? How are lifter and valve clearances? How are valve seats? Again, now is the time to remedy these things...
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BobUkPipedream
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Re: Measuring piston clearance after rebore

Post by BobUkPipedream » Wed Mar 29, 2023 3:29 pm

The lifters were okay in hole size, but I am putting in new std lifters to get round any dishing. The valves stem holes were a bit worn so oversize valves and recut seats.

The babbit was good overall, and even after bedding to 1.5 thou, I still have quite a few shims. I did fit a new centre cap just because I could and a main cap to get runout back to new. Generally I don’t think the engine did many miles as triple gears were good. I did put new triple gear pins in, but they were only a bit worn. The original bores were only a bit worn too, it felt a bit like sacrilege to change to Alu pistons, but at the time, I felt it would be best for the crank.

I wished I had a proper machine shop so that I could do it all myself, so much easier and satisfying. The most stressful thing for me is relying on other people for those machining jobs.


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Re: Measuring piston clearance after rebore

Post by Scott_Conger » Wed Mar 29, 2023 3:47 pm

You made me smile...the most stressful thing for ME is relying on ANYONE for ANYTHING, so I feel your angst.
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Re: Measuring piston clearance after rebore

Post by speedytinc » Wed Mar 29, 2023 3:59 pm

"everything seems to just turn into a mini nightmare and every time I buy something or get machining done, it goes wrong in some way; the new bent Scat crank was just one small episode in a list of silly problems."

Welcome to the reality of the hobby.
Model T mechanical's are a can of worms.
Isnt that to be expected on a 100 + year old thing that has gone thru dozens of "mechanics/wrench slingers" hands?
I, personally love the challenge with everyone I get into.


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Re: Measuring piston clearance after rebore

Post by RGould1910 » Thu Mar 30, 2023 10:40 am

Amen to avoid having others do work on your car. I'm convinced that shabby workmanship is the reason we do as much as we can ourselves. When I started in this hobby, the only tools I owned were carried in a tool pouch. Gradually I learned how to do most things needed to restore a car. I still farm out some things but am darn careful who I choose to do the work.i could tell you some real horror stories!

The other thing. There is nothing wrong or imprecise using long feeler gauges as you did. I've know respected auto machinists who do the same. The proof is in the results you found.

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